2. Chapter 6
Batteries
Battery Principles • Battery Types • Battery Systems • Battery Installation

3. Batteries are collections of cells that produce electricity through electrochemical reactions. Cells can be configured into batteries of many different shapes and sizes.
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 Figure 6-1. A cell is the basic unit in a battery that stores electrical energy in chemical bonds and delivers this energy through chemical reactions.

4. Many components are common to various battery types.
A battery cell consists of one or more sets of positive and negative plates immersed in an electrolyte. See Figure 6-2. 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 electrode. The amount of active material in a battery determines 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.

5. Higher temperatures and slower discharge rates result in increased battery capacity.
Temperature and discharge rate typically affect capacity, especially of lead-acid batteries. Warmer batteries are capable of storing and delivering more charge than colder batteries. See Figure 6-3. However, higher operating temperatures also 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 system 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.

6. Electrochemical reactions within a cell produce a flow of electrons from the negative terminal to the positive terminal when they are connected electrically.
At the negative terminal, the active material in the negative plate reacts with the electrolyte to form a new material that releases excess electrons. See Figure 6-4. 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.

7. Slower discharge rates remove more energy from a battery than faster discharge rates.
Discharge rate is the ratio of nominal battery capacity to the discharge time in hours. For example, a 5 A discharge current for a nominal 100 Ah battery would be a C/20 discharge rate. The designation C/20 indicates that 1/20th 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. See Figure 6-5. 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.

8. The state of charge and depth of discharge of a battery always add up to 100%.
The state of charge (SOC) is the percentage of energy remaining in a battery compared to its fully charged capacity. See Figure 6-6. 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 discharged is at 25% state of charge.

9. When PV arrays are used to charge batteries, seasonal insolation variations (in addition to loads) affect depth of discharge values.
The average daily depth of discharge is the average percentage of the total capacity that is withdrawn from a battery each day. See Figure 6-7. If the load varies seasonally, such as in a dusk-to-dawn lighting system, the average daily DOD will be greater in the winter months due to the longer nights increasing the loads. If the loads are constant, the average daily DOD will be greater in the winter due to low temperatures that lower the available battery capacity. Depending on the rated capacity and the average daily 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.

10. Batteries exhibit higher self-discharge rates at higher temperatures.
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 electrochemical mechanisms and losses. The rate of self-discharge differs among battery types and increases with battery age and temperature. 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 Figure 6-8.

11. The charging reaction within a cell is the reverse of the discharge reaction restoring the reactants and electrolyte to their original forms.
The electrons passing through in the opposite direction reverse the chemical reactions and restore the active materials and electrolyte to their original compositions. See Figure 6-9. Charge rate is the ratio of nominal battery capacity to the charge time in hours expressed in the same way as discharge rate. For example, a charging rate of C/50 to a 100 Ah battery applies 2 A of current until the battery reaches a specific fully charged voltage.

12. Bulk, absorption, and float charging control battery voltage with the charging current during a multiple-stage charging cycle.
During charging, the battery voltage rises sharply then stabilizes. See Figure Voltage rises again, first very slowly, then faster. Voltage stabilizes again at the fully charged voltage level. Batteries can be charged in one stage or multiple stages. Three common stages of normal battery charging are bulk charging, absorption charging, and float charging. Equalizing charging is an additional, special type of charging.

13. The freezing point of sulfuric acid electrolyte changes at various states of charge because of changes in specific gravity.
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. See Figure As the battery becomes discharged, the specific gravity decreases, resulting in a higher freezing point for the electrolyte.

14. Sulfation reduces the capacity of a lead-acid cell by locking away active material and electrolyte as crystals.
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 Figure Sulfation also increases internal resistance within the battery, making it more difficult to charge.

15. Stratification results when the specific gravity of the electrolyte is higher at the bottom of a cell than at the top.
If physically tall battery cells are not adequately charged, 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 Figure 6-13.

16. Batteries are divided into classes based on their applications and their discharge and cycle characteristics.
Secondary batteries are often classified as traction; starting, lighting, and ignition (SLI); or stationary batteries. See Figure The differences in their design and materials result in different discharge and cycle characteristics.

17. Catalytic recombination caps (CRCs) recombine oxygen and hydrogen into water, which then drains back into the battery.
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. See Figure 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 itself 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%.

18. Captive-electrolyte batteries are sealed and contain electrolyte that is immobilized.
Captive electrolyte is electrolyte that is immobilized. See Figure 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 cannot 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.

19. The characteristics of the types of batteries commonly used in PV systems vary between different designs.
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 some types do not tolerate extreme temperatures well and some require frequent maintenance. See Figure 6-17.

20. System requirements and characteristics of battery types must be considered when choosing a battery.
Each battery type has design and performance features suited for particular applications. System designers must consider the advantages and disadvantages of different battery types with respect to the requirements of a particular system. Considerations include lifetime, deep cycle performance, tolerance to high temperatures and overcharge, and maintenance requirements. See Figure 6-18.

21. Connecting batteries in series increases system voltage.
Just like PV modules in an array, batteries are first connected in a series string by connecting the negative terminal of one battery, to the positive terminal of the next battery. See Figure Because there is only one path for the current to flow, the circuit current remains the same through all the batteries in series. 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. If batteries or cells with different capacities are connected in series, the capacity of the string is limited by the lowest-capacity battery. Therefore, it is recommended that mixing batteries of different age, type, or capacity in series be avoided.

22. Connecting batteries in parallel increases system capacity.
Batteries are connected in parallel by connecting all of the positive terminals together and all of the negative terminals together. See Figure 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.

23. Series and parallel connections can be combined to produce a desired system voltage level and capacity.
Series strings of batteries can also be connected in parallel in the same way. See Figure 6-21.