Batteries, as electrical energy storage medium, are very important and delicate part of standalone solar PV systems. They are important because without energy storage, a solar PV system will not be able to deliver the energy to the load when there is no sunlight.

In the case of standalone systems, we need electrical energy for running our appliances in non-sunshine hours, while in the case of grid connected PV systems, we do not require any energy storage. Grid, if operational, provides the energy whenever it is required. In standalone PV systems, batteries are delicate because the misuse or non-optimal  use of batteries can reduce their life significantly.

These days, as the cost of solar PV modules are decreasing, the cost of Battery is becoming a significant part of the overall solar PV system cost. Thus, from cost perspective as well the batteries are becoming more important in solar PV systems. Due to the importance of batteries in solar PV system, significant efforts are made here to give all technical as well as practical details about batteries.

6.1  Some Basics about Batteries

In our day- to-day life, we frequently use things like torch, calculator, mobile phone, radio, electronic watch, etc. All these things are portable electronic devices and they require movable supply of energy. The DC (direct current) supply is needed to run these devices. The conventional form of AC (alternating current) supply available to us cannot be applied to these devices directly. To provide the power supply to these devices, a device having electrical energy stored in it is used. These devices having stored electrical energy or stored charge is known as battery. The charged stored in the batteries can be used to supply the energy to appliances when required. A battery stores electrical energy (charge) in the form of chemical energy. When a battery is used, the chemical energy stored  is converted into electrical energy.

A Simple analogy can be drawn between battery as electrical energy storage medium and water tank as water storage medium. We all know the water tank is used to store large quantity of water and we can use water from tank when needed. Water storage tanks come in different shapes like cylindrical and cuboid and different storing capacities like 500 litres, 1000 litres, 10000 litres etc. The quantity of water to be stored determines the size of water tank to be used. Similarly, depending on the electrical energy requirement, size of the battery (charge storage capacity) and its terminal Voltage is determined.

A battery is a two terminal device. One terminal I called positive (+) and the other terminal is called negative (-). In the charged condition, there is a voltage difference between two terminals. This voltage difference drives the current in appliances when connected. For giving supply to a device from  battery; the positive and negative terminals of a battery is connected to the corresponding terminals of the device.

6.1.1  Rechargeable Batteries

Batteries used in solar PV systems require frequent charging or refilling of charge. Again an analogy can be drawn with regular refilling of water tank in our house. The water tanks in our houses and commercial buildings need to be refilled on regular basis. As per the usage of water, the level of water decreases in the tank and after sometime the tank becomes empty. To avoid disruption to the continuous supply of water, the tank has to be refilled as soon as it gets empty. This process of emptying water from tank and refilling the tank is repeated frequently, almost on daily basis. 

Similar to water tank, batteries are ‘charge storage tanks’. Also, similar to the emptying and refilling cycle of water tank, the electrical energy of batteries gets emptied (while in use) which we need to refill again. This refilling of electrical energy in batteries is technically known as “charging”. Actually, during the charging process, electrical energy supplied from outside to the batteries get converted into chemical energy. The process of consumption of electrical energy from batteries is technically known as ‘discharging’. During the discharging process, chemical energy stored in batteries gets converted into electrical energy which is supplied to appliances. In this way, the working of batteries typically consists of two processes; charging and discharging. Single charging and single discharging process together is called one charging-discharging cycle.

Some batteries allow repeated charging-discharging cycles while others do not. The batteries which allow repeated charging-discharging cycle are known as ‘rechargeable batteries’. The rechargeable batteries are widely used in the solar PV systems. The batteries in PV system play an important role in the storage of electrical energy and providing continuous electric current supply irrespective of absence/ presence of the sun and the change in weather. Without batteries, it is difficult to think of ‘standalone solar PV systems’

6.2  How does a battery work ?

Generally, a battery is made of a combination of two or more units of electrochemical cells (voltaic cells) connected together in series or parallel combination. It is called electrochemical cell, because it deals with the electrical and chemical energy. The electrochemical cell, in general, is also termed cell. A single unit of an electrochemical cell consists of two half-cells as shown in Figure 6.1. Each half cell consists of an electrode and an electrolyte. The two half-cells  are electrically connected to each other by salt bridge. The electrodes in the two half-cells are of different metals. In each half-cell, a chemical reaction occurs at the metal electrode. The operation of the cell involves two chemical reactions.

One is oxidation reaction and the other is reduction reaction, commonly called the Redox reaction which converts the chemical energy into electrical energy as shown in Figure 6.1. Oxidation is a process in which electrons are lost or released, and the reduction is a process in which electrons are accepted or gained and both can be represented by the following expressions :

6.2.1  Charging & Discharging of batteries

Both oxidation and reduction reactions take place at the time of charging and discharging. While charging, the oxidation reaction takes place at negative terminal (cathode) and reduction reaction takes place at positive terminal (anode) and vice-versa during discharging (Figure 6.2). A charging process and subsequent discharging process together are defined as one charging-discharging cycle or a single cycle.

When the battery is completely filled with charge up to its maximum level, it is said to be fully charged. When the charge is completely used or finished, the battery is said to be fully discharged.

6.2.2  Components of a Battery Cell

It can be seen from the discussion in Section 6.2 that operation of a battery requires anode (positive electrode), cathode (negative electrode), electrolyte, and salt bridge ( Figure 6.2). The role of each of these components is briefly described below:

• Anode :  It is generally referred as positive terminal or positive node or positive lead. It is the electrode which gives up electrons to the external circuit, as a result the electrode is oxidized during the discharging reaction.

• Cathode : It is generally referred as negative terminal or negative node or negative lead. It is the electrode which gains electrons from the external circuit, as a result of which the electrode is reduced during the discharging reaction.

• Electrolyte : It is a medium which provides conductivity to ions between anode and cathode. One can say that an electrolyte is a medium through which current flows internally in a battery. An electrolyte is typically a liquid, such as water or other solvents with dissolved salts, acids or alkalis.

• Salt bridge: It is a porous material used to keep the two electrodes connected but yet keep them separate from each other; otherwise the chemical reaction would stop. It is also referred as a separator.

6.3  Types of batteries

There are varieties of batteries that are available in the market for several types of applications. Each battery type is more suited for one particular application. The type of battery is identified by the chemistry of materials used in making it. The batteries are broadly divided into two categories:

1. Non-rechargeable batteries or primary batteries, and 
2. Rechargeable batteries or secondary batteries.

6.3.1  Non-rechargeable Batteries

In the non-rechargeable batteries, the electrochemical reaction is not reversible. This type of batteries is used for one time and once discharged, they cannot be charged again.

The non-rechargeable batteries are the most convenient, simple, easy to use and require less maintenance. These types of battery are portable and are made in various sizes and shapes. The size are normally referred as size A, AA, AAA, C, D, etc. and the shapes of such batteries can be of several types like coin, cylindrical, cuboid, etc. These batteries are mainly used in transistors, toys, torches, etc. These batteries have high shelf life, reasonable cost, energy and good power density. Generally, these high shelf life, reasonable cost , energy and  good power density. Generally, these batteries are available in small capacities, typically below 20 Ah (Ampere-hour). These batteries can be operated in a wide range of temperatures; -400C to 700C.

The most common example of non-rechargeable battery is Zinc Chloride battery, commonly known as pencil cell (shown in Figure 6.3). This battery can be used only once and it cannot be recharged. Other examples of non-rechargeable batteries are magnesium cells, Aluminum Cells, Alkaline-manganese dioxide cells, Mercuric oxide cells, etc. All of them are used for different applications. The batteries that are required in solar PV systems need to be charged and discharged regularly, therefore, non-rechargeable batteries are not used in standalone solar PV systems.

6.3.1  Rechargeable Batteries (Secondary Batteries)

The batteries in which the conversion of chemical energy into electrical energy (discharging) and the reverse process, that is, conversion of electrical energy into chemical energy (charging) can take place are called the rechargeable battery or secondary battery.

The rechargeable batteries are the most widely used batteries in the world. These batteries are used for various applications, such as starting, lighting and ignition (SLI) in automotives, standby power supply, electronic appliances like DVD player, mobile phones, camera, camcorder, laptops etc. These batteries are available in a wide range of charge storage capacities in the market and can be easily procured.

The commonly available rechargeable batteries are listed below and their pictures as shown in Figure 6.4.

• Lead Acid
• Nickel Cadmium (NiCd)
• Nickel Metal Hydride (NiMH)
• Lithium Ion (Li-ion), and
• Lithium Ion Polymer (Li-ion polymer)

We can see in Figure 6.4, the size of each battery is different. The physical size of a battery also reflects its energy storage or charge storage capacity. The batteries of small charge capacities are small in size and high charge capacities are large in size. The lead-acid batteries are normally big in size which means that they can store large amount of charge. Due to this reason, the lead  acid batteries are commonly used for solar PV applications. The lead acid batteries are actually one of the most widely used rechargeable batteries used for large amount energy storage application. They are also mostly used for solar PV applications.

6.4 Parameters of batteries

One must choose appropriate batteries foe application in the solar PV systems. The choice is determined by the various characteristics of batteries , suitable for given need. The characteristics of batteries are defined by a set of battery parameters. These parameters include charge storage capacity, terminal voltage, rate at which batteries can be charged and discharged, the cost of battery, the number of times the charging-discharging cycle can be carried out in a battery lifetime and so on. In practice, these parameters effect the performance of the battery. For design, installation and maintenance of the battery or group of batteries (called battery bank) knowing and understanding the battery parameters are very important. Ideally, we would like our battery to work for many years and should give us required energy in all conditions. On every battery casing, the important parameters are displayed by the manufacturers. Based on the parameters values mentioned by the manufacturer, the selection of a battery can be made. Various parameters related to battery are discussed in this section.

The standard parameters of a battery specified by the manufacturers are following :

• Battery terminal voltage (in volts),
• Charge storage capacity (in Coulomb or ampere-hour or Ah),
• Depth of discharge (in percentage),
• Number of useful charging-discharging cycle (in number),
• Life cycle (in years),
• Self discharge (in %), etc.

6.4.1  Battery Terminal Voltage (V)

Batteries supply electrical energy to the load by flowing current to the loads/devices connected across its terminals. The electrical energy transfer from battery to load is possible only when there is voltage difference between two terminals. Thus a battery’s terminal voltage is the voltage difference between its two electrodes.

The voltage difference between the battery terminals is driving force for current to flow. For a given appliance, an appropriate level of terminal voltage must be available, otherwise the device does not work. For instance, sometimes 6V and sometimes 12  V. Some devices may also need higher voltages. Thus, battery terminal voltage is one of the important parameter that determines the choice of the battery.

For solar PV system application, there are batteries which are available with 6V and 12 V ratings. Each battery is made up of cells. The terminal Voltage of cells is determined by the material they are made up of. Normally, the cell voltage is not large to give us required 6 V or 12 V. Therefore, many cells are connected together in series to get higher voltage. In a 12V Lead-acid batteries, 6 cells are connected together.

The battery terminal voltage changes with the condition of battery. The terminal voltage increases when battery gets charged, the terminal voltage decreases when battery gets discharged. It also depends on several  other conditions.

6.4.2  Battery  Open Circuit Voltage & Terminal Voltage

The terminal voltage of battery is maximum when it is fully charged and when no current is flowing. The condition of no current flow is equivalent to open circuit. Therefore, the maximum terminal voltage of battery is also referred as open circuit voltage of battery or V0 It is also referred as e.m.f. (electromotive force) of the battery or Vemf.

When current flows through battery, its terminal voltage is normally lower than the open circuit voltage, V0 . This happens because of the internal resistance of the battery. Due to its own resistance, some voltage drop occurs inside the battery. This voltage drop is equal to the current flowing through the battery multiplied by the internal resistance of the battery or equal to I X R I (as shown in figure 6.5(a)). The battery and its symbol used in electrical circuit diagram are shown in Figure 6.5 (b). Therefore, when current is flowing, the actual available terminal voltage will be different between open circuit battery voltage (Vo) and internal voltage drop in battery I X R I . This voltage drop I X R I occurs inside the battery.

In this way, the battery terminal voltage can be written as :

Where,

Vbattery = Battery voltage or terminal voltage (in volts) ;

V0 = Open circuit voltage(in volts);

I = Current flow when load is connected to a battery (amperes); and

R I = Internal resistance of battery (ohms).

The battery has its internal resistance  due to many factors. A battery consists of many cells. The factors like the contacts within the cells, metal plates / electrodes and salt bridge separator, even the electrolyte creates  some internal resistance. The schematic representation of open circuit voltage and terminal voltage of battery when current is flowing is given in figure 6.6.

6.4.3  Technologies for Battery Terminal Voltage

There are various voltage technologies associated with battery like Open-circuit voltage, Nominal or operating voltage, and Cut-off voltage, and these technologies are briefly described here:

• Open Circuit Voltage : It is also called theoretical voltage because this is the maximum possible voltage at output terminals of battery when circuit is open.
• Nominal terminal voltage or operating voltage : It is actual voltage available at the output terminals of the battery on which load can operate. The standard battery nominal voltages available are 1.5V, 3V,6V, 12V, 24V,48V, etc.
• Cut-off voltage: It is a voltage up to which the load can be operated and below which the battery should be disconnected from the load in order to prevent it from over discharge.

Figure 6.7 shows and points up the voltage and capacity values of sealed lead-acid battery along with the other parameters.

 6.4.4  Battery Storage Capacity (C)

The capacity of a battery is the capacity to store the charge in the battery. It is the product of current (in amperes) it can deliver for a given time (in hours), i.e., Ampere X Hour (Ah). One ampere-hour (Ah) is the amount of charge delivered when constant current of one ampere (A) is used for one hour (h). In this chapter, we will express battery capacity in term of ampere-hour. The capacity of a battery is given by the expression shown below :

Capacity (C) = Current (A) X Hour (h)

The capacity of non-rechargeable batteries normally varies in few mAh (milli Ah) to several Ah range. The capacity of rechargeable batteries can vary from few Ah to thousands  of Ah.

The capacity of batteries depends on temperature. The same battery will have different capacity at different temperatures. The capacity is specified at standard test conditions of 250 C. Hence, the capacity value measured at installation sites may vary from the values given by the manufacturer because of the change in temperature at particular location. Therefore, while selecting the battery for a given purpose, temperature of the location should be taken in account.

After knowing charge storage capacity and terminal voltage of the battery, we can find out some other information’s about the battery.

How much current battery can supply ?

We must always remember, in practical, there is no source which can supply an unlimited amount of current. Similarly, battery is a device that can only supply fixed amount of current. The capacity rating helps in finding out how much current a battery can provide, if duration of the current drawn (or discharge duration) is given. The current drawn can be given as:

The above expression suggests that, for a given battery capacity, if the discharge duration is long, we can draw small current from battery, but if our discharge duration is small, we can draw large current from battery. This is analogue to water tank, from a water tank of fixed capacity, if we draw water slowly, we can draw water for long time , but if we draw water fastly then the tank will get emptied soon. Thus, by choosing small discharge duration, it is possible to draw a large amount of current. But drawing large current from battery must be avoided. As large current cause large voltage drop inside the battery due to internal resistance and terminal voltage available outside decreases.

Here, the question is, what current is large current? Normally, for batteries used for solar PV systems, the discharge duration is 10 to 20 hours. Corresponding this discharge duration is 10 to 20 hours. Corresponding  this discharge duration, the magnitude of current drawn will be considered normal current. But if you are discharging battery in 1 to 2 hours then the current drawn will be considered large current. The amount of current that can be drawn from a battery is discussed more in details in section 6.4.5 and 6.4.6.

How much energy is stored in battery ?

If we know the terminal voltage of the battery and its charge storage capacity, we can obtain how much electrical energy is stored in the battery. The electrical energy is given in terms of the product of charge capacity and voltage. Thus, energy stored in a battery can be given by the following expression :

Energy (watt-hour) = Capacity (Ah) X Voltage (V)

The above expression indicates that large capacity battery and higher terminal voltage battery stores higher amount of electrical energy.

What is the power of  the battery ? 

Power for any device is defined as product of voltage and current. In case of battery if we multiply the terminal voltage of battery with discharge current we will get the power of the battery. Thus, battery power can be written as :

Battery power (watt) = Terminal Voltage (V) X Current drawn (A)

6.4.5  State of Charge (SoC) and Depth of Discharge (DoD)

In practical applications, all the charge stored in a battery cannot be used for running load. Only some percentage of total charge stored can be used for running the load is referred as Depth of discharge (DoD). 50% DoD means that only 70% of the total stored charge can be used. In general, we want higher DoD for the batteries which are used in solar PV systems. Therefore, normally, deep discharge batteries are preferred for which the allowable DoD is 100%. Normally, the batteries used for SLI (starting, lighting & ignition, for instance, our car batteries) applications have small DoD, about 50%. The Li-ion batteries have DoD of 80% to 90%.

As the depth of discharge of battery increases (due to use of its stored charge), the terminal voltage of the battery decreases. Fully charged battery will have higher terminal voltage as compared to discharged battery. Table 6.3 shows the voltage level at different DoD level in terms of percentage.

Manufacturers specify allowable DoD level for their batteries. The battery should not be discharged below manufacturers specified level in order to prevent damage to the battery. If the batteries are discharged below their DoD rating, then the life of the batteries decreases very fast. It means that if the life of the battery in 3 years and it is continuously discharged below its DoD limit, then battery may stop functioning in 6 months only. For practical application. It is better to take batteries to 50% of the DoD specified by manufacturer. The tolerable limit of DoD is determined from the charging / discharging efficiency of a battery. The typical tolerable DoD of various types of batteries is given in Table 6.4. 

State of Charge

Rechargeable batteries have to be often charged for reuse. For such batteries, the time of charging is decided by the present values of charge level or present state of charge (SoC). The SoC indicates level of charge, i.e. percentage of total charge that is stored at this time in a battery. Thus, if 60% of the total charge storage capacity is still there in the battery then the battery’s SoC is 60%.

The DoD is another way of showing SoC. The DoD is the inverse of SoC. Both DoD and SoC are expressed in percentage. Present SoC when subtracted from 100% gives the present value of DoD. This can be written in the following way :

DoD (%) = 100 % – SoC (%)

SoC (%) = 100 % – DoD (%)

For example suppose a battery after sometime of usage has SoC 70%. This indicates that its present DoD = 100% – 70% = 30%. As we keep using battery, its DoD percentage increases and the SoC decreases. The schematic representation in Figure 6.8 shows the different levels of SoC and DoD.

It is discussed earlier that as the SoC decreases, the open circuit voltage & terminal voltage of the battery decreases. In other words, for higher SoC, the battery will have higher terminal voltage and for lower SoC, the battery will have lower terminal voltage. Thus, when a fully charged battery is utilized to supply the charge to a load, and as the amount of stored charge in the battery decreases, its terminal voltage keeps decreasing. In this way, at any stage of discharge, if we measure the terminal voltage, we can estimate the state of charge of a battery. For a lead-acid battery, the relationship between the open circuit voltage and SoC is given in Figure 6.9. 

6.4.6  Charging / Discharging Rate of C-rating

It has been discussed earlier that the terminal voltage (when current is drawn) of the battery is less than the open circuit voltage (when no current is drawn) due to voltage drop in internal resistance of the battery. The voltage drop due to internal resistance of the battery. The voltage drop due to internal resistance is I X Ri. The large current drawn from the battery causes large voltage drop, meaning less terminal voltage is available for the load. Discharging battery at high rate (high current flowing into the battery) is also not safe. The amount of current (discharge current) drawn from battery plays a very important role in service life or back-up time of the battery. As we increase the load current the battery discharges at faster rate (as shown in Figure 6.10). Over –discharging of battery leads to decrease in capacity and life span as well as it causes mechanical damage to the battery.

One cycle of battery means one charging plus one discharging cycle. Typically, the life cycle of a lead-acid battery is 500-800 cycles. Generally, manufacturer gives the values of maximum charging-discharging current and voltage. The battery can be charged with the following three methods :

1. Constant Voltage.
2. Constant Current or
3. Both.

A lead acid battery accepts all of these methods of charging is inbuilt in the good charge controllers. Therefore, appropriate charge controllers must be used for good life of the battery.

In order to ensure proper charging / discharging of batteries, manufacturers specify the charging /discharging current rates in terms of C-rating. The C-rating specify in how many hours a given battery should be charged or discharged. The C-rating value is obtained by dividing the battery capacity (Ah) by the suggested number of hours taken to fuly charge the battery completely (or to rach 100% SoC) or time taken to reach the full tolerable DoD of the battery. The expression for C-rating is as follows:

Where, Capacity © is in ampere-hour (Ah), and time for full charge or full tolerable discharge (t) is in hours (h).

Consider, a battery of capacity C and time for full charge or discharge is 1 hour, then C-rating will be C/1 or 1C. Similarly, If t=10 hours, then C-rating is C/10.

6.4.7  Battery Efficiency

The charging voltage of any rechargeable battery is greater than the discharging voltage. The charging voltage is the sum of battery e.m.f. and voltage drop due to the battery’s internal resistance. The discharging voltage is the difference of battery e.m.f. and voltage drop due to the battery’s internal resistance of the battery, the discharged energy is always less than the charging energy. Typically, a lead- acid battery is 80% to 90% efficient in doing charge transfer. The expression for the charge transfer efficiency is given below :

The energy efficiency is typically 65% to 70% for a lead-acid battery. Charge transfer efficiency / ampere-hours is usually used to calculate the panel array needed to charge the battery bank.

6.4.8  Operating Temperature

The operating temperature of battery is one of the factors which effect the performance of the battery. As the temperature decreases, chemical activity and the internal resistance of the cell increases, in turn, this reduces the voltage and current capacity of the battery. As the temperature reduces, the available capacity of battery decreases but internal resistance of the battery increases which results in reducing available terminal voltage. Typically, the operating temperature range of a lead-acid battery is -150C to 600C. Note that battery ratings are given for 250C temperature.

When a 100Ah battery is discharged  at 250C temperature, it will provide its maximum capacity, but when the same battery operates at 200  It gives less capacity in comparison  to capacity given at 250C. As the temperature decreases, the battery capacity also decreases. The decrease in capacity is indicated by the SoC of a battery . If  SoC of a battery (with same charging status ) is measured at two different temperature, say  400C and  200C, then the battery will show different SoC in both cases, as shown in Figure 6.11. At 400C, the battery will show higher SoC as compared to 200C. This point should be taken care of while designing a SPV system. The battery capacity can also decrease at higher temperatures due to deterioration in chemical activities. Normally, the best battery performance is obtained between 200C and  400C.

6.4.9  Life Cycle

One charging and discharging operation of a battery is referred as one cycle of battery. A battery cannot be used for infinite number of charge-discharge cycles. Due to each charge-discharge cycle, the capacity of a battery decreases slightly. Therefore, depending the material and technology used for making a battery, the batteries can be used only for a certain number of cycles. The usable number of charge-discharge cycles of a battery is referred as life cycle of the battery. After the life cycle, the battery capacity decreases below acceptable level.

After each charge-discharge cycle, the battery capacity decreases by some amount. When, after certain usage, the battery capacity decreases to 80% of the initial capacity, It is considered the end of life for the battery. The life cycle of a battery is defined as the number of charging and discharging cycles it can perform when its capacity reaches below 80% of its initial nominal capacity. If battery is charged and discharged daily, then one cycle is equivalent of one day, and one year is equivalent to 365 cycles. In this way, the life of a battery can be given in terms of the number of cycles or number of years of operation. The life cycle for non- rechargeable batteries is 1 cycle because only one time the chemical energy is converted into electricity. For typical rechargeable batteries, life cycle ranges from 500 to 1500 cycles. The degradation of performance of a batteries occurs due to ageing effect which includes the shedding of active material from plates. This induces gradual decrease in performance of the battery.

DoD directly affects the life cycle of a battery . In case, the batteries are discharged below the lowest DoD limit, there is a danger, the batteries may get damaged when recharged. This, in turn, will shorten the life span of batteries. As the DoD increases, the life of the battery decreases fast. For instance, a Lead-acid battery if discharged only 10% daily and then charged again, the battery may work for 5 years. But, if the same battery is discharged 30% daily then it may work for 3 years only. If the same battery is discharged 80% daily, it may only work for less than one year. Allowable DoD limit depends on the types of battery material and the construction of its two electrodes (or plate) called anode and cathode. A typical life of a lead-acid battery is given in terms of the number of cycles of charge and discharge. It is clear from the figure that, if we discharge our batteries more on regular basis, then the life of the battery is shorter.

The lead-acid batteries come in different electrode design. Depending on the design of a battery, the life of the battery can be different. The DoD of a lead-acid battery for different electrode designs and possible life time of batteries are given below in Table 6.8.

  6.4.10  Self Discharges / Shelf Life

Self discharge is the charge consumed when battery is not in use for a long time, i.e., sits on the shelf. The reason for self discharge is that the electrochemical process takes place within the cell. Self discharge is equivalent to the application of a small external load. Self discharge of batteries should be as low as possible. The self discharge rate of battery increases with increase in temperature of battery. Therefore, it is recommended  to store batteries at lower temperatures in order to reduce self discharge. Self discharge of a battery is directly influenced by the chemistry of battery. The charge stored stays in a battery depends on the chemistry of battery. Li-ion cells typically lose 25% of the stored charge in three months when stored at about 300C, the lead-acid battery would  lose 50% of the stored charge in 3 to 4 months, while the Ni-Cd battery would lose the same charge in just 6 weeks.

6.5 Comparison of Various Rechargeable Batteries

With PV system in focus, we focus more on the typical features, components and other related information of lead-acid battery. The typical values of voltage (e.m.f.), efficiency, self discharge, number of cycles and life of rechargeable batteries are tabulated in Table 6.9.

6.6 How to Select a Battery ?

Knowing the different categories and the types of battery is not sufficient to select a battery for given requirement. The selection of a battery is done by seeing the battery parameters. The parameter values for voltage rating, current rating, capacity rating, number of charging-discharging cycles and shelf life differs from battery-to-battery. These values also differ from one manufacturer to another. So, we must have good understanding of the battery parameters to identify a proper battery for an application before buying or using it. 

In the market many types of batteries are available. While designing a solar PV system comprises batteries, the one problem often faced is a proper choice of battery from many available batteries. This problem can be simplified by making a list of minimum requirements, conditions and limitations:

6.6.1 Types of Battery

Based on application, the type of a battery is chosen. For example, primary batteries are used in portable devices which run on very low current, such as electronic watch, torch and radio. Primary batteries are used for one time only. They are cheap and easily disposable. Some applications require high current to run devices. In such cases, the secondary batteries or rechargeable batteries are used which provide high current as they have high capacity and can be recharged a number of times. The rechargeable batteries are costly compared to other primary batteries, but they are cost effective over long time use.

6.6.2 Voltage and Current

A battery should be selected according to the voltage and current requirements of the instrument to which it is connected. This is discussed in more detail in Chapter 7.

6.6.3 Temperature requirement

The batteries are designed to operate at certain temperature, normally about 250C. In practice, the prevailing temperature or ambient temperature may be different.  Sometimes, the ambient temperature can be above 450C and sometimes, it can be negative or at freezing temperature. The battery output current and its capacity depends on the temperature. At higher temperature, batteries can be deliver higher current as compared to lower temperature. At lower temperatures, like below 200C, the chemical reactions in battery become slower and the battery can only deliver lower current than designed value. At higher temperatures, the battery may provide higher current but its life decreases when operating temperatures are high. At very high temperature, some batteries can be dangerous causing explosion and fire can be occur. Therefore, depending on the operating temperature of battery, appropriate battery must be chosen for a given applications. The manufacturer datasheet provides the information about safe operating temperature and how the capacity and current will change with temperature for a given battery.

6.6.4 Shelf Life

Batteries shelf life is directly affected by the rate of self-discharge. Self discharge occurs all the time, even when the battery is not used. Some batteries have extremely low self discharge rates, such as most of the lithium battery whereas, some nickel metal hydride batteries can lose up to 4% of their capacity every day.

6.6.5 Charge-Discharge Cycle (If Rechargeable)

The number of charge-discharge cycle of a battery depends on the type of battery. Normally, for rechargeable applications, battery with large number of charge-discharge cycle should be chosen. For instance, for standalone solar PV systems, batteries with large number of charge-discharge cycles are chosen.

6.6.6 Cost

Costing is based on the type of batteries selected and it’s pricing by different manufacturers. Also, it depends on the features of the battery. Generally, the maintenance free and advanced technology batteries are expensive. Normally, the efforts are made to optimize the battery requirement and reduce the cost. In some cases, it may require that system must perform in all conditions, therefore, the over sizing of the battery is done. In such cases, the cost of the battery is not important but the reliable system performance is given importance. But if the system requirement is not crucial, then we should try to minimize the battery expenses  in a system.

6.6.7 Availability

Before selecting  a battery, we should check its availability in the market. If a battery, rarely available, is used in a system, it may create problems while replacing or buying new one for the system. So, a battery that is readily available, easy in transportation and can be bought even in remote places, Should be selected.

6.7 Batteries for Photovoltaic (PV) Systems

The batteries used for PV systems must be rechargeable, allow deep discharge, should have long life span, easily serviced and have high capacities and low self-discharge. Lead-acid batteries are rechargeable, typically, they have capacities in the range of 1 to 12000 Ah, they have 500 to 800 charge-discharge cycles and life time is of about 2 to 3 years. The lead acid-battery performs very well for a wide range of temperatures, from -150C to 600C. The replacement and maintenance of lead-acid batteries are simple. All these factors make lead-acid batteries a good choice for PV applications. There are varieties of lead-acid batteries available. In general, the lead-acid batteries are classified into two categories : 

1. Liquid Vented
2. Sealed or VRLA (Valve Regulated Lead-Acid) It is of two types :

• Absorbed glass mat (AGM) battery
• Gel battery (gel cell)

6.7.1 Liquid Vented

Liquid vented batteries are generally used in automobiles. When a battery is charged and discharged, the hydrogen and oxygen gas is produced due to chemical reactions. The gas produced during charging-discharging cycle must be vented out of the battery. In the process of venting out the gas, some water from the battery is lost. This loss of water must be supplied to the battery again, otherwise the battery will become dry and will not function properly. Therefore, for such batteries, some maintenance like regular water refilling is required.

6.7.2 Sealed or VRLA (Valve Regulated Lead-Acid) 

Sealed or valve regulated lead acid (VRLA) batteries do not have any caps from where the gases produced in the charge-discharge cycle can exit as in case of liquid vented batteries. The VRLA batteries are designed in a manner that hydrogen and oxygen produced in the battery  are recombined again to produce water. The valve is provided in the battery to regulate the pressure inside the battery. Since during charge-discharge cycle no water is lost, no refilling of water is required for sealed or VRLA batteries. They are maintenance free. Sealed batteries have the electrolyte or acid either gelled or put into a sponge-like glass mat. They have the advantage / disadvantage of being completely liquid – tight. They can be operated in any position, even sideways or upside down, and acid will not leak.

Absorbed Glass Mat (AGM) battery

In this type of battery, the fibrous silica glass mat is used to suspend electrolyte. It forms semi solid (Gel) electrolyte along  with empty pockets which helps in recombination of gas like hydrogen generated during the charging process. This decreases the risk of hydrogen explosion.

Gel Battery

The gel battery is similar to Absorbed Glass Mat Battery. The only difference here is the silica gel is used instead of fibrous silica glass.

 6.8  Design of Lead-acid Batteries

Lead acid batteries are manufactured based on the application requirements. The requirements can be of different types, such as large number of charging and discharging cycles (long life), deep discharge capability in each cycle  (high DoD) and high current supplying capacity. The distinguishing feature from one design to other design is their structure of electrodes or  plates (where chemical reactions take place in batteries). In PV systems, the batteries are very frequently charged  and discharged. The batteries selected for PV systems should meet the high number of charging-discharging cycle requirement. Hence sealed Lead-acid Deep cycle battery is the ideal choice used widely in the PV systems. Deep cycle batteries can be deeply discharged allowing to use most of its capacity (i.e .,̴50% of its capacity).

Electrode design

A typical Lead-acid battery consists of electrodes, electrolyte, separator and polypropylene mono-block casing. For constructing batteries, two important techniques, flat-plate design and tubular electrode design are used.

The charge storage capacity of a battery depends on the rate of chemical reactions at the electrode. The large surface area of the electrode allows more chemical reaction at electrode and hence large current supply capacity. Therefore, a battery used for SLI (starting, lighting and ignition) where high current is required for short time, flat plate design of electrode is used. In the case of large capacity requirement, a large volume of active material on electrodes is required in which the electrodes are made thicker. Also, when we discharge battery to deep levels (deep discharge batteries) the active material on electrode gets lose and falls off the electrode reducing the capacity of the battery. In order to avoid the loss of active material from electrode in charge-discharge cycle in deep discharge battery with tubular electrode design is used.

Flat-plate designs are mostly used for manufacturing starting, lighting and ignition (SLI) Lead acid battery. The tubular electrode designs are used in batteries for deep discharge cycle. Tubular positive plates are made of 20-30 tubes connected together by a connector bus as shown in Figure 6.13(b). The manufacturing steps are almost same for all batteries except electrodes design.

Electrical design

The lead –acid batteries generally come with voltage ratings 6V, 12V and 24V. Figure 6.14 shows a typical 12V lead-acid battery configuration; the battery consists of six cells connected in series. Each cell with e.m.f. is 2V and capacity is 1C. The cells connected in a string gives voltage 12 V and capacity 1C.

6.8.1  How Battery is Constructed ?

To introduce the battery preparation steps, we take an example of typical 12 volt battery. The battery manufacturing process involves about 7 stages, as mentioned here :

1. Casting the positive and negative grids. 
2. Applying the active material (pasting) on the grids.
3. Covering the positive plate with microporous separator.
4. Combine the positive and negative plate. 
5. Arrange the plate sets and form plate blocks.
6. Forming cells and filling the acidic solution.
7. Finalizing battery: carrying out initial charge, testing the mechanical and electrical reliability, closing the vents, cleaning and sticking the label and other related cautionary information.

It must be noted that the number of stages in the line of process may differ depending on the manufacturer’s adopted method. The battery manufacturing in line process in detail is as follows :

Stage 1 Casting grids :  Firstly, the positive grids are casted using melting procedure and negative grids from expanded metal respectively. The grid serves the purpose of holding the active material and conduct electricity between active material and the terminal.

Stage 2 Applying active material on grid : Secondly, a layer of lead dioxide as an active material is applied for positive plate. A layer of metallic lead as an active material is applied for negative plate. Here, by means of a Cementation process, the active material gains mechanical (structural) strength and retention properties through the grid and active mass.

The preparation of the active material precursor involves a sequence of mixing and curing process using lead oxide (PbO + Pb), sulfuric acid, and water. Curing is a process that makes the paste into a cohesive, porous mass. This develops a bond between the paste and the grid. At the end of pasting process we get positive and negative plates.

Stage 3 Putting separator : The positive plates are put in separators with a microporous cover. The separators cover each positive plate like an envelope so as to prevent it coming into contact with a negative plate and causing internal short circuits 

Stage 4 Forming Plate Set : Positive plate with microporous separator and negative plate are arranged together to form a single unit plate set.

Stage 5 Forming plate block : Several  plate sets are placed together in a single group. Then all the positive plates in a group are connected together by a small strap which forms positive plate pack. Similarly, all the negative plates in a group are connected together by a small trap which forms a negative plate pack. The number of sets in a group depends on the capacity of unit cell that is to be made. One positive plate pack and one negative plate pack form a unit plate packs in a plate block has cell connectors which act as cell terminals.

Stage 6 Connecting cells to form battery: The plate blocks are placed in a polypropylene monoblock casing consisting of six chambers. Each chamber with a plate block placed vertically forms a unit cell of 2 V. Each cell has one positive and one negative connector (cell connector). As shown in Figure 6.15, the red and yel