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Objectives: • Identify the principal functions and features of charge controllers. • Define charge regulation and load control setpoints. • Explain how temperature compensation affects setpoints. • Differentiate between various types of charge controller algorithms and switching designs. • Understand the impacts of control algorithms and setpoints on system and battery performance. • Identify concerns and requirements for charge controller applications and installation. • Identify the requirements for PV systems operating without charge control. CHARGE CONTROLLER FEATURES A charge controller is a device that regulates battery charge by controlling the charging volt age and /or current from a DC power source, such as a PV array. A charge controller in a PV system maintains a battery at its highest possible state of charge while protecting the battery from overcharge by the array and overdischarge by system loads. This maximizes available battery capacity and cycle life. However, charge controllers alone can't ensure that a battery receives a full charge. The array must be large enough to adequately supply the load and charge the batteries during periods of low insolation. Charge controllers orchestrate the interaction between major PV system components. They manage energy flow between the array, battery bank, and electrical loads. After system sizing, charge controllers have the greatest impact on overall system performance. Some charge controllers also include load control and other energy management features, and may indicate system status through LEDs or LCD displays. 1. Charge controllers manage interactions and energy flows between a PV array, battery bank, and electrical load. PV ARRAY; BAT BANK; DC LOAD Charge controllers are characterized by their means and methods of charge regulation and the battery conditions (setpoints) used to trigger charge control functions. Charge controllers are rated for specific voltage and current levels. Many include a number of additional features and functions to control system loads, display and record system data, indicate system operational status, or otherwise enhance the ability to manage and optimize the performance of the system. Battery Charging Charge controllers manage the array current delivered to a battery, and consequently affect the way a battery accepts charge. Charge acceptance is the ratio of the increase in battery charge to the amount of charge supplied to the battery. Charge acceptance is also known as charge efficiency. For example, if a charge cycle delivers 60 Ah in order to gain 54 Ah of additional battery charge, the charge acceptance is 90% (54 Ah ÷60 Ah = 0.90 or 90%). Since a given amount of capacity always requires more charge to overcome losses and inefficiencies, charge acceptance is always less than 100%. Charge controller designs use various methods to regulate current and voltage during battery charging. Single-stage charging processes are simpler, but relatively inefficient. Multistage charging changes the applied volt age and /or current in steps to bring the battery to a higher state of charge. This process is more sophisticated, requiring additional electronics, but it greatly improves charge acceptance. See 2. Multistage methods can charge batteries faster and limit excessive overcharge and gassing. Overcharge is the ratio of applied charge to the resulting increase in battery charge. For ex ample, a battery that accepts 60 Ah and regains 54 Ah of charge receives an overcharge of 111% (60Ah ÷ 54Ah = 1.11 or 111%). Overcharge is the inverse of charge acceptance. The amount of overcharge required depends on the type of battery and its condition and state of charge. It also depends on the rate and manner in which charge is delivered to the battery, which is governed by the charge controller. Charge acceptance is generally greater than 90% for most batteries at normal charge rates up to about 90% state of charge. Then it decreases significantly due to internal losses as the battery reaches full charge. Correspondingly, a higher overcharge is required to return a battery to full state of charge from a shallow depth of discharge than from a greater depth of discharge. During the final stages of charging, some of the current sup plied to the battery is consumed by gassing and other internal battery losses, lowering charge efficiency. Overcharge Conditions It should be noted that the term “overcharge” may be used to describe two different conditions of battery charging. First, normal charging requires that more charge be applied to the battery than the expected gain in state of charge. This type of overcharge is necessary because some of the charge is lost through various battery inefficiencies and gassing. As an analogy, imagine a cup with a small hole at the bottom. As water is poured in (charging), a small amount leaks out (losses). Ultimately, the amount of water in the cup (state of charge) is slightly less than the amount of water that was poured in (applied charge). Therefore, a little extra water (overcharge) is needed to reach a certain level. The second definition of overcharge applies to batteries that are already fully charged. When the water level reaches the top of the cup (full charge), the pouring (charging current) should be stopped or reduced to a very small amount. (A very small charging current replaces the charge lost through self-discharge.) If the pouring (current) is not stopped or reduced, the cup will overflow (overcharge). This overcharge results in excessive gassing and can dam age the battery. NORMAL OVERCHARGING: CHARGE LOSSES and SELF-DISCHARGE; APPLIED CHARGE; CHARGING CURRENT; STATE OF CHARGE; OVERCHARGING ABOVE FULL CHARGE: CHARGING CURRENT; CHARGE LOSSES and SELF-DISCHARGE TIME SINGLE-STAGE low on the charge controller. In properly sized systems, deficit charge should be recoverable when insolation and load conditions return to normal. However, if lead-acid batteries experience repeated deficit charge cycles and are left at partial state of charge for long periods, they will become progressively more difficult to charge due to sulfation and /or stratification. When charged, voltage will rise more quickly in batteries with sulfation or stratification than in normal batteries, falsely indicating a high state of charge. Recovery from this condition can sometimes be accomplished by an equalization charge. Undercharged batteries are a major cause of PV system failures and inability to meet system loads. Deficit charge is a charge cycle in which less charge is returned to a battery bank than what was withdrawn on discharge. Deficit charge can occur in PV systems during periods of low insolation and high load, particularly for marginally sized systems. It may also be caused by the regulation voltage being set too Overcharge Protection During high insolation periods, energy generated by an array may exceed the electrical load demand. The excess may charge a battery, but if it is already at a high state of charge, then the charging current must be carefully controlled to avoid overcharging. In this case, overcharge is the condition of a fully charged battery continuing to receive a significant charging current. A charge controller protects a battery from over charge through charge regulation. This involves either interrupting or limiting the current flow from the array when the battery approaches a full state of charge. Active means of overcharge protection are required for most PV systems using batteries. According to the NEC, any PV system employing batteries shall have equipment to control the charging process unless the maximum array charge current multiplied by one hour is less than 3% of the rated battery capacity in ampere- hours. This equates to a charge rate of C/33. For example, a 100 Ah battery requires charge control if the maximum array charging current exceeds 3A. Without charge control, the current from the array will flow into a battery proportionally to the irradiance. When a battery is fully charged, unregulated charging will cause the battery voltage to reach exceedingly high levels, causing severe gassing, electrolyte loss, internal heating, and accelerated grid corrosion. If a battery is not protected from overcharge, it will likely lose capacity and fail prematurely. REGULATION VOLTAGE; CHARGING CURRENT IS ON OR OFF; REGULATION VOLTAGE; CHARGING VOLTAGE; BATTERY VOLTAGE; TIME MULTISTAGE 2. Single-stage battery charging is simpler, but multistage battery charging brings batteries to a higher state of charge. Array switching can be done on either the positive or negative leg of the array circuit However, charge controllers that switch the negative leg may not be grounded to the negative leg, which can affect controller regulation functions. 3. Charge controllers protect batteries from overcharge by terminating or limiting charging: Overcharge Protection; ARRAY; CHARGE CONTROLLER ARRAY CURRENT; LOW STATE V OF CHARGE; HIGH STATE OF CHARGE Overcharge Protection During periods of low insolation or excessive load demand, a marginally sized array may not produce sufficient energy to charge the battery. Overdischarge is the condition of a battery state of charge declining to the point where it can no longer supply discharge cur rent at a sufficient voltage without damaging the battery. A charge controller protects a battery from overdischarge through load control, disconnecting electrical loads when the battery reaches a low voltage (low state of charge) condition. See 4. Overdischarge protection limits battery depth of discharge and prevents damage to the battery. Since some loads will operate improperly, or not at all, at lower than normal voltages, this feature also protects them. Under cold conditions, overdischarge protection also prevents a battery from freezing. Once the battery is charged to a certain level, the loads are reconnected to the battery. Some controllers combine the charge regulation and load control functions in order to protect batteries from both overcharge and overdischarge, while others have only one function. Also, these charge and discharge functions can be combined with other electrical components, such as inverters, to form multifunction power conditioning units (PCU). Status Information Most charge controllers provide information on the operational state of the system. For charge regulation, this may include digital displays or LED lights to indicate when the battery is charging and when it reaches full charge. Other indicators may be used to alert operators of a low battery voltage or low state of charge condition, such as a red light or an audible alarm. See 5. More advanced controllers may include meters to monitor battery and array currents and voltages or track the amount of energy supplied to and from a battery. This system status information can be extremely useful for improving load management and optimizing system performance. BATTERY BANK; RGING CURRENT IS FULL ARRAY CURRENT; LOW STATE OF CHARGE; ARRAY; CHARGE CONTROLLER; HIGH STATE OF CHARGE CHARGE CONTROLLER ALLOWS CURRENT TO LOAD; CHARGE CONTROLLER DISCONNECTS LOAD, INTERRUPTING DISCHARGE CURRENT 4. Charge controllers protect batteries from overdischarge by disconnecting loads at low battery voltage. Circuit Protection Some charge controllers incorporate features to protect the unit or loads from damage due to high currents or improper wiring, such as reversed array or battery polarity. Overcurrent devices, such as a fuse or circuit breaker, protect the controller from array or load currents significantly higher than its rating. Some controllers also protect system loads by sensing when the battery is disconnected and then disconnecting the array, preventing potentially high array open-circuit voltages or unregulated currents from damaging the loads. Advanced Load Control Charge controllers also may provide other load control functions besides battery overdischarge protection. For example, charge controllers in PV lighting systems may activate and control the lighting load based on ambient light conditions or time of day. The array can operate as a light sensor, indicating nighttime conditions when the controller measures the array current or voltage drop to a preset low level. Alternatively, clocks or timers integrated into charge controllers can activate loads for prescribed periods. CHARGE (LOAD) CONTROLLER; CHARGE (LOAD); BATTERY BANK; DISCHARGING CURRENT; DC LOAD OF CHARGE; HIGH STATE OF CHARGE; DC LOAD; LOW STATE OF CHARGE; CHARGE CONTROLLER; AMPS WATTS VOLTS 5. Most charge controllers include displays or LEDs to indicate battery voltage, state of charge, and /or present operating mode. Charge Controller Status In small PV systems, the charge controller may also simplify wiring by providing conductor termination points for the array, battery, and load circuits Most charge controllers have easy to use screw terminals for making connections. CHARGE CONTROLLER TYPES Charge controllers are characterized by the way they regulate charging current to a battery. A charge control algorithm is a programmed series of functions that a charge controller uses to control current and /or voltage in order to maintain battery state of charge. Charge controllers use a variety of algorithms and switching methods to provide charge regulation. In conjunction with the charge control setpoints, these algorithms are selected or modified to optimize battery charging, array energy utilization, load avail ability, and overall system performance. Most charge controllers use solid-state switching elements, typically transistors. An interrupting-type charge controller is a charge controller that switches the charging current ON and OFF for charge regulation. Interrupting-type charge controllers are simple designs, but not widely used in PV systems. Since the array current is switched abruptly, they have difficulty avoiding excessive overcharge and are best used with the more tolerant flooded, open-vent batteries. A linear-type charge controller is a charge controller that limits the charging current in a linear or gradual manner with high-speed switching or linear control. Linear-type charge controllers provide a more consistent charging process, so they can generally charge batteries more efficiently and faster. Linear-type controllers are compatible with more types of batteries. Shunt Charge Controllers Unlike batteries, PV devices are current- limited by nature, so PV modules and arrays can be short-circuited without any harm. A shunt charge controller is a charge controller that limits charging current to a battery system by short-circuiting the array. The array is short-circuited through a shunt element inside the charge controller, moving the array’s operating point on the I-V curve very near the short-circuit condition and limiting the power output. All shunt controllers must also include a blocking diode in series between the battery and the shunt element to prevent the battery from short-circuiting. See 6. Because there is some voltage drop through the shunt element and blocking diode, the array is never completely short-circuited, resulting in some power dissipation within the controller. For this reason, most shunt controllers include a heat sink to dissipate power and are generally limited to array source circuits with currents of 20 A or less. Shunt-Interrupting Charge Controllers. A shunt-interrupting charge controller is a charge controller that suspends charging cur rent to a battery system by completely short- circuiting the array. When the battery voltage falls, the controller reconnects the array to resume charging the battery. This ON/OFF cycling of the array current may occur once over a period of several minutes, but at higher charge rates and as the battery approaches full state of charge, the cycling may occur every few seconds. Shunt-Linear Charge Controllers. A shunt- linear charge controller is a charge controller that limits charging current to a battery sys tem by gradually lowering the resistance of a shunt element through which excess current flows. When the battery state of charge and array current are high enough during the day, a shunt-linear controller maintains the battery at the regulation voltage. In some designs, a comparator circuit in the controller monitors the battery voltage, and makes corresponding adjustments to the resistance of the shunt element to regulate array current. A Zener power diode can also be used for simple shunt-linear designs. Shunt charge controllers are so named because they divert array current to a parallel (or shunt) path when charge regulation s required This is the same as short circuiting the array Although most electrical power sources are extremely dangerous to short circuit, it can be done safely with PV devices because they are inherently current-limited Series Charge Controllers A series charge controller is a charge controller that limits charging current to a battery system by open-circuiting the array. As the battery reaches full state of charge, a switching element inside the controller opens, moving the array’s operating point on the I-V curve to the open-circuit condition and limiting the power output. This method works in series between the array and battery, rather than in parallel as for the shunt controller. See 7. Shunt Charge Controllers 6. Shunt charge controllers regulate charging current by short-circuiting the array. Series Charge Controllers: BATTERY BANK; LOAD; DC LOAD; DISCONNECT ELEMENT; BATTERY BANK 7. Series charge controllers regulate charging current by opening the circuit from the array. Because a series controller open-circuits rather than short-circuits the array, as in a shunt-controller, no blocking diode is needed to prevent the battery from short-circuiting when the controller regulates. Since no blocking diode is used, most series controllers open the circuit between the array and battery at nighttime to prevent reverse current losses from the battery through the array. Series-Interrupting Charge Controllers. A series-interrupting charge controller is a charge controller that completely open-circuits the array, suspending current flow into the battery. When the battery voltage falls, the controller closes the series switching element to reconnect the array and resume charging the battery. Similar to shunt-interrupting charge controllers, the rate of the ON/OFF cycling is dependent on charge rate and battery state of charge. Series-Linear Charge Controllers. A series- linear charge controller is a charge controller that limits charging current to a battery system by gradually increasing the resistance of a series element. This limits current flow into the battery and maintains the battery at the voltage regulation setpoint (as long as array current is sufficiently high). The series regulation element acts like a variable resistor and loads the array at lower current and power levels by operating the array to the right of the maximum power point of its I-V curve. Since series-linear controllers also dissipate power, they generally require a heat sink. Series-Interrupting, Pulse-Width-Modulated (PWM) Charge Controllers. A series-interrupting, pulse-width-modulated (PWM) charge controller is a charge controller that simulates a variable charging current by switching a series element ON and OFF at high frequency and for variable lengths of time. A PWM charge controller pulses the full charging current and varies the width of the pulses to regulate the amount of charge cur rent flowing into the battery. See 8. The frequency of the pulses is several hundred hertz and the pulses may last only a couple of milliseconds. When the battery is partially charged, the current pulse is essentially ON all the time. To simulate a lower charging current as the voltage rises, the pulse width is decreased. For example, if the pulses switch the full charging current so that it is ON half the time and OFF half the time, the resulting current effectively simulates a charge current at 50% of the full current. Pulse-Width Modulation 8. Pulse-width modulation (PWM) simulates a lower current level by pulsing a higher current level ON and OFF for short intervals. Pulse-width modulation can either convey information over a communications channel or control the amount of power sent to a load. Additional applications of PWM include motor controls, telecommunications, audio amplifiers and synthesizers, light dimmers, and voltage regulators. Similar to the series-linear algorithm in performance, power dissipation within PWM controllers is considerably lower than for other interrupting-type designs. PWM charge controllers can be used with all battery types, and the controlled manner in which power is applied to the battery makes them especially preferred with sealed batteries. PWTvI charge controllers automatically adjust for varying battery charge acceptance over time, help prevent electrolyte stratification and sulfation, and minimize equalization requirements. FULL CURRENT PULSES OF VARYING WIDTH; PULSE FREQUENCY Maximum Power Point Tracking (MPPT) Charge Controllers Newer charge controllers incorporate the latest in power electronics and microprocessor controls to optimize system performance and allow greater flexibility in system design. A maximum power point tracking (MPPT) charge controller is a charge controller that operates the array at its maximum power point under a range of operating conditions, as well as regulates battery charging. An MPPT circuit monitors array output and dynamically changes its resistance or input voltage to move the operating point on the array’s I-V curve toward the maximum power point. See 9. The power is then transformed by a DC-DC converter circuit into another voltage and current required by the load, in this case a battery. Maximum Power Point Tracking 9. Maximum power point tracking manipulates the load or output voltage of an array in order to maintain operation at or near the maxi mum power point under changing temperature and irradiance conditions. MPPT charge controllers improve the battery- charging performance of PV systems. For example, a nominal 48 V battery bank is typically charged by four 36-cell crystalline silicon modules connected in series to ensure adequate battery-charging voltage under the warmest of operating conditions. Under cool conditions, however, the maximum power voltage of this array may be higher than 65 V, while the battery voltage may be 50 V to 55 V during charging. Without MPPT control and DC-DC conversion, the array operates at the lower battery voltage, which is a potential power loss up to about 20%. MPPT charge controllers ensure the highest possible array power utilization, and ensure adequate battery-charging voltages under any condition and for a multitude of array and battery voltages. Diversionary Charge Controllers Since stand-alone PV systems are designed to supply power to the loads and charge the battery system under worst-case conditions, they often have excess energy available during periods of high insolation or low load. Conventional charge controllers waste this excess power by disconnecting or limiting array current. A diversionary charge controller is a charge controller that regulates charging current to a battery system by diverting excess power to an auxiliary load. Instead of wasting excess power, the auxiliary load provides a useful output with the diverted power. See 10. The diversion load is an auxiliary load that is not a critical system load, but is always available to utilize the full array power in a useful way to protect a battery from overcharge. Examples of diversion loads include loads with inherent energy storage, such as resistance water heaters, and loads that are especially useful under high insolation, such as ventilation fans or irrigation pumps. The inverters in battery-based interactive PV systems are also considered diversion loads. Since constant availability of the diversion load can't be guaranteed, the NEC requires that a second independent means of charge control be used to prevent overcharging the battery wherever diversionary charge control is used. This is usually accomplished by including a conventional series controller. This helps ensure that if any part of a diversionary charge control system fails, the batteries will not be overcharged and create a potentially hazardous condition. NEC® requirements also state that for DC diversion loads, the load voltage rating shall be greater than the maximum battery voltage, and the power ratings of the load shall be at least 150% of the power rating of the array. VOLTAGE; MPPT CIRCUIT CHANGES ARRAY VOLTAGE TO MAXIMIZE POWER Diversion FULLY CHARGED BATTERY BANK; FLOAT CHARGE CURRENT; SMALL DISCHARGE CURRENT WHEN MOST NORMAL LOADS NOT NEEDED 10. Diversionary charge controllers regulate charging current by diverting excess power to an auxiliary load when batteries are fully charged. Hybrid System Control Since the PV array in hybrid systems is not sized to meet all the loads by itself, the auxiliary energy sources are required to provide supplemental battery charging. A hybrid system controller is a controller with advanced features for managing multiple energy sources. In many cases, hybrid system control functions are integrated into multifunction PCUs. See 11. A principal feature of many hybrid controllers is the ability to start and control an engine generator. Battery voltage is typically used to control engine generator operation, though it can also be controlled based on time of day, load currents, or scheduled cycles of equalization charging or engine exercising. Once a condition requiring engine generator operation occurs, the controller automatically activates the engine starting circuit. For diesel engines, the controller warms the glow plugs for a short period prior to starting. To limit the wear and tear on the starter, the cranking time and the number of starting attempts can also be controlled. If the engine does not start quickly, the controller indicates this condition with an LED or an error message, suggesting that engine service is required. After the engine starts, the controller may allow a short warm-up period before the full load is connected. The engine generator may operate for a specific amount of time or until the battery voltage reaches a certain level, then shuts down until needed again. If the engine generator is not needed often, the controller runs a regular programmed exercise cycle. The exercise cycle operates the engine for a few minutes at a set interval, such as once a week, to keep it in good working order. ARRAY; DIVERSIONARY CHARGE CONTROLLER Hybrid systems can include any combination of alternative energy sources, such as a PV array and a wind turbine. 11. Controllers designed for hybrid PV simultaneously. The operation of engine generators can be controlled to maximize the use of array energy and set preferred times of operation. For example, the engine generator may not be allowed to operate early in the day or at a high state of charge when it may cause the array controls to regulate. Also, it may not be desirable to have noisy engine generators operating at night. However, excessively high load currents or extremely low battery voltage may temporarily override these controls to permit battery charging or operation of loads directly from the generator. Ampere-Hour Integrating Charge Controllers Charge controllers can also regulate charge current based on parameters other than battery voltage. An ampere-hour integrating charge controller is a charge controller that counts the total amount of charge (in ampere-hours) into and out of a battery and regulates charging cur rent based on a preset amount of overcharge. See 12. For example, a lighting load may discharge 72 Ah from the battery each night. To charge the battery, an ampere-hour charge controller set for 115% overcharge would allow 83 Ah (72 Ah x 115% = 83 Ah) of charge to flow back into the battery before terminating charging. ARRAY WIND TURBINE ENGINE GENERATOR HYBRID SYSTEM; CONTROLLER AC INPUT; BATTERY BANK; ENGINE CONTROL and MONITORING CIRCUIT LOAD systems must manage multiple current sources Charge Controllers TIME (HR) 12. Ampere-hour charge controllers track the cumulative number of ampere-hours applied to a battery bank and discontinue charging at a preset total. CHARGE CONTROLLER SETPOINTS A charge controller setpoint is a battery condition, commonly the voltage, at which a charge controller performs regulation or switching functions. Setpoints are associated with either charge regulation or load control. Charge controller setpoints define the range of permissible battery and system voltages. The setpoints greatly affect battery performance, life, and load availability, so their proper specification and adjustment is vital. Simple interrupting-type charge controllers use two basic setpoints for charge regulation and two basic setpoints for load control. Additional setpoints may be available on more advanced charge controllers. Charge Regulation Setpoints Optimal charge regulation setpoints ensure that the battery is maintained at the highest possible state of charge without overcharging and over a range of conditions. Two charge regulation setpoints regulate the charging function. A higher voltage setpoint is used to disconnect the array from charging the battery and a lower voltage setpoint is used to reconnect the array and resume battery charging. See 13. Charge Regulation Setpoints TIME 13. Charge regulation setpoints are the voltage levels at which the charge controller interrupts or reconnects the charging current from the array to the battery bank. Voltage Regulation (VR) Setpoint. The voltage regulation (VR) setpoint is the voltage that triggers the onset of battery charge regulation because it is the maximum voltage that a battery is allowed to reach under normal operating conditions. The VR setpoint may also be called the regulation voltage. When a battery reaches the VR setpoint, a controller will either discontinue or limit the array charge current to prevent overcharging the battery. For charge controllers that provide multistage charging, the VR setpoint defines the beginning of absorption charge. The optimal VR setpoint depends on several factors, especially the controller algorithm, temperature, and type of battery. See 14. VR setpoints for PWM and linear-type charge controllers are generally lower than those for interrupting controllers because they are more efficient at charging. The VR setpoints used in PV systems are typically much higher than the regulation volt ages recommended by battery manufacturers. This is because the battery must be charged within a limited time in a PV system (during sunlit hours), while battery manufacturers allow for the much longer charge times possible in other applications. If the VR setpoint in a typical PV system were set at the manufacturer’s recommended regulation voltage, the batteries in most PV applications would never fully charge. A higher setpoint allows a battery to be charged in a shorter time, but must be low enough to avoid excessive overcharge and gassing. In small, stand-alone PV systems, such as an electronic sign, the charge controller is especially important. VOLTAGE REGULATION HYSTERESIS; VOLTAGE REGULATION SETPOINT; ARRAY; RECONNECT; VOLTAGE SETPOINT Consult battery and charge controller manufacturer’s literature to determine optimal regulation setpoints * in V at 25°C Per Cell Flooded open-vent lead-acid 2.43 to 2.47 Sealed valve-regulated lead-acid (VRLA) 2.37 to 2.40 Flooded pocket plate nickel-cadmium 1.45 to 1.50 Per Nominal 12V Battery 14.6 to 14.8 14.2 to 14.4 14.5 to 15.0 Per Cell 2.40 to 2.43 2.33 to 2.37 1.45 to 1.50 12 V Battery 14.4 to 14.6 14.0 to 14.2 14.5 to 15.0 14. The optimal voltage regulation setpoint depends on the types of battery and charge controller. Array Reconnect Voltage (ARV) Setpoint. For interrupting-type charge controllers, once the array current is disconnected at the voltage regulation setpoint, the battery voltage will begin to decrease. When the battery voltage decreases to a predefined voltage, the array is again reconnected to the battery and charging resumes. The array reconnect voltage (ARV) setpoint is the voltage at which an interrupting- type charge controller reconnects the array to the battery and resumes charging. [ Battery charging in PV systems is considered opportunity charging because a battery can be charged only during sunlit hours Therefore, PV system charge controllers use different battery charging setpoints than do conventional battery chargers that are connected to a constant power source, such as the utility.] The array current cycles into the battery in an ON/OFF manner because it is disconnected at the VR setpoint and reconnected at the ARV setpoint. As the battery is brought up to a higher state of charge, the duration of each cycle becomes shorter, and the array current remains disconnected from the battery for a longer time. Linear- and PWM-type charge controllers maintain a battery at the regulation voltage by reducing, rather than completely disconnecting, the charging current. The array current is controlled to hold the battery at the regulation voltage as it completes charging. Therefore, these charge controllers do not have a clearly definable ARV setpoint. Voltage Regulation Hysteresis (VRH). The voltage regulation hysteresis (VRH) is the voltage difference between the voltage regulation (VR) setpoint and the array reconnect voltage (ARV) setpoint. The VRH applies only to interrupting-type charge controllers. For example, a controller with a YR setpoint of 14.5 V and an ARV setpoint of 13.5 V has a voltage regulation hysteresis of 1.0 V. The VRH is vitally important to the performance of interrupting-type charge controllers. If the VRH is too large, the array current remains disconnected for long periods, wasting energy from the array and making it difficult to fully charge the battery. If the VRH is too small, the array will cycle ON and OFF rapidly, which limits the life of switching elements. The battery charge and discharge rates also affect the optimal VRH. Battery voltage changes much more quickly at high rates than at lower rates, suggesting a larger VRH value. In general, a smaller VRH is required for PV systems that do not have daytime loads to ensure the array remains charging a sufficient amount of time. Most interrupting-type controllers use VRH values between 0.4 V and 1.4 V based on a nominal 12 V battery system. Since ARY setpoints do not apply to linear or PWM designs, VRH does not either. Load Control Setpoint Load control protects batteries from overdischarge by turning the load circuits ON and OFF as needed. See 15. Load control features are common on charge controllers, or may be added as a separate controller. Low-Voltage Disconnect (LVD) Setpoint. The low-voltage disconnect (LVD) setpoint is the voltage that triggers the disconnection of system loads to prevent battery overdischarge because it is the minimum voltage a battery is allowed to reach under normal operating conditions. At given system discharge rates, the LVD setpoint determines the battery’s maximum allowable depth of discharge, which affects its life, available capacity, and electrolyte freezing point. Load disconnect circuits typically use solid-state switches, but since disconnects occur considerably less frequently in PV systems than charge regulation cycles, simple electromechanical relays are sometimes used. If the load current exceeds the rating of the controller’s switching element, higher-rated auxiliary relays maybe activated by the primary control relay to handle the full load current. Many charge controllers also have a visual or audible signal to alert system operators of the low voltage disconnect condition. Load disconnections should generally be infrequent in most PV systems. Proper sizing of the battery and array to meet the loads are keys to minimizing load disconnections. If a load disconnect event occurs in a marginally sized system, many days or weeks may be required to fully charge the battery. The LVD setpoint involves a tradeoff between battery life and load availability. Disconnecting loads from a battery at a voltage that is too low results in excessively deep battery discharges and limits battery life, but improves load availability in the short-term. However, disconnecting loads at too high a voltage protects the battery better, but sacrifices load availability during low insolation periods. In general, the LVD setpoint in PV systems is selected to discharge deep-cycle batteries to no more than 75% to 80% depth of discharge, based on their rated capacity. Typical LVD setpoint values used for nominal 12 V lead- acid batteries at discharge rates lower than C130 are between 11.2 V and 11.5 V. High discharge rates require a lower LVD setpoint to achieve the same depth of discharge. For shallow-cycle batteries, such as SLI batteries, or batteries that are subjected to freezing temperatures, a higher LVD setpoint should be used to limit the depth of discharge. Load Reconnect Voltage (LRV) Setpoint. The load reconnect voltage (LRV) setpoint is the voltage at which a charge controller reconnects loads to the battery system. After the controller disconnects the load from the battery at the LVD setpoint, the battery voltage rises quickly, then more gradually until its open- circuit voltage is reached, additional charge is provided b the array or another source, the battery voltage rises even more. When a controller senses the battery has reached the LRV, the load is reconnected to the battery. The LRV setpoint also affects battery health and load availability, and defines the state of charge at which loads are reconnected to the battery. The LRV setpoint should be high enough to ensure the battery has been some what charged before reconnecting the loads, but low enough so that loads do not remain disconnected for unreasonably long periods. Some controller designs “lock out” loads until the next day or when the controller senses that the array is again charging the battery. This prevents the unwanted condition of multiple load disconnects and reconnects occurring in one evening, for example, in a PV lighting system. Load Control Setpoints:
15. Load control setpoints are the volt age levels at which the charge controller interrupts or reconnects the discharging current from the battery bank to the loads. Critical Loads Some PV systems powering loads of varying importance use multiple LVD setpoints and load controls to disconnect loads at progressively lower battery voltages. This preserves battery charge for the most important loads. A noncritical load is disconnected from the battery system by the charge controller at a higher LVD setpoint to retain some useful battery charge for critical loads. A critical load is connected to power for as long as possible, perhaps even past the normal low-voltage load disconnect point. A load controller may have separate connections for loads of different priorities, each with different LVD setpoints, or an extremely critical load may be connected directly to the battery system so it can't be disconnected. However, even critical loads will operate only until the battery system completely discharges. Complete discharge can damage the batteries, so these systems must be carefully designed and operated. For example, a PV lighting system with several independent lighting fixtures may protect a battery from severe discharges by disconnecting some of the lighting loads when the battery reaches a certain depth of discharge, reducing lighting levels instead of disconnecting the entire system. Multiple load disconnect setpoints are sometimes used for remote medical clinics where vaccine refrigerators are the most critical loads and are set to disconnect at a battery depth of discharge greater than 90%, where noncritical loads are disconnected at a much higher setpoint and state of charge. LRV setpoints used in small PV systems vary between 12.5 V and 13.0 V (at 25°C) for most nominal 12 V lead-acid batteries. Under typical charge and discharge rates in PV systems, this ensures that the battery reaches 25% to 50% state of charge before the load is reconnected. Systems with dynamic loads or with very high charge or discharge currents may use different setpoints to tailor the settings to the desired battery and load performance. Low-Voltage Disconnect Hysteresis (LVDH). The low-voltage disconnect hysteresis (LVDH) is the voltage difference between the low- voltage disconnect (LVD) and load reconnect voltage (LRV) setpoints. If the LVDH is too small, the load may cycle ON and OFF rapidly at low battery state of charge, possibly damaging the load or control elements, and extending the time it takes to fully charge the battery. If the LVDH is too large, the load may remain OFF for extended periods while the array more fully charges the battery. The reduced cycling improves battery health, but also limits load availability during low insolation periods. The proper LVDH selection for a given system will depend on load availability requirements, and the battery’s charge and discharge rates. Float Charge Setpoint Multistage charge controllers with an absorption charge cycle may also include a float charge setpoint, which is a slightly lower voltage than the VR setpoint. When the battery completes the normal charge, the controller lowers the voltage to the float charge setpoint. Float charging holds the battery at a fixed voltage by limiting charge current. Similar to battery manufacturers’ specified float voltages, float charge voltages in PV systems can be maintained for an extended period without overcharging the battery. Commonly, float voltage for a lead-acid battery is around 13.5Vto 13.8 V (at 25 °C) for a nominal 12 V battery, or 2.25 V to 2.3 V per cell. Dedicated lighting controllers include settings for when a lighting load should be turned ON and OFF Equalization Setpoint Equalization is a controlled overcharge for a few hours, which is only performed on open- vent batteries. Charge controllers may provide an automated or user-activated equalization charge cycle. During an equalization cycle, the VR setpoint is increased to a higher level, where it remains for a predetermined amount of time before returning to its normal level or the slightly lower float voltage. See 16. Controllers that rely on only an array for the equalization charge may require several days or weeks to complete the prescribed time at the equalization voltage, especially during periods of high load or low insolation. With oversized arrays or generator-powered chargers, equalization can be accomplished in shorter periods. Equalization settings include the equalization voltage setpoint, cycle frequency, and duration. Equalization voltages for flooded lead-acid batteries may be as high as 2.5 V to 2.58 V (at 25°C) per cell, which corresponds to 15V to 15.5V for a nominal 12 V battery. The frequency and duration of equalization charges depend on the type of battery and its condition, and availability of the array and other DC charging sources. Typically, manufacturers recommend an equalization charge for flooded lead-acid batteries once every few weeks for about 3 hr. Setpoint Adjustments Appropriate charge controller setpoints are crucial to maximizing battery life and system performance, and are arguably more important than the type of controller design. A relatively simple design with properly adjusted setpoints will work better than a sophisticated controller with improper settings. A difference of a few tenths of a volt in the VR setpoint can have a significant effect on the battery system. For some controllers, setpoints are established by the manufacturer for a specific battery type and are not adjustable in the field. However, many controllers allow adjustment of the setpoints during installation, though the NEC requires manual means of setpoint adjustment to be inside the charge controller so they are not easily accessible for unqualified persons to make inappropriate changes. Setpoints may be adjusted with potentiometers that allow a range of settings, or with DIP switches or jumpers for discrete setpoint increments or battery types. See 17. For microprocessor-based charge controllers, including those integrated with inverters, setpoint adjustments are made through soft ware programming. Equalization Charge 16. The equalization setpoint brings the battery voltage to a level that is higher than the normal charge regulation voltage. 17. Setpoints are adjusted with controls inside the charge controller. In general, for any type of battery, higher setpoints are required for interrupting-type charge controllers than the more advanced controller algorithms. Since interrupting-type algorithms do not apply charging current as smoothly as more sophisticated algorithms, batteries must be charged to higher voltage to accept charge applied in this way in a similar amount of time. Also, for any type of controller, recommended VR setpoints for flooded open- vent lead-acid batteries are always higher than for sealed VRLA batteries. Flooded batteries can tolerate more overcharge than sealed batteries because water can be added to replenish the electrolyte. Charge controller and battery manufacturer’s data must be reviewed before making any determination on setpoints. However, manufacturer’s recommendations are only guidelines, and atypical systems or con figurations may require different setpoints. When in doubt, setpoints should be set slightly low to be conservative, and the battery system should be carefully monitored for the first few discharging and charging cycles, preferably under a range of temperature and insolation conditions. Small adjustments can be made until charging and performance is optimized. When the optimal setpoints are established for a particular system, the values should be recorded in the system documentation and on a label near the charge controller. Temperature Compensation Temperature affects battery charging characteristics, so some charge controllers automatically adjust setpoints to compensate for this factor. When battery temperature is low, temperature compensation increases the YR setpoint to allow the battery to reach a moderate gassing level and fully charge. When battery temperature is high, the VR setpoint is lowered to minimize excessive overcharge and electrolyte loss. Temperature compensation helps ensure that a battery is fully charged during cold weather and not overcharged during hot weather. Temperature compensation is also used for LVD setpoints. Charge controllers with temperature compensation use a sensor to measure temperature. For small systems and controls located in the same thermal environment, temperature sensing within the charge controller is generally satisfactory for approximating battery temperature. For larger systems or systems with batteries located in a different thermal environment than the controller, external temperature probes should be used. See 18. Temperature probes must be securely attached to a battery case to ensure the most accurate battery temperature measurement. If temperature probes become detached from a charge controller, the controller should regulate at the nominal setpoints. Setpoint voltages are usually based on a nominal battery temperature of 25°C (77°F). Temperature-compensated charge controllers automatically adjust the setpoints based on established coefficients. For lead-acid batteries, the widely accepted temperature coefficient for the regulation voltage is -5 mV/°C/cell (-0.005 V/°C/cell). The temperature compensation coefficient is negative because increases in temperature require a reduction in the setpoint voltages. Temperature coefficients for other battery chemistries differ from that for lead-acid types, but most are negative values. T Sensors 18. Temperature probes are placed between batteries and connect to a charge controller to compensate the charge regulation setpoint. A setpoint voltage at battery temperatures other than 25°C can be calculated with the following formula: V_comp = temperature-compensated setpoint voltage (in V) V_set = nominal setpoint voltage at 25°C (mV) C = temperature compensation coefficient (in V/°C/cell) That = battery temperature (in °C) n = number of battery cells in series For example, consider a PV system using a 12 V lead-acid battery (6 cells in series) and a charge controller with a nominal VR setpoint of 14.4 V at 25°C. What should the YR setpoint be at 5°C? Conversely, if the same battery operates at 40°C (104°F), the YR setpoint is reduced from 14.4 V to 13.95 V. This amounts to a 1.05 V difference in the YR setpoint for a 12 V battery operating over a temperature range of 5°C to 40°C (41°F to 104°F). For higher-voltage battery systems, the compensated YR setpoint will vary even more. For a nominal 48 V system, the YR setpoint will vary by more than 4.2 V over the same 35°C (63°F) temperature range. CHARGE CONTROLLER APPLICATIONS Charge controllers used in PY systems must be properly specified, selected, and conFigr.d based on the application requirements. They must have appropriate current and voltage ratings, and must be compatible with other equipment in the system. The selection and sizing of charge controllers in PV systems involves consideration of many factors. See 19. Charge Controller Selection Criteria:
19. Many criteria should be considered when selecting charge controllers for PV systems. Ratings For NEC code compliance, charge controllers used in PV systems must be listed to the UL Standard 1741, Inverters, Converters, and Controllers for Use in Independent Power Systems. The testing associated with this product listing ensures that charge controllers are safe and suitable for the intended operating conditions. Solar World industries America PV systems in developing countries are often small systems consisting of a small array, a few batteries, and a simple charge controller that controls a single load, such as a light. Charge controllers for PV systems are produced in a variety of system/battery volt age ratings, commonly 12 V, 24 V, and 48 V. Higher voltage charge controllers handle the same power levels at lower currents and are generally more efficient. Many types of charge controllers can be reconFigr.d for different voltages, making them flexible for use in a variety of system designs. Charge controllers should be sized for greater than the maximum voltage expected from the PV system, usually the open-circuit voltage of the array under the coldest conditions. Charge controllers are also rated for maximum array and load currents. The charge controller current rating must be specified to 125% of the maximum PV circuit current. For example, a controller rated for 40 A can accommodate an array with short circuit rating of 32A (40A ÷ 125% = 32 A). Similarly, a charge controller with load control function must have a load current rating at least 125% of the maximum expected load current. The load current rating may be different from the array current rating. Location When the temperature compensation sensor is inside the charge controller, the charge controller should be as close to the batteries as possible so that they are at the same ambient temperature. If this type of charge controller is located in a cooler space, the setpoints will be too high, and if it is in a warmer space, the setpoints will be too low. However, charge controllers should not be installed in the same enclosure or immediately above flooded lead-acid batteries. Battery gases may deteriorate the controller electronics over time and , in extreme cases, a controller could ignite if an explosive amount of battery gases accumulated without proper ventilation. To minimize their operating temperature, charge controllers should be installed out of direct sunlight and with unobstructed airflow around the heat sinks. Charge controllers should also be protected from excessive moisture. Voltage Drop Installing charge controllers close to batteries also minimizes voltage drop. As charging current increases on the conductors between the charge controller and battery terminals, the voltage drop increases. Since many charge controllers sense battery voltage with the conductors used to deliver charging current, the measured voltage is slightly higher than the actual battery voltage. See 20. This prematurely activates charge regulation, causing the battery to be undercharged. Excessive voltage drop also affects the load control setpoints. The discharge current causes the measured voltage from the charge control ler to be lower than the actual battery voltage. This results in the loads being disconnected at an actual battery voltage that is slightly higher than the LVD setpoint. The voltage drop during charging and discharging effectively compresses the actual operating voltage range of the battery, reducing the available battery capacity. Some controllers, particularly ones designed for higher currents, include additional terminals for conductors to sense battery voltage. See 21. This is called a four-wire or Kelvin measurement. No current flows in these conductors, so there is no voltage drop. There fore, a precise battery voltage is measured and setpoints are activated at the correct voltages. When a charge controller does not have separate voltage sense leads, voltage drop can be minimized by using oversized conductors and locating the controller close to the batteries. Baltimore Electrical JATC Installing charge controllers physically near the battery bank and other system components keeps voltage drop low, improving system performance. Multi Arrays For large arrays with multiple sub-array source circuits, multiple independent charge control lers can be used instead of a single, larger controller. The array charging current is limited in a stepped manner as each controller begins to regulate each sub-array. See 22. This configuration improves overall system reliability in the event of the failure of any individual controller. This design can also facilitate later system expansion by allowing for the addition of a new array source circuit and charge controller. As long as the charge rate for that circuit is limited to no more than 3% of the battery bank capacity, one source circuit may be left unregulated for float charging. Multiple Battery Banks Although rarely, a PV system can be con Figr.d to charge multiple separate battery banks with a single array. This situation usually results from a system expansion or from incorporating an isolated emergency standby battery or loads with different operating voltages. The battery banks may be different sizes, types, and ages, or perhaps different voltages. Charge acceptance and charge requirements may be different between the battery banks, leading to one battery bank being undercharged and the other being overcharged if the same charge controller and setpoints are used. Voltage Drop CHARGE CONTROLLER REGULATES CHARGING TOO EARLY; CHARGE CONTROLLER DISCONNECTS LOADS; TOO EARLY; CURRENT DISCHARGING 20. Long conductors between a charge controller and a battery bank have resistance that causes voltage drops. Voltage drops affect the voltage measured at the charge controller, which triggers overcharge and overdischarge protections too early. In addition to temperature compensation, some charge controllers also compensate for high charge or discharge currents, which also affect battery voltage. Careful component selection and system configuration can still effectively optimize the performance and life of multiple battery banks in these situations. Charge controller manufacturer’s literature will specify allowable alternative configurations for their equipment. Separate charge controllers are the best method for charging multiple battery banks from a single array. Each controller must be rated to handle the entire array current, and blocking diodes are required between each charge controller and the array to prevent the battery banks from operating at the same voltage. See 23. In this configuration, each charge controller can be set for the specific battery types and charge each battery bank appropriately. Since the battery banks are isolated by the blocking diodes and independent charge controllers, system loads may be connected to either battery bank, but not both. 21. Some charge controllers use additional battery voltage sense conductors to avoid the effect of voltage drop on setpoints. 22. Larger PV systems often use independent charge controllers for each array source circuit. Some charge controllers can be used alone to charge multiple battery banks if the volt ages are sufficiently close, though this is not recommended by most charge controller manufacturers. In this configuration, a resistor is added between the array and battery positive terminals on the charge controller and blocking diodes are added between the positive battery terminal of the charge controller and the positive terminal of each battery. See 24. This allows the charge controller to distribute the appropriate charging current to each battery bank. The batteries are also isolated from one another in the controller output circuit, instead of on the array input side when using separate charge controllers. Multiple battery banks are often used because the system must power loads of different voltages. However, there are alternative strategies for this situation. Lower voltages can be tapped from a higher voltage battery bank, though this can cause equalization problems for anything but very small loads. The best solution is to conFigr. the battery bank for the highest voltage load and use a DC-DC converter to supply lower voltages. PV Systems A charge controller is required in nearly all PV systems using batteries. However, some small systems can be specially designed to operate without a charge controller. A self-regulating PV system is a type of stand-alone PV s stem that uses no active control systems to protect the battery, except through careful design and component sizing. See 25. When maintenance is infrequent or prohibitively expensive, such as for remote applications, eliminating the need for a sensitive electronic charge controller can simplify the system design, lower costs, and improve reliability. Self-regulating PV systems must be designed so that the battery is never over charged or over-discharged under any operating conditions. The expected operating conditions, such as temperature, insolation, and discharge rates, must be well understood for this type of system to function safely and successfully. The energy supply from the array, the energy capacity of the battery, and the energy demand of the load must be carefully balanced. 4 Battery Banks with Multiple Charge Controllers ARRAY; CHARGE CONTROLLER; DIODES TO LOADS; CHARGE CONTROLLER; JUNCTION BOX; TO LOADS banks from a single array. 23. Separate charge controllers are usually recommended for charging independent battery Banks with One Charge Controller ARRAY CHARGE CONTROLLER — DIODES; TO LOADS JUNCTION BOX; RESISTANCE 24. In some cases, a single charge controller may be used to charge independent battery banks. 25. To balance the charge to and from the battery in a self-regulating PV system, the load must be well defined, the battery must be oversized, and the array current must be self-limiting. Electrical Load. For self-regulating PV systems to work properly, the electrical load must be well defined and have a consistent day-to day energy requirement. If the load is not used daily, the array may overcharge the battery. The load must also be consistent and automatically controlled. The load establishes the battery and array sizes needed in any stand-alone PV system, but is particularly important for self-regulating systems. Environmental Conditions. Applications with widely varying insolation or extreme temperature variations are generally not suit able for self-regulating systems. Seasonal variations in insolation require a larger array to meet the load requirements, which can overcharge the battery. Similarly, temperature changes affect battery charging and array voltage, which can't be automatically compensated for in self-regulating systems. Battery and Array Sizing. As with any stand-alone PV system, the array for a self- regulating system must be large enough to supply the loads and charge the battery under the worst-case conditions of high load demand and low insolation. If the array is too small, the battery may discharge completely. How ever, an array that is too large can overcharge the battery. To prevent overcharge when the battery is fully charged, the maximum array charge rate must be very low. Since the size of the array can't be reduced, charge rates are reduced by increasing the capacity of the battery bank (adding more batteries). Increasing battery capacity also increases system autonomy, lowers the average daily depth of discharge, and prolongs battery cycle life. Self-Regulating PV Systems ARRAY; CHARGE INTO BATTERY BALANCES DISCHARGE; FROM BATTERY LOAD According to the NEC, any PV system with batteries must employ charge control whenever the charge rate exceeds C133. More conservatively, charge rates of C/100 or less are considered low enough to be tolerated for extended periods when most types of batteries are already fully charged. Low-Voltage Modules. Low-voltage modules are an important part of a self-regulating system. Low-voltage modules take advantage of the fact that the module current falls off sharply as the voltage increases above the maximum power point. By matching the maxi mum power voltage with the battery charge regulation voltage, charge current is naturally limited as the battery approaches full state of charge, helping prevent battery overcharge. Common standard module designs include 36 cells in series and produce a maximum power voltage of about 17 V at STC. This volt age provides an acceptable charging current for nominal 12 V lead-acid batteries at higher operating temperatures and with moderate voltage drop through conductors and equipment. These modules always operate to the left of the “knee” on their I-V curve, producing maximum power current or greater. Low-voltage modules include 29 or 30 cells in series and have a maximum power voltage of about 15 V. At typical operating temperatures, the maximum power point falls within the range of typical battery voltages. As a battery approaches full state of charge and the module voltage passes the maximum power point, the module current falls sharply as the voltage increases further. See 26. Since module voltage and battery charge voltage requirements both decrease with increasing temperature, the voltages are reasonably matched for moderate temperature variations. 26. Low-voltage modules control current into a battery bank without a charge controller because their current automatically falls as the battery reaches full charge. Since PV devices are greatly affected by temperature, the output of a low voltage module can easily move outside the range of safe battery voltages at various temperatures In cool weather, the battery may be severely overcharged and in warm weather, the battery may be undercharged Self-regulating systems using low voltage modules should therefore be used only in locations where the temperature variance is small • Charge controllers manage the interactions between the array, battery bank, and loads; are the heart of stand-alone PV systems; and have a profound affect on battery life, load availability, and overall system performance. • Charge controllers regulate battery charging and protect batteries from being overcharged or over-discharged. • Additional features may be incorporated in charge controllers to provide enhanced system control functions. • Charge controllers use various algorithms and switching designs to manage battery state of charge. • Interrupting-type charge controllers regulate charging current by switching it ON or OFF. • Linear-type charge controllers use a variable resistance to regulate charging current in a gradual manner. • Shunt charge controllers control charging current by short-circuiting the array. • Series charge controllers control charging current by open-circuiting the array. • Pulse-width modulation (PWM) simulates a variable current by switching a full current ON and OFF at high speed for varying lengths of time. • Diversionary charge controllers divert the array current to an auxiliary load when the battery bank is fully charged. • Charge controller setpoints are the battery conditions that trigger charge control actions. • Charge regulation setpoints determine when the array’s charging current will be applied to the battery bank. • Load control setpoints determine when the loads will be operating, preventing the battery bank from being over-discharged. • Optimal charge controller setpoints are determined by the charge control algorithm and type of battery. • Setpoints should be adjusted for temperature because temperature affects battery charging characteristics. • Charge controllers must have the appropriate voltage and current rating for the application requirements. • Long conductors between the charge controller and the battery bank produce a voltage drop that affects the accuracy of voltage measurement by the controller. • Large arrays with multiple source circuits use individual charge controllers for each circuit. • Multiple battery banks can be charged from one array, with either multiple charge controllers or one specially conFigr.d charge controller. • Self-regulating PV systems do not require active charge control because the charging and discharging cur rents are carefully balanced. • A charge controller is a device that regulates battery charge by controlling the charging voltage and /or cur rent from a DC power source, such as a PV array. • Charge acceptance is the ratio of the increase in battery charge to the amount of charge applied to the battery. • Overcharge is the ratio of applied charge to the resulting increase in battery charge. Overcharge can also refer to the condition of a fully charged battery continuing to receive a significant charging current. • Deficit charge is a charge cycle in which less charge is returned to a battery bank than what was withdrawn on discharge. • Overdischarge is the condition of a battery state of charge declining to the point where it can no longer sup ply discharge current at a sufficient voltage without damaging the battery. • A charge control algorithm is a programmed series of functions that a charge controller uses to control current and /or voltage in order to maintain battery state of charge. • An interrupting-type charge controller is a charge controller that switches the charging current ON and OFF for charge regulation. • A linear-type charge controller is a charge controller that limits the charging current in a linear or gradual manner with high-speed switching or linear control. • A shunt charge controller is a charge controller that limits charging current to a battery system by short- circuiting the array. • A shunt-interrupting charge controller is a charge controller that suspends charging current to a battery system by completely short-circuiting the array. • A shunt-linear charge controller is a charge controller that limits charging current to a battery system by gradually lowering the resistance of a shunt element through which excess current flows. • A series charge controller is a charge controller that limits charging current to a battery system by open- circuiting the array. • A series-interrupting charge controller is a charge controller that completely open-circuits the array, suspending current flow into the battery. • A series-linear charge controller is a charge controller that limits charging current to a battery system by gradually increasing the resistance of a series element. • A series-interrupting, pulse-width-modulated (PWM) charge controller is a charge controller that simulates a variable charging current by switching a series element ON and OFF at high frequency and for variable lengths of time. • A maximum power point tracking (MPPT) charge controller is a charge controller that operates the array at its maximum power point under a range of operating conditions, as well as regulates battery charging. • A diversionary charge controller is a charge controller that regulates charging current to a battery system by diverting excess power to an auxiliary load. • The diversion load is an auxiliary load that is not a critical system load, but is always available to utilize the full array power in a useful way to protect a battery from overcharge. • A hybrid system controller is a controller with advanced features for managing multiple energy sources. • An ampere-hour integrating charge controller is a charge controller that counts the total amount of charge (in ampere-hours) into and out of a battery and regulates charging current based on a preset amount of overcharge. • A charge controller setpoint is a battery condition, commonly the voltage, at which a charge controller performs regulation or switching functions. • The voltage regulation (VR) setpoint is the voltage that triggers the onset of battery charge regulation because it is the maximum voltage that a battery is allowed to reach under normal operating conditions. • The array reconnect voltage (ARV) setpoint is the voltage at which an interrupting-type charge controller reconnects the array to the battery and resumes charging. 202 Photovoltaic Systems • The voltage regulation hysteresis (VRH) is the voltage difference between the voltage regulation (VR) setpoint and the array reconnect voltage (ARV) setpoint. • The low-voltage disconnect (LVD) setpoint is the voltage that triggers the disconnection of system loads to prevent battery overdischarge because it is the minimum voltage a battery is allowed to reach under normal operating conditions. • The load reconnect voltage (LRV) setpoint is the voltage at which a charge controller reconnects loads to the battery system. • The low-voltage disconnect hysteresis (LVDH) is the voltage difference between the low-voltage disconnect (LVD) and load reconnect voltage (LRV) setpoints. • A self-regulating PV system is a type of stand-alone PV system that uses no active control systems to protect the battery, except through careful design and component sizing. 1. What is the difference between charge acceptance and overcharge? 2. How do charge controllers protect battery banks from overcharge and overdischarge? 3. Explain the differences between interrupting-type charge controllers and linear-type charge controllers. 4. Explain the differences between shunt and series charge controllers. 5. How does PWM technology regulate charging current? 6. How does a diversionary charge controller utilize excess energy? 7. Explain four primary charge controller setpoints. 8. What are the possible consequences of a charge controller hysteresis that is too narrow or too large? 9. Why must charge controller setpoints be temperature-compensated? 10. How does the distance between the charge controller and battery bank affect the regulation functions of a charge controller? 11. How do low-voltage modules control charging current? Next: Inverters |