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Objectives: • Identify major battery components and their functions. • Differentiate between the basic types and classifications of batteries. • Understand the operation of batteries and their discharging and charging characteristics. • Understand how temperature, discharge and charge rates, and electrolyte specific gravity affect battery capacity and life. • Understand major principles and considerations for designing battery banks. BATTERY PRINCIPLES A battery is a collection of electrochemical cells that are contained in the same case and connected together electrically to produce a desired voltage. See Ill. 1. A battery cell is the basic unit in a battery that stores electrical energy in chemical bonds and delivers this energy through chemical reactions. Battery designs can vary by type and manufacturer, but many share the same basic components and store electricity using similar electrochemical reactions. Ill. 1. Batteries are collections of cells that produce electricity through electrochemical reactions. Cells can be configured into batteries of many different shapes and sizes. The word cell can refer to the smallest electricity unit. In both PV modules and batteries. Also, many people use the word battery when they mean cell For example, AA is a common size of cells not battery, for consumer electronics Cells are connected to each other inside the battery case with inter-cell connectors. Ill. 2. Many components are common to various battery types. SEPARATOR; NEGATIVE PLATE; CASE; POSITIVE CELL TERMINAL; NEGATIVE TERMINAL; ACTIVE MATERIAL; CELL PARTITIONS; INTERCELL CONNECTOR; CELLS POSITIVE PLATE Battery Design: A battery cell consists of one or more sets of positive and negative plates immersed in an electrolyte solution. A plate is an electrode consisting of active material supported by a grid framework. Active material is the chemically reactive compound on a battery cell plate. The amount of active material in a battery is proportional to the energy storage capacity of a battery. The grid is a metal framework that supports the active material of a battery cell and conducts electricity. See Ill. 2. In each cell, there can be a single pair or, more commonly, several pairs of positive and negative plates. When there are multiple pairs, all the positive plates are connected together and all the negative plates are connected together. Thicker plates tolerate deeper discharges over long periods while maintaining good adhesion of the active material to the grid, resulting in longer battery life. Thinner plates allow more pairs per cell, maximizing surface area for delivering high currents. However, thinner plates are less durable and are not designed for deep discharges. Electrolyte is the conducting medium that allows the transfer of ions between battery cell plates. Electrolyte may be in a liquid or gelled form. An insulating separator keeps the plates from making electrical contact and short-circuiting, but is porous to allow the flow of electrolyte ions between the plates. The chemical reactions during battery charging produce gases. Some batteries include cell vents to allow the gases to escape to the atmosphere. Other batteries are sealed and do not allow gases to escape under normal conditions, but include pressure-relief vents. These vents remain closed under normal conditions, but open when the battery pressure increases beyond a threshold, often the result of over charging or high-temperature operation. Battery cases contain cells in individual chambers and give the battery structure. Cases are typically made from strong and durable plastics, which do not react with the electrolyte. The case includes terminals, vents, cell partitions, and intercell connectors that connect plate assemblies between cells. Terminals are the external electrical connections to a battery. Connections to loads, charging circuits, or other batteries are made at the terminals. Clear battery cases allow easier monitoring of electrolyte levels and battery plate condition. Steady-State: A cell or battery that is not connected to a load or charging circuit is at steady-state. Steady-state is an open-circuit condition where essentially no electrical or chemical changes are occurring. At steady-state, there is electrical potential between the positive and negative terminals, but electrons can't flow until a load is connected between the terminals. The open-circuit voltage is the voltage of a battery or cell when it is at steady-state. The open-circuit voltage of a fully charged lead-acid cell is about 2.1 V. Capacity Capacity is the measure of the electrical energy storage potential of a cell or battery. Several physical factors affect the capacity, including the quantity of active material; the number, design, and dimensions of the plates; and the electrolyte concentration. Operational factors affecting capacity include discharge rate, charging method, temperature, age, and condition of the cell or battery. Capacity is commonly expressed in ampere-hours (Ah), but can also be expressed in watt-hours (Wh). For example, a battery that delivers 5 A for 20 hr has delivered 100 Ah. If the battery averages 12 V during discharge, the capacity can also be expressed as 1200 Wh (100 Ah x 12 V = 1200 Wh). Temperature and discharge rate may affect capacity, especially of lead-acid batteries. Warmer batteries are capable of storing and delivering more charge than colder batteries. See Ill. 3. However, high temperatures decrease the useful life of a battery. Manufacturers generally rate lead-acid battery performance and cycle life at 25°C (77 F). For the best trade-off between capacity and lifetime, the s stem should be designed for the recommended discharge rate and the battery should be located where the average temperature will be close to the manufacturer's recommendation. Any differences from the rated conditions affect the actual capacity of the battery. Envelope separators are pockets that encapsulate the entire plate except for the electrical connections at the top. The envelopes prevent pieces of active material from creating internal short circuits if they break away from the grid and fall to the bottom the cell. Ill. 3. Higher temperatures and slower discharge rates result in increased battery capacity Discharging Discharging is the process of a cell or battery converting chemical energy to electrical energy and delivering current. Discharging removes energy from a battery. Discharging begins when a load is connected to the battery at the positive and negative terminals. At that moment, a chemical reaction begins that causes electrons to flow from the negative terminal to the positive terminal. At the negative terminal, the active material in the negative plate reacts with the electrolyte to form a new material that releases excess electrons. At the positive terminal, the active material in the positive plate reacts with the electrolyte to form a new material, which requires extra electrons to complete the reaction. When the battery is discharging, the excess electrons at the negative terminal are conducted outside the battery, through the load, to the positive terminal to complete the reactions at the positive plate. Small, demonstration PV systems may include the batteries to simulate the operation of stand-alone systems. The voltage between the terminals is highest at the beginning of a discharge cycle and gradually falls. The voltage of a battery system is not constant but ranges from a few volts above its nominal voltage to a few volts below. For example, a nominal 12 V lead-acid battery is between 12.6 V and 13 V at steady-state when fully charged. The battery voltage falls to a voltage between 10.8 V and 11 V during discharge, depending on the discharge current. The cutoff voltage is the minimum battery voltage specified by the manufacturer that establishes the battery capacity at a specific discharge rate. Below the cutoff voltage, there is no further usable capacity. Discharge rate is expressed as a ratio of the nominal battery capacity to the discharge time in hours. For example, a 5A discharge for a nominal 100 Ah battery would be a C/20 discharge rate. The designation C/20 indicates that ½oth of the rated capacity is discharged per hour, or that the battery will be completely discharged after 20 hr. Capacity is directly affected by the rate of discharge. Lower discharge rates are able to remove more energy from a battery before it reaches the cutoff voltage. Higher discharge rates remove less energy before the battery reaches the same voltage. See Ill. 5. State of Charge (SOC). The state of charge (SOC) is the percentage of energy remaining in a battery compared to the fully charged capacity. Discharging a battery decreases the state of charge, while charging increases the state of charge. For example, a battery that has had three-quarters of its capacity removed is at 25% state of charge. See Ill. 6. Depth of Discharge (DOD). Depth of discharge (DOD) is the percentage of withdrawn energy in a battery compared to the fully charged capacity. For example, a battery that has had three-quarters of its capacity removed is at a 75% depth of discharge. By definition, the depth of discharge and state of charge of a battery add up to 100%. Two common qualifiers for depth of discharge in PV systems are the allowable DOD and the average daily DOD. Discharging Reaction: Ill. 4. Electrochemical reactions within a cell produce a flow of electrons from the negative terminal to the positive terminal. POSITIVE- PLATE ACTIVE MATERIAL; NEGATIVE-PLATE ACTIVE MATERIAL; POROUS SEPARATOR Discharge Rate: Ill. 5. Slower discharge rates remove more energy from a battery than faster discharge rates. CUTOFF VOLTAGE; NOMINAL VOLTAGE Battery cells are essentially devices for converting between chemical energy and electrical energy. When a load is connected to the cell or battery, the discharging reaction uses the energy of chemical bonds to free electrons and cause them to flow. State of Charge vs. Depth of Discharge: Ill. 6. The state of charge and depth of discharge of a battery always add up to equal 100%. Allowable DOD. The allowable depth of discharge is the maximum percentage of total capacity that is permitted to be withdrawn from a battery. The allowable DOD may be as high as 80% for deep-cycle traction batteries or as low as 15% for automotive batteries. The allowable DOD is determined by the cutoff voltage and discharge rate. Average Daily DOD. The average daily depth of discharge is the average percentage of the total capacity that is withdrawn from a battery each day. See Ill. 7. If the load varies seasonally, such as in a lighting system, the average daily DOD will be greater in the winter months due to greater loads at night. If the loads are constant, the average daily DOD will be greater in the winter due to low temperatures that lower the rated battery capacity. Depending on the rated capacity and the aver age daily energy load, the average daily DOD may vary from only a few percent in systems designed with high autonomy to as high as 50% for marginally sized battery systems. Autonomy. In stand-alone PV systems, battery system capacity is generally several times greater than needed for the average daily load requirements in order to account for days with below-normal insolation. Autonomy is the amount of time a fully charged battery system can supply power to system loads with out further charging. Autonomy is expressed in days. Battery systems are typically sized for an autonomy period of two to six days. Autonomy may be greater for applications involving a critical load, public safety, or highly variable insolation. Lower average daily DOD or greater battery system capacity results in longer autonomy. Avg. Depth of Discharge: Self-Discharge. Self-discharge is the gradual reduction in the state of charge of a battery while at steady-state condition. Self-discharge is also referred to as standby or shelf loss. Self-discharge is a result of internal electro chemical mechanisms and losses. The rate of self-discharge differs among battery types and increases with battery age. Self-discharge rates are typically specified in percentage of rated capacity per month. Higher temperatures result in higher self-discharge rates, particularly for lead-antimony designs. See Ill. 8. In operation, self-discharge is usually in significant compared to normal system loads. However, if the PV system is small or self- discharge is otherwise a concern, continuous application of a very small charge current counters the effects of self-discharge and keeps a battery at full charge. Charging A cycle is a battery discharge followed by a charge. Charging is the process of a cell or battery receiving current and converting the Ill. 7. When arrays are used to charge bat teries, seasonal insolation variations affect depth of discharge values; electrical energy into chemical energy. Charging is done by applying an electrical current to the cell or battery in a direction opposite to the discharge. In order for the battery to accept current, the voltage of the charging source must be higher than the battery voltage. For example, a nominal 12 V lead-acid battery is charged with about 14.4 V and reads about 12.6 V when fully charged. Self-Discharge Rates Ill. 8. Batteries exhibit high self-discharge rates at higher temperatures. The electrons passing through in the opposite direction reverse the chemical reactions and restore the active materials and electrolyte to their original compositions. See Ill. 9. Charge rate is quantified in the same way as discharge rate. For example, a charging rate of C/SO to a 100 Ah battery applies 2 A of cur rent until the battery reaches a specific fully charged voltage. During charging, the battery voltage rises sharply then stabilizes. Voltage rises again, first very slowly, then faster, and may pass the gassing voltage. The gassing voltage is the voltage level at which battery gassing begins. Voltage stabilizes again at the fully charged voltage level. Batteries can be charged in one stage or multiple stages. Three stages of battery charging are bulk charging, absorption charging, and float charging. Equalizing charging is an additional type of charging. Chg. Reaction: Ill. 9. The charging reaction discharge reaction; within a cell is the reverse of the discharge reaction. Bulk Charging. Bulk charging is battery charging at a relatively high charge rate that charges the battery up to a regulation voltage, resulting in a state of charge of about 80% to 90 See Ill. 10. This is also called normal charging. Single-stage charging is bulk charging only. This mode is simple to implement, but does not fully charge the battery. Multiple-stage charging includes additional charging modes to reach a higher state of charge. Absorption Charging. Once a battery is nearly fully charged, most of the active material in the battery has been converted to its original form, and current regulation is required to limit the amount of overcharge supplied to the battery. Absorption charging is battery charging following bulk charging that reduces the charge current to maintain the battery voltage at a regulation voltage for a certain period. The absorption charging period can be preset or adjustable, and is usually 1 hr to 3 hr. Absorption charging charges another 10% to 15% of battery capacity. Ill. 10. Bulk, absorption, and float charging control battery voltage with the charging current during a multiple-stage charging cycle. Multiple-Stage Charging: Battery types that can't be recharged use electrochemical reactions that are not easily reversible. This makes them impractical for PV systems but commonly used in consumer electronics due to their low cost. Float Charging. When the battery reaches nearly 100% state of charge, the charging voltage is lowered slightly to the float voltage and the charging current is set to a very low rate. Float charging is battery charging at a low charge rate that maintains full battery charge by counteracting self-discharge. The float charge rate must not exceed the self-discharge rate or the battery will be overcharged. Float charging is also called trickle charging or finish charging. A charge controller remains in float charge mode until the array current is no longer high enough to maintain the battery at float voltage. Equalizing Charging. An equalizing, or refreshing, charge is used periodically to maintain consistency among individual lead-acid cells. Equalizing charging is current-limited battery charging to a voltage higher than the bulk charging voltage, which brings each cell to a full state of charge. Equalizing charging is a controlled overcharge to produce gassing, ensuring that every cell is fully charged. Equalizing a battery helps to prevent electrolyte stratification, sulfation, and cell voltage inconsistencies that develop during normal battery operation, and maintains battery capacity at the highest possible levels. For batteries that are deeply discharged on a daily basis, an equalizing charge is recommended every one or two weeks. For batteries less severely discharged, equalizing may only be required every one or two months. An equalizing charge is typically maintained until the cell voltages and specific gravities remain consistent for a few hours. Only flooded open-vent batteries need equalization. Sealed or valve-regulated batteries can be damaged by equalization. Gassing and Overcharge When a battery is nearly fully charged, essentially all of the active materials have been converted to their fully charged composition and the cell voltage rises sharply. Further charging at this point results in gassing. Gassing is the de composition of water into hydrogen and oxygen gases as the battery charges. Hydrogen forms at the negative plate and oxygen forms at the positive plate. The gases bubble up through the electrolyte and may escape through cell vents, resulting in water loss from the electrolyte. Some level of gassing is required to achieve full charge, but gassing must be carefully controlled to prevent excessive water loss. Water may need to be periodically added to the electrolyte to maintain the proper acid concentration, though this can't be done for all types of batteries. Gassing is useful for gently agitating the electrolyte, which ensures that its concentration is uniform. However, excessive gassing can dislodge active materials from the grids, increase battery temperature, or expose the plates, which can permanently damage the battery. The gassing voltage is a function of battery chemistry, temperature, charge rate, voltage, and state of charge. As battery temperature decreases, the corresponding gassing voltage increases, and vice versa. At a battery temperature of 0°C (32°F) the gassing voltage of lead-acid cells increases to about 2.5 V. The effect of temperature on the gassing voltage is the reason why the charging process should be temperature compensated. This ensures that batteries are fully charged in cold weather and not overcharged during warm weather. At a given temperature, the gassing volt age is the same for all charge rates. However, since charge rates affect capacity, gassing begins at a lower state of charge at higher charge rates. For example, at a charge rate of C/20 at 25°C (77°F), the gassing voltage of about 2.35 V per lead-acid cell is reached at about 90% state of charge. At a charge rate of C/5, the gassing voltage is reached at about 75% state of charge. Overcharge is the ratio of applied charge to the resulting increase in battery charge. For ex ample, a 100 Ah battery may require 110 Ah of charge to account for charge loss due to gassing. The overcharge in this case is 110%. Varying amounts of overcharge are required for different battery types and from different states of charge. Generally, more overcharge is required to charge a battery from a shallow DOD than from a greater DOD, since the final stages of charging are the least efficient. Some batteries may require as much as 120% overcharge while many others require only about 110%. Electrolyte concentration Specific gravity is the ratio of the density of a substance to the density of water. By definition, water has a specific gravity of 1.00. The electrolyte in a fully charged lead-acid battery is a solution of approximately 25% sulfuric acid and 75% water (by volume). Ions from the acid are consumed by the active material during discharging and released during charging, so the concentration of the acid in the water changes with state of charge. Therefore, the specific gravity of the electrolyte is related to the battery state of charge, though it is also affected by temperature. In a fully charged lead-acid battery, the specific gravity of the electrolyte typically ranges between 1.25 and 1.28 at a temperature of 25°C (77°F). When the battery is fully discharged, the electrolyte is essentially water, with a specific gravity near 1.00. Therefore, electrolyte specific gravity can be used to estimate battery state of charge. However, temperature will also affect specific gravity, so readings must be adjusted to compare to the expected values at 25°C (77°F). Concentrated sulfuric acid has a very low freezing point (as low as -70°C -94°F]), while water has a freezing point of 0°C (32°F). Therefore, the freezing point of the electrolyte also varies with the specific gravity of the electrolyte. As the battery becomes discharged, the specific gravity decreases, resulting in a higher freezing point for the electrolyte. See Ill. 11. Batteries may be designed so that individual cells can be removed for transport and reassembly if necessary. Ill. 11. The freezing point of sulfuric acid electrolyte changes at various states of charge because of changes in specific gravity. Electrolyte Freezing Points In extreme climates, the specific gravity of the electrolyte is often adjusted outside the typical range to compensate for the effects of temperature extremes. In warm climates where freezing temperatures do not occur, the specific gravity may be reduced to below 1.25. Lower concentrations also lessen the degradation of the separators and grids, prolonging the useful service life of the battery. However, lower electrolyte specific gravity decreases the storage capacity and discharge rate performance, though these factors are usually offset by the higher operating temperatures. In very cold climates, the electrolyte may be susceptible to freezing, particularly during winter when the batteries may not be fully charged due to below-average insolation. If the electrolyte freezes, it will expand, causing irreversible damage to the battery. The specific gravity of the electrolyte may be increased slightly, up to 1.30. Increasing the electrolyte concentration lowers the freezing point and accelerates the electrochemical reactions, improving the capacity at low temperatures. However, high specific gravity can reduce the useful service life of a battery. While the specific gravity can be used to estimate the state of charge of a lead-acid battery, low or inconsistent specific gravity readings between series-connected cells in a battery may also indicate sulfation, stratification, or a lack of charge equalization between cells. Sulfation. Sulfation is the growth of lead sulfate crystals on the positive plate of a lead-acid cell. Sulfation is a side effect of normal battery aging, but can be accelerated by prolonged operation at partial states of charge. Sulfation decreases the electrolyte concentration and the available active material, and therefore the capacity of the cell. See Ill. 12. Sulfation also increases internal resistance within the battery, making it more difficult to charge. Battery temperature is often monitored by an electronic sensor placed between adjacent battery cases. Sulfation: Ill. 12. Sulfation reduces the capacity of a lead-acid cell by locking away active material as crystals. During normal battery discharging, the active materials on both the positive and negative plates are converted to lead sulfate. If the battery is charged soon after being discharged, the lead sulfate converts easily back into the active materials. However, if a lead-acid battery remains at a low state of charge for prolonged periods (weeks), the lead sulfate crystallizes on the positive plate, which inhibits conversion back to active materials during charging. The crystals essentially reduce the amount of active material, decreasing the capacity of the battery. If lead sulfate crystals grow too large, they can also cause physical damage to the plates. Sulfation is a common problem with lead- acid batteries in some PV systems because the batteries often operate at partial states of charge for extended periods. To minimize sulfation, the array should be sized to charge the battery during the month with the highest load to insolation ratio. Also, the additional energy sources in hybrid systems are effective at keeping batteries fully charged. Stratification. During charging, electrolyte acid ions form on the plates and gradually descend to the bottom of the cell. Over time, the electrolyte can develop a greater acid concentration at the bottom of the cell than at the top. Stratification is a condition of flooded lead-acid cells in which the specific gravity of the electrolyte is greater at the bottom than at the top. See Ill. 13. If left unmixed, the reaction processes vary between the bottom and the top of the plates, decreasing battery performance and increasing corrosion. If the battery has a transparent case, it may be possible to see more degradation on the bottoms of the plates than the tops. Stratification is generally the result of low charge rates or undercharging, which does not produce the gassing needed to agitate the electrolyte. Tall stationary cells are particularly prone to stratification when charged at low rates. Periodic equalization gassing thoroughly mixes the electrolyte and minimizes stratification problems. Battery Life Battery life depends on many design and operational factors, including battery materials, operating temperatures, cycle frequency, depth of discharges, average state of charge, and charging methods. Battery life is expressed in terms of cycles or years. The end of the useful life of a battery is marked by loss of capacity that increases the daily DOD beyond an acceptable level. End-of-life reductions in battery capacity may occur gradually or abruptly, depending on the type of failure. Exact quantification of battery life in PV systems is difficult due to the number of variables involved, but certain trends can be used to estimate battery life. As long as a battery is not overcharged, overdischarged, or operated at excessive temperatures, the lifetime of a battery is roughly proportional to its aver age state of charge. For example, a typical flooded lead-acid battery that is maintained above 90% state of charge will provide two to three times more charge/discharge cycles than a battery maintained at an average 50% state of charge. Stratification: LOW-SPECIFIC GRAVITY ELECTROLYTE. Ill. 13. Stratification results when the specific gravity of the electrolyte is higher at the bottom of a cell than at the top. For safety and weight reasons, batteries are sometimes drained after initial charging by the manufacturer and shipped in a dry condition. The acid and water are then added during installation. Operating temperature has a significant affect on battery life. The electrochemical reaction rates double for every 10°C (18°F) increase in temperature. Therefore, battery life decreases by half for every 10°C (18°F) increase. Higher operating temperatures also accelerate corrosion of the positive plate grids, resulting in greater gassing and electrolyte loss. Lower operating temperatures generally increase battery life, but capacity is significantly reduced, particularly for lead- acid batteries. In areas of severe temperature variations, batteries should be located in insulated or otherwise temperature-regulated enclosures. BATTERY TYPES Many types and classifications of batteries are manufactured, each with specific design and performance characteristics suited for particular applications. In PV systems, lead-acid batteries are most common due to their wide availability in many sizes, low cost, and well-understood performance characteristics. In a few critical, low-temperature applications, nickel-cadmium batteries are used, but high initial cost limits their use in most PV systems. Other battery designs may be used in small PV applications but are not common. Electrical storage batteries are either primary or secondary batteries. A primary battery is a battery that can store and deliver electrical energy but can't be recharged. Carbon-zinc and lithium batteries typically used in consumer electronic devices are examples of primary batteries. Primary batteries are not suitable for PV systems. A secondary battery is a battery that can store and deliver electrical energy and can be charged by passing a current through it in an opposite direction to the discharge current. Lead-acid batteries commonly used in automobiles and PV systems are examples of secondary batteries. + + + Battery Disposal and Recycling Batteries are hazardous items because they contain toxic materials such as plastics, lead, and acids that can harm humans and the environment. For this reason, laws have been established that dictate requirements for battery disposal and recycling. In some cases, battery manufacturers provide guidelines for battery disposal through local distributors. Battery manufacturers may also accept batteries for recycling. In most areas, batteries may be taken to approved recycling centers. Under no circumstances should batteries be disposed of in landfills or burned. At a recycling center, the battery is broken into pieces and placed in a vat. The lead and heavy materials fall to the bottom, while the plastic rises to the top and the acid is drained off. The materials are separated and each undergoes a different recycling process. The plastic pieces are washed, dried, and sent to a plastic recycler. There the pieces are melted together and extruded into small plastic pellets of uniform size. The pellets are sold to battery case manufacturers for use in new cases. The lead plates and other parts are cleaned and then melted in smelting furnaces. The molten lead is poured into ingot molds. Large ingots weighing about 2000 lbs are called hogs. Smaller ingots weighing 65 lbs are called pigs. After a few minutes, the impurities float to the top of the still-molten lead in the ingot molds and are scraped away. When the ingots are cool, they are removed from the molds and sent to battery manufacturers where they are melted again to produce new lead plates and other parts for new batteries. More than 97% of all battery lead is recycled. Old battery acid can be handled in two ways. The acid can be neutralized with an industrial compound similar to household baking soda. The resulting liquid is mostly water that is treated, cleaned, and tested to ensure that it meets clean water standards. The other method is to process the acid into sodium sulfate, an odorless white powder that is used in laundry detergent, glass, and textile manufacturing. + + + Battery Class Secondary batteries are often classified as traction; starting, lighting, and ignition (SLI); or stationary batteries. The differences in their design and materials result in different discharge and cycle characteristics. See Ill. 14. Traction Batteries. A traction battery is a class of battery designed for repeated deep- discharge cycle service. Because they are often used in electrically operated vehicles and equipment such as golf carts, forklifts, and hybrid automobiles, traction batteries are also called motive power batteries. These batteries have fewer plates per cell than other types of batteries, and the plates are thick and durable. Traction batteries are very popular in PV systems for their deep-cycle capability, long life, and durability. Starting, Lighting, and Ignition Batteries. A starting, lighting, and ignition (SLI) battery is a class of battery designed primarily for shallow-discharge cycle service. These batteries have many thin plates per cell, allowing the battery to deliver high currents for short periods, and are most often used to power automobile starters. SLI batteries are not recommended for PV applications because they are not designed for deep discharges. However, SLI batteries are sometimes used for PV systems in developing countries where they are the only type of battery available locally. When depth of discharges are limited, SLI batteries may provide up to about two years of useful service in small, stand-alone PV systems. Stationary Batteries. A stationary battery is a class of battery designed for occasional deep-discharge, limited-cycle service. These batteries are designed to last a long time on standby service, so they are commonly used in uninterruptible power supplies (UPSs) to provide back-up power to computers, telephone equipment, and other critical loads or devices. Stationary batteries are not recommended for most PV applications because they are designed for very few charging and discharging cycles. Ill. 14. Batteries are divided into classes based on discharge and cycle characteristics. Manufacturer expectations for battery life may not be applicable in PV systems because most manufacturers do not rate battery performance under the same cycling and depth of discharge conditions as are found in PV systems Flooded electrolyte Batteries Flooded electrolyte is electrolyte in the form of a liquid. The electrolyte in flooded lead-acid cells is a sulfuric acid and water solution and the electrolyte in flooded nickel-cadmium cells is an alkaline solution of potassium hydroxide and water. Flooded batteries can be classified as either open-vent or sealed-vent types. Open-Vent Batteries. Open-vent batteries al low charging gases to freely escape. However, water must be added periodically to replenish water lost through gassing and maintain the correct concentration. Open-vent batteries have removable caps for the addition of water. Distilled or de-mineralized water must be used to replenish electrolyte because even normal tap water has impurities that can taint a battery and result in premature loss of capacity. A catalytic recombination cap (CRC) is a vent cap that reduces electrolyte loss from an open-vent flooded battery by recombining vented gases into water. CRCs contain particles of a catalyst such as platinum or palladium. A catalyst is a substance that causes other substances to chemically react but does not participate in the reaction. The catalyst causes a reaction between the hydrogen and oxygen that are generated by the battery during charging. The gases recombine in the CRC to form water, which drains back into the battery. CRCs on open-vent flooded lead- antimony batteries reduce electrolyte loss by as much as 50% in subtropical climates. See Ill. 15. CRCs also generate heat during the re combination process. Increasing temperature in CRCs can be used to detect gassing in the battery. If CRCs for different battery cells are at significantly different temperatures during charging (meaning that some cells are gassing and others are not), an equalization charge may be required. Sealed-Vent Batteries. Sealed-vent flooded batteries have nonremovable caps on the cells that allow only excess charging gases to escape through pressure-relief vents. Under controlled charging, the pressure relief vents remain closed and the process is contained. Excessive overcharge, however, can increase the internal gas pressure to the point of opening the pressure relief vents and releasing gas. This results in a permanent loss of water that can't be replenished. Manufacturers account for only a small loss of water by including reserve electrolyte. Catalytic Recombination Caps: CATALYST OXYGEN; BATTERY CASE; NEGATIVE PLATE; ELECTROLYTE HYDROGEN Ill. 15. Catalytic recombination caps re combine oxygen and hydrogen into water, which then drains back into the battery. Captive electrolyte Batteries Captive electrolyte is electrolyte that is immobilized. See Ill. 16. Batteries with captive electrolyte are sealed and often referred to as valve-regulated lead-acid (VRLA) batteries because they include pressure-relief vents. Captive electrolyte can't be replenished, so these batteries are intolerant of excessive overcharge. However, captive-electrolyte batteries feature internal gas recombination. This process recombines the oxygen gas from the positive plates and the hydrogen gas from the negative plates back into water, replenishing the electrolyte. Captive-electrolyte batteries are popular because they are spill-proof and easily transported. Captive electrolyte is also less susceptible to freezing than flooded electrolyte. Since captive-electrolyte batteries do not require water additions, they are ideal for remote applications where maintenance is infrequent or unavailable. The two most common types of captive electrolyte are the gelled electrolyte and absorbed glass mat electrolyte designs. Ill. 16. Captive-electrolyte batteries are sealed and contain electrolyte. That is immobilized. Gelled Electrolyte. Electrolyte is gelled by adding silicon dioxide to the electrolyte. The mixture is added to the battery as a warm liquid and turns to gel as it cools. Cracks and voids develop within the gelled electrolyte during the first few cycles, providing paths for the gases to move around, facilitating recombination. Excessive gases, however, escape through the pressure-relief vents, drying and shrinking the gel, causing it to lose contact with the plates and reducing capacity. Some gelled electrolyte has a small amount of phosphoric acid added to improve the deep-discharge cycle performance of the battery by minimizing grid oxidation at low states of charge. Absorbed Glass Mat Electrolyte. The electrolyte in an absorbed glass mat (AGM) battery is a liquid absorbed into fiberglass mats that are sandwiched in layers between the plates. The fiberglass mats also act as separators between the plates. This arrangement also allows the oxygen and hydrogen gases to migrate and re combine as water. AGM batteries are intolerant of high operating temperatures. Recommended charge regulation methods for gelled batteries also apply to AGM batteries. Lead-Acid Batteries Lead-acid batteries are the most common type of batteries used in PV systems. Lead- acid batteries are generally inexpensive and widely available in many capacities from 10 Ah to over 1000 Ah. Their deep-cycle characteristics make them ideal for PV applications, but they do not tolerate extreme temperatures well and may require frequent maintenance. See Ill. 17. In a lead-acid cell, the active material is lead dioxide (Pb0 in the positive plates and metallic sponge lead (Pb) in the negative plates. The electrolyte is a solution of sulfuric acid (H and water. The lead-acid electrochemistry produces a cell with a nominal voltage of 2 V. Six cells are configured in series to produce a battery with a nominal voltage of 12 V. Lead-acid battery types are differentiated by the elements alloyed with lead in the plate grids, and the physical form of the electrolyte. The most common types of grid alloys for lead- acid batteries are lead-antimony, lead-calcium, and hybrids. Lead-Antimony Batteries. Lead-antimony battery plate grids include 3% to 6% of antimony (Sb), which provides excellent deep- discharge performance. Lead-antimony grids also limit the shedding of active material and last the longest of the lead-acid batteries at higher temperatures. Disadvantages of lead- antimony batteries include a high self-discharge rate and possibly frequent water additions due to gassing. Lead-antimony batteries are a robust design with thick plates and are classified as traction batteries. They are well suited to PV applications due to their deep-cycle capability and tolerance of abuse. Most lead-antimony batteries are flooded, open-vent types with removable caps to permit water additions. The frequency of water additions can be minimized by using catalytic recombination caps or having cases with excess electrolyte reservoirs. Lead-Calcium Batteries. Lead-calcium battery plate grids include 3% to 6% calcium (Ca), which reduces gassing and lowers the self-discharge rate. Less water is lost in lead-calcium batteries than in lead-antimony batteries, so they generally require less maintenance. Disadvantages of lead-calcium batteries include poor charge acceptance after deep discharges, shortened battery life and capacity at higher operating temperatures, and intolerance of overcharging. Lead-calcium batteries can be either captive-electrolyte or flooded-electrolyte designs. Lead-calcium batteries make up the majority of captive-electrolyte designs and are commonly used in PV applications. Flooded lead-calcium batteries with open vents are stationary batteries, which are not suitable for most PV applications. Flooded lead-calcium batteries with sealed vents are primarily SLI batteries developed as "maintenance-free" automotive starting batteries. They are maintenance-free in the sense that they incorporate sufficient re serve electrolyte to operate over their expected service life without additional water. They have a low cost, but they are designed for shallow cycles and generally do not last long in most PV applications. These batteries can be used in small stand-alone PV systems such as in rural homes and lighting systems, but must be care fully charged and discharged to achieve useful performance and life. Hybrid Batteries. A hybrid battery is a battery that uses a combination of plate designs to maximize the desirable characteristics of each. The most common type of hybrid battery uses lead-calcium positive plate grids and lead-antimony negative plate grids. This design combines the advantages of the lead-calcium and lead-antimony designs, including good deep-cycle performance, low water loss, and long life. However, stratification and sulfation can be problems with these batteries, and they must be maintained accordingly. These batteries are sometimes used in PV systems with larger capacities and deep cycle requirements. Hybrid batteries are usually flooded designs, although models using gelled electrolyte have been developed. = = = Battery chemistry Battery chemistry is described with chemical equations, which show how the molecules or ions of the active material and electrolyte combine and /or split into different materials while either releasing or absorbing electrons. In a lead-acid battery during discharging, the negative plate reaction is the following: Pb0 + HS0 + 3ft + 2e - PbSO + 2H The positive plate reaction is: Pb + HS0 - PbSO + H + 2e During charging, the reactions reverse. The materials on the right side of the equation change into the materials on the left. For this reason, the reaction arrows are usually shown as "< Therefore, in a nickel-cadmium battery, the negative plate reactions are: Cd + 20H Cd(OH) + 2e The positive plate reactions are: 2NiO(OH) + 2H + 2e 2Ni(OH) + 20H = = = Ill. 17. The characteristics of lead-acid batteries vary between different designs. Flooded electrolyte; Captive electrolyte; Lead-antimony; Lead-calcium open-vent; Lead-calcium sealed-vent Lead-antimony/lead-calcium Lead-calcium sealed-vent Lead-antimony/lead-calcium Ill. 18. Two types of nickel-cadmium batteries have different performance characteristics. Captive electrolyte sintered plate; Flooded electrolyte pocket plate Nickel-Cadmium Batteries Nickel-cadmium (Ni-Cd) batteries are secondary batteries with several advantages over lead- acid batteries that make them well suited for PV systems, including long life, low maintenance, excessive discharge tolerance, excellent low- temperature capacity retention, and noncritical voltage regulation requirements. The main disadvantages of nickel-cadmium batteries are high initial cost and limited availability com pared to lead-acid designs. See Ill. 18. A nickel-cadmium cell consists of positive plates of nickel hydroxide (NiO(OH)) and negative plates of cadmium (Cd), immersed in an alkaline potassium hydroxide (KOH) electrolyte solution. The concentration, and therefore the specific gravity, of the electrolyte does not change during discharging or charging reactions. The nominal voltage for a Ni-Cd cell is 1.2 V, requiring 10 Ni-Cd cells to be configured in series for a nominal 12 V battery. The volt age of a Ni-Cd cell remains relatively stable until the cell is almost completely discharged, then drops off dramatically. Ni-Cd batteries can accept charge rates as high as C/l, and are tolerant of continuous overcharge up to a C/15 rate. Two primary types of Ni-Cd batteries are sintered plate and pocket plate batteries. Sintered Plate Ni-Cd Batteries. Sintered plate Ni-Cd batteries are made by heat- processing active materials and rolling them into a cylindrical metallic case. The electrolyte is immobilized, preventing leakage and allowing operation in any orientation. These batteries are commonly used in consumer electronic devices. The main disadvantage of sintered plate designs is the memory effect, in which a battery that is repeatedly discharged to only a percentage of its rated capacity will eventually "memorize" this cycle pattern and limit further discharge, resulting in loss of capacity. In some cases, the memory effect can be partially reversed, regaining some capacity, by conducting special charge and discharge cycles. However, sintered plate Ni-Cd batteries are not recommended for PV applications. Pocket Plate Ni-Cd Batteries. Pocket plate Ni-Cd batteries are large flooded batteries available in capacities up to and over 1000 Ah. These batteries can withstand deep discharges and temperature extremes much better than lead-acid batteries, and they do not experience the memory effect associated with sintered plate batteries, so they are suitable for use in PV systems. Similar to flooded lead-acid de signs, these batteries require periodic water additions. The main disadvantage of pocket plate Ni-Cd batteries is high initial cost. However, because of their long lifetimes, the life cycle cost can be the lowest among the battery types for PV applications. BATTERY SYSTEMS Battery selection and system design involves many decisions and compromises. Choosing the best battery for a PV application depends on many factors and involves a careful review of the battery specifications with respect to the particular application needs. Some decisions on battery selection may be easy to make, such as required capacity, physical properties; and cost, while other decisions are much more difficult and involve compromises between desirable and undesirable battery features. Battery selection: Each battery type has design and performance features suited for particular applications. Sys tem designers must consider the advantages and disadvantages of different battery types with respect to the requirements of a particular sys tem. Considerations include lifetime, deep cycle performance, tolerance to high temperatures and overcharge, and maintenance requirements. See Ill. 19. Batteries of different types or ages should not be mixed in the same bank. = = = Battery Safety Batteries are potentially dangerous because they contain hazardous materials and chemicals and store a large amount of electrical energy. Depending on the battery system, certain safety precautions are required. Personnel Protection When maintaining batteries, personnel should wear protective clothing such as aprons, ventilation masks, goggles or face shields, and gloves to protect from acid spills or splashes and fumes. Safety showers and eyewashes may be required if batteries are located close to personnel. A fire extinguisher should be located in close proximity to the battery area. In some applications, automated fire sprinkler systems may be required for safety and to protect facilities and expensive load equipment. OSHA regulations, building codes, and other applicable standards must be consulted before installing battery systems. Electrical Hazards Batteries can deliver extremely high short-circuit currents, as high as several thousand amperes for moderate-size battery banks. These currents can cause severe electrical burns and can weld tools and other metallic equipment in the circuit. High-voltage battery banks must be isolated into lower-voltage segments for service and maintenance. To reduce the potential for short circuits, live battery terminals must be protected with plastic or rubber covers. When covers are removed for battery maintenance, service personnel should not wear any watches, rings, bracelets, or other jewelry that could potentially come into contact with the battery terminals. All tools used around batteries should have insulated handles to prevent electrical contact. Electrolyte Hazards The electrolyte in lead-acid batteries contains sulfuric acid, which is highly caustic and can destroy clothing and burn skin. For these reasons, protective clothing such as rubber or plastic aprons and face shields should be worn when working with batteries. To neutralize sulfuric acid spills or splashes on clothing, the spill should be rinsed immediately with a solution of baking soda or household ammonia and water. In nickel-cadmium batteries, the electrolyte is a potassium hydroxide solution that can be neutralized with a mixture of vinegar and water. If electrolyte is accidentally splashed in the eyes, the eyes should be forced open and flooded with cool, clean water for fifteen minutes. If acid electrolyte is ingested, large quantities of water or milk should be drunk, followed by milk of magnesia, beaten eggs, or vegetable oil. A physician should be called immediately. When preparing an electrolyte solution from concentrated acid and water, the acid should always be poured slowly into the water while mixing. The water should never be poured into the acid. Appropriate nonmetallic funnels and containers should be used when mixing and transferring electrolyte solutions. Explosion Hazards: During charging, batteries may produce explosive mixtures of hydrogen and oxygen gases that may be present for several hours after charging. Keep sparks, flames, burning cigarettes, or other ignition sources away from batteries at all times. Active or passive ventilation techniques are suggested and often required, depending on the number of batteries located in an enclosure and their gassing characteristics. The use of battery vent caps with a flame arrester feature lowers the possibility of a catastrophic battery explosion. Improper charging and excessive overcharging may increase the possibility of battery explosions. Batteries should never be connected or disconnected at the terminals while under load. This can cause arcing, which could cause a battery explosion. All loads and charging sources should be disconnected at points away from the batteries before servicing.
= = = Battery Banks A battery bank is a group of batteries connected together with series and parallel connections to provide a specific voltage and capacity. Battery banks can be configured to produce many different voltages. Required battery bank voltage is often determined by load or inverter input voltage requirements. Most PV systems with batteries operate at 12 V, 24 V. or 48 V. For larger loads, it is recommended to use a higher voltage, which lowers the system currents. For example, a 120 W DC load operating from a 12 V battery draws 10 A, but when operating from a 24 V battery, a similar 120W load draws only 5 A. Lower system current reduces the size and cost of conductors, fuses, disconnects, and other current-handling components in the PV system. Ill. 19. System requirements and characteristics of battery types must be considered when choosing a battery. Battery Selection Criteria: SYSTEM REQUIREMENTS System configuration Discharge current Daily depth of discharge Autonomy Accessibility Temperature BATTERY CHARACTERISTICS Energy storage density Sealed or unsealed Allowable depth of discharge Charging characteristics Life cycles Electrolyte specific gravity Freezing susceptibility Sulfation susceptibility Gassing characteristics Self-discharge rate Maintenance requirements Size and weight Terminal configuration Auxiliary hardware availability Manufacturer reputation Cost Warranty Ill. 20. Connecting batteries in series increases system voltage. Batteries in Series Ill. 21. Connecting batteries in parallel increases system capacity. Ill. 21. Connecting batteries in parallel increases system capacity Pasted plates are flat grids covered by active material with a peanut butter-like consistency; The active material fills in the spaces between the ribs of the grid; Tubular plates consist of rows of thin vertical rods with active material plated around them, forming tubes Series Connections. Just like modules in an array, batteries are first connected in series by connecting the negative terminal of one battery to the positive terminal of the next battery, for as many batteries as are in the series string. Because there is only one path for the current to flow, the circuit current remains the same as the individual battery current. For batteries of similar capacity and voltage connected in series, the circuit voltage is the sum of the individual battery voltages, and the circuit capacity is the same as the capacity of the individual batteries. See Ill. 20. If batteries or cells with different capacities are connected in series, the capacity of the string is limited by the lowest-capacity battery. Parallel Connections. Batteries are connected in parallel by connecting all of the positive terminals together and all of the negative terminals together. Batteries connected in parallel provide more than one path for current to flow, so currents add together at the common connections. The current of the parallel circuit is the sum of the currents from the individual batteries. The voltage across the circuit is the same as the voltage across the individual batteries, and the overall capacity is the sum of the capacities of each battery. See Ill. 21. Series strings of batteries can also be connected in parallel in the same way. It is generally recommended that batteries be connected in as few parallel strings as possible. Slight voltage differences between the batteries may occur due to the length, resistance, and integrity of the parallel connections. These voltage differences can lead to inconsistencies in the charge received by each string, eventually causing unequal capacities within the system. The strings with the lowest circuit resistance will be exercised more than the strings of batteries with greater circuit resistance. The batteries in strings that receive less charge may begin to sulfate prematurely. For PV systems with large capacity requirements, larger batteries allow configurations of one series string rather than several parallel strings. When batteries must be configured in parallel, the connections between the battery bank and the PV power system should be made from opposite sides of the battery bank to improve the distribution of charge and discharge from the bank. See Ill. 22. Enclosures Electrical codes and safety standards generally require batteries to be installed in an enclosure separated from controls or other PV system components. Battery enclosures must be of sufficient size and strength to hold the batteries and can be located below ground if necessary to prevent freezing. If the enclosure is located above ground, shading or a reflective coating should be used to limit the direct exposure to sunlight. Avoiding extreme temperature swings improves battery performance, extends battery life, and decreases the need for maintenance. Large battery banks may be installed in dedicated battery rooms. Batteries may be installed on racks to secure the batteries and provide added structural support. An enclosure may be insulated or may have cooling or heating mechanisms to protect batteries from extreme temperatures. Passive cooling enclosures reduce battery temperature without the use of active components such as motors, fans, or air conditioners. Active temperature regulation requires electrical power, increasing the complexity, size, and cost of the PV system. Racks. Battery racks must be made from rigid materials, either metallic or nonmetallic, that are resistant to deterioration from electrolyte. The parts directly contacting the batteries must be electrically insulated or made from nonconductive materials. Batteries must be arranged to provide sufficient working space for inspection and maintenance. Electrolyte Containment. Battery systems should include some means to contain electrolyte in the event of a spill. Individual batteries may have double-walled cases to contain electrolyte if the inner case is breached. Pans or trays underneath the batteries or liquid-tight battery bank enclosures also provide containment in the event of a spill. Ventilation. Because batteries can produce toxic, corrosive, and explosive mixtures of gases, ventilation of the battery enclosure is usually required. For small battery systems charged at low rates, accumulation of gases is minimal, and natural ventilation may be adequate. When properly regulated, VRLA batteries may not require special ventilation and may even be used indoors. Otherwise, fans can be used to provide mechanical ventilation. Under no circumstances should batteries be kept in an unventilated area or located in an area accessible to unqualified persons. Ill. 22. Series and parallel connections can be combined to pro duce a desired system voltage level and capacity. Batteries in Series and Parallel === EXAMPLE PV SYSTEM Location: Baltimore, MD (39.3°N, 76.7°W) Type of System: Utility-interactive with battery backup Peak Array Power: 960 W DC Date of Installation: September 2004 Installers: Apprentices and journeymen Purposes: Training, supplemental electrical power Baltimore Electrical JATC Training Center The Baltimore Electrical JATC facility features a pre-engineered utility-interactive PV system with battery backup. The system includes eight utility- interconnected PV modules and a battery backup system. The system four 12 V batteries supply power to the building during utility outages. Training was the primary purpose for installing the system. Almost all of the electrical components are housed inside a rolling cabinet, facilitating demonstrations a building, the batteries remain close to room temperature. A temperature probe between the batteries helps the charge controller adjust charging operations as needed. Maintenance is minimal. The impact of the additional PV power on the facility's utility bills is currently small. However, the JATC is considering its options for a larger PV system in the future. The training and experience from the current system might then be applied to the future project to offset a more significant portion of their electricity use and reduce their dependence on the utility. Find instruction on the various parts. The cabinet includes the inverter; charge controller, batteries, breakers, disconnects, and other BOS components. By disconnecting the two main conductors, one for the DC power from the array and the other for AC power out to the building electrical system, the cabinet can be easily moved around for training. The batteries are housed in a ventilated section of the cabinet so that toxic and explosive gases can't accumulate. Since the cabinet is located inside the Most of the electrical components are housed in a cabinet that can be used for teaching purposes. The PV system includes four 12 V batteries connected/n series to supply power to the building in the event of a utility outage. = = = = Overcurrent Protection and Disconnects Like any other electrical system, battery systems must have proper DC-rated overcurrent protection and disconnects to protect system conductors and to isolate the battery bank from the rest of the system for testing and maintenance. Batteries can deliver extremely high discharge currents, so overcurrent protection devices must have the appropriate interrupting ratings. SUMMARY: • Batteries are collections of electrochemical cells electrically connected together in series. • Electrochemical reactions produce a flow of electrons from the negative terminals to the positive terminals of a cell. • Physical factors affecting capacity include the quantity of active material; the number, design, and dimensions of the plates; and the electrolyte concentration. • Operational factors affecting capacity include discharge rate, charging method, temperature, age, and condition of the cell or battery. • Discharging and charging rates are based on the amount of charge applied in one hour. • Charging is done by applying an electrical current to the cell or battery in a direction opposite to the discharge. In order for the battery to accept current, the voltage of the charging source must be higher than the battery voltage. • State of charge is the available capacity within a battery, while depth of discharge is the capacity that has been removed. • Bulk charging quickly charges a battery up to the regulation voltage, and absorption charging slowly completes the charge. • Equalization is a controlled overcharge that ensures that each cell is fully charged. • Overcharging causes water in the electrolyte to form gases, which escape through cell vents. • Water lost from gassing of open-vent batteries must be replaced. • Water lost from gassing of sealed or captive-electrolyte batteries can't be replenished. • Temperature can have significant effects on capacity, electrolyte specific gravity, self-discharge, gassing voltage, voltage setpoints, and battery life. • The concentration, or specific gravity, of the electrolyte can indicate state of charge and the general health of a cell. • Sulfation and stratification are common battery problems that can decrease battery performance and life. • Uncontrolled discharging and charging can result in loss of battery capacity and life. • The most common batteries used for PV applications are traction batteries. TERMS: • Electrolyte can be in a free-flowing liquid form (flooded) or an immobilized form (captive). • Lead-acid batteries are highly suitable for most PV applications but usually require a lot of maintenance. • Some nickel-cadmium batteries are suitable for most PV applications and have good temperature tolerance but can be expensive and difficult to obtain. • Required battery bank voltage is often determined by load or inverter input voltage requirements. Most PV systems with batteries operate at 12 V. 24 V, or 48 V. • Batteries can be connected in series and parallel combinations to produce a desired system voltage level and capacity. • It is generally recommended that batteries be connected in as few parallel strings as possible. • Electrical codes and safety standards generally require batteries to be installed in an enclosure separated from controls or other PV system components. • A battery is a collection of electrochemical cells that are contained in the same case and connected together electrically to produce a desired voltage. • A battery cell is the basic unit in a battery that stores electrical energy in chemical bonds and delivers this energy through chemical reactions. • A plate is an electrode consisting of active material supported by a grid framework. • Active material is the chemically reactive compound on a battery cell plate. • The grid is a metal framework that supports the active material of a battery cell and conducts electricity. • Electrolyte is the conducting medium that allows the transfer of ions between battery cell plates. • Steady-state is an open-circuit condition where essentially no electrical or chemical changes are occurring. • The open-circuit voltage is the voltage of a battery or cell when it is at steady-state. • Capacity is the measure of the electrical energy storage potential of a cell or battery. • Discharging is the process of a cell or battery converting chemical energy to electrical energy and delivering current. • The cutoff voltage is the minimum battery voltage specified by the manufacturer that establishes the battery capacity at a specific discharge rate. • The state of charge (SOC) is the percentage of energy remaining in a battery compared to the fully charged capacity. • Depth of discharge (DOD) is the percentage of withdrawn energy in a battery compared to the fully charged capacity. • The allowable depth of discharge is the maximum percentage of total capacity that is permitted to be withdrawn from a battery. • The average daily depth of discharge is the average percentage of the total capacity that is withdrawn from a battery each day. • Autonomy is the amount of time a fully charged battery system can supply power to system loads without further charging. • Self-discharge is the gradual reduction in the state of charge of a battery while at steady-state condition. • A cycle is a battery discharge followed by a charge. • Charging is the process of a cell or battery receiving current and converting the electrical energy into chemical energy. • The gassing voltage is the voltage level at which battery gassing begins. • Bulk charging is battery charging at a relatively high charge rate that charges the battery up to a regulation voltage, resulting in a state of charge of about 80% to 90%. • Absorption charging is battery charging following bulk charging that reduces the charge current to maintain the battery voltage at a regulation voltage for a certain period. • Float charging is battery charging at a low charge rate that maintains full battery charge by counteracting self-discharge. • Equalizing charging is current-limited battery charging to a voltage higher than the bulk charging voltage, which brings each cell to a full state of charge. • Gassing is the decomposition of water into hydrogen and oxygen gases as a battery charges. • Overcharge is the ratio of applied charge to the resulting increase in battery charge. • Specific gravity is the ratio of the density of a substance to the density of water. • Sulfation is the growth of lead sulfate crystals on the positive plate of a lead-acid cell. • Stratification is a condition of flooded lead-acid cells in which the specific gravity of the electrolyte is greater at the bottom than at the top. • A primary battery is a battery that can store and deliver electrical energy but can't be recharged. • A secondary battery is a battery that can store and deliver electrical energy and can be charged by passing a current through it in an opposite direction to the discharge current. • A traction battery is a class of battery designed for repeated deep-discharge cycle service. • A starting, lighting, and ignition (SLI) battery is a class of battery designed primarily for shallow- discharge cycle service. • A stationary battery is a class of battery designed for occasional deep-discharge limited-cycle service. • Flooded electrolyte is electrolyte in the form of a liquid. • A catalytic recombination cap (CRC) is a vent cap that reduces electrolyte loss from an open-vent flooded battery by recombining vented gases into water. • A catalyst is a substance that causes other substances to chemically react but does not participate in the reaction. • Captive electrolyte is electrolyte that is immobilized. • A hybrid battery is a battery that uses a combination of plate designs to maximize the desirable characteristics of each. • A battery bank is a group of batteries connected together with series and parallel connections to provide a specific voltage and capacity. QUIZ: 1. What is the difference between a battery and a cell? 2. Describe the chemical reactions that take place when a load or conductor is connected to the positive and negative terminals of a battery. 3. What factors affect the capacity of a cell or battery? 4. Explain the relationships between electrolyte specific gravity, freezing point, and state of charge (SOC). 5. Why are traction batteries ideal for most PV applications? 6. What are some relative advantages and disadvantages of captive-electrolyte batteries? 7. How does average daily DOD affect autonomy? 8. How does discharge rate affect battery capacity? 9. How can series and parallel connections be used to design a battery system with a specific voltage and capacity? Next: Charge Controller Prev: |