Home > Electrical Machines > Batteries > Batteries in Photovoltaic Systems – Applications & Maintenance

Batteries DC Circuits Electrical Design Electrical Energy Renewable Energy Solar Power

Batteries in Photovoltaic Systems – Applications & Maintenance

0 36 minutes read

Batteries: Fundamentals, Applications and Maintenance in Solar PV (Photovoltaic) Systems

In a standalone photovoltaic system battery as an electrical energy storage medium plays a very significant and crucial part. It is because in the absence of sunlight the solar PV system won’t be able to store and deliver energy to the load.

During non-sunshine hours we need this stored energy in a standalone system to run our appliances, whereas in a grid-tied system if the grid is operational then the required amount of energy can be provided by the grid in case of low or even non-sunshine hours.

With the advance in technology and the increase in the market, the cost of solar PV modules is decreasing whereas the cost of batteries is becoming a significant part of a standalone system. Non-optimal use of batteries can result in the reduced life of such a significant device in the system.

Thus, here in this article, we are going to see some important practical and technical details of batteries as well as their utilization and maintenance in the solar photovoltaic system.

Battery Fundamentals

Our portable electronic devices like smartphones, smartwatches, laptops, torches, and power banks, etc all these things require some portable supply of energy to use these devices. The conventional AC supply available cannot be used to run such devices hence we need a portable DC supply. So, to provide such a portable DC supply a device having electrical energy stored known as the battery is used.

The charge stored in the battery is utilized to supply the required power to those portable electronic and electrical devices whenever it is necessary. Electrical energy is stored in a form of chemical energy and when required this chemical energy is converted into electrical energy to fulfill the application’s energy demand.

The utilization of the battery is similar to that of the water storage tank, turn the tap on when water is required and turn it off when water is not required, and restore the water when the tank is empty. In battery instead of water the charge is stored and depending on the energy required the charge storage capacity (size of the battery) is determined.

A battery is a two-terminal device, one is the positive terminal and the other is the negative terminal. There is a voltage difference in these two terminals when the battery is charged. It is due to this difference in the terminals that drive the current when it is connected to an electrical or electronic appliance.

In the market, there are different types of batteries available which come in various shapes, sizes, voltage ratings, storage capacities, charge-discharging cycles, shell life, and technologies. There are rechargeable and non-rechargeable batteries.

The batteries used in smartphones, smartwatches, laptops, and torches, etc are of low capacity whereas batteries used in electric vehicles, motors, PV systems, and other renewable energy systems are of high capacity. So, depending on the application a particular type of battery is chosen.

Related Post: Why Battery Rated in Ah (Ampere hour), Not in VA.

Rechargeable Batteries

Just like refilling a storage water tank, a battery is also required to restore the charge in a standalone solar PV system. The charge level of the battery drops as it is utilized for fulfilling the load demand just as in the case of a storage water tank where the water level drops when it is utilized.

As soon as the charge level just like that of water level drops it is required to restore the charge continuously to avoid any disruption in the operation of the system. This process of restoring the charge or refilling the electrical energy in the batteries is known as “Charging”.

In this process of charging the electrical energy is supplied to the batteries where it is converted into chemical energy. The process of consuming the electrical energy from the batteries is known as “Discharging”. In the process of discharging the chemical energy from the batteries is converted into electrical energy which is supplied to the load.

Thus, the battery has a charging and discharging process. One charging and discharging process of a battery are known as one charge cycle of the battery. Some batteries have multiple charge-discharge cycles while some don’t, the batteries with multiple cycles are known as “Rechargeable Batteries”.

Such rechargeable batteries with many cycles are widely applicable in solar PV applications as they ensure the continuity of the power to the load in the presence of low or even no sunlight, without which the implementation of a standalone solar PV system would be very unreliable and difficult.

Related Post: Difference Between a Battery and a Capacitor

Working of a Battery

A battery is a combination of two or more units of voltaic cells (electrochemical cells) that are connected in series or parallel. It deals with electrical and chemical energy hence the name electrochemical cell, it is also termed as simply a “cell”.

Now, this single unit of a voltaic cell or electrochemical cell consists of two half cells as shown in figure 1 below. Each half-cell has an electrode and an electrolyte. This two half-cell which makes a single unit of electrochemical cells are connected electrically to each other by a salt bridge and the electrodes of different metals are used in the two half-cells as shown in figure 1.

A chemical reaction occurs at the metal electrode in each half-cell. Two chemical reactions occur during the cell’s operation; one is an oxidation reaction and the second is a reduction reaction. The process in which the electrons are lost or released is known as oxidation and the process in which the electrons are accepted or gained is known as reduction.

This process of oxidation and reduction happens at the time of charging and discharging of the electrochemical cell. During charging, the oxidation takes place at the cathode terminal (negative terminal) and reduction takes place at the anode terminal (anode terminal) as shown in figure 2.

A charging and discharging process together is called a single cycle. A fully charged battery is when it is filled with charge and it is said to be completely discharged when its charge is completely utilized.

Related Post: Calculation & Design of Solar Photovoltaic Modules & Array

Components of the Battery Cell

As we saw various components of the battery cell such as anode, cathode, salt bridge, etc we will see the functions of these components below:

Anode: It is referred to as positive lead or positive terminal or positive node. This positive electrode gives up the electrons to the external circuit which results in oxidation during the discharging process.

Cathode: It is referred to as negative lead or negative terminal or negative node. This negative electrode gains electrons from the external circuit which results in the reduction of the electrons during the discharging process.

Electrolyte: The electrolyte acts as a conducting medium for the ions between the anode and the cathode. An electrolyte is also referred to as a medium through which the current flows internally in a battery. It is water with other dissolved solvents along with acids, alkalis, and salts.

Salt bridge: Sometimes referred to as a separator. It is a porous material that isolated the two electrodes but yet keeps them connected from each other otherwise the chemical reaction would stop.

Types of Batteries

There are a variety of batteries available in the market and each of them is suitable for a particular application. They are broadly classified as rechargeable and non-rechargeable batteries.

Non-rechargeable Batteries: This type of battery is one time usable i.e. the electrochemical reaction is non-rechargeable batteries is not reversible. Hence, such a battery cannot be charged again. They are popular because easy to use, simple, portable, requires no or even less maintenance, and are available in different sizes and shapes.

The sizes are referred to as A, AA, AAA, C, D, etc, and shapes such are cylindrical, coin, cuboid, etc such are mainly applicable in wristwatches, toys, etc. These batteries are available in a capacity below 20 Ah and can operate in a temperature range of – 40 ^OC to 70^OC. They have high shell life and are cheaper. One of the most commonly known non-rechargeable batteries is the pencil cell i.e. the zinc chloride battery.

There are other such batteries used for different applications such as Aluminum cells, magnesium cells, mercuric oxide cells, etc. The battery which is utilized for the solar PV application requires frequent charge and discharge operation to supply the load demand. Thus, the non-rechargeable batteries are not suitable for Solar PV operation.

Rechargeable Batteries: The batteries which are capable of converting chemical energy into electrical energy and can do the vice-versa process i.e. convert the chemical energy into electrical energy are called are rechargeable batteries.

These batteries are the most widely used in the world for various applications such as automotive power supply, lighting, camera, electric vehicles, smartphones, laptops, and solar PV systems, etc. They are easily manufactured and can store charge in a wide range of capabilities.

Some of the commonly used batteries are Nickel-metal oxide, lead-acid, nickel-cadmium, lithium-ion polymer, etc. The charge storage capacity of the battery is reflected by its physical size. Small size batteries have small storage of charge while large size batteries have high storage of charge.

One of the most commonly used batteries in the solar PV system is the lead-acid battery. They are big as they can store high charge and this is the reason, they are most commonly used in the solar PV system.

Related Post: Types of Batteries and Cells and Their Applications

Battery Parameters

Different parameters of the battery define the characteristics of the battery, which include terminal voltage, charge storage capacity, rate of charge-discharge, battery cost, charge-discharge cycles, etc. so the choice to select batteries for a particular solar PV system application is determined by its various characteristics.

All the above-mentioned parameters affect the performance of the battery and it is important to know and understand these parameters to install, utilize, and maintain a battery pack in a solar PV system. These battery parameters are displayed by the manufacturers and based on this we can select a battery or a set of batteries for our solar PV application. The following are the parameters mentioned by the manufacturers;

Related Post: Parameters of a Solar Cell and Characteristics of a PV Panel

Battery Terminal Voltage

The electrical energy is supplied to the load when it is connected across its terminal and current starts flowing through it. The energy flow to the load can only be possible if there is a voltage difference across the battery terminal.

This voltage difference between the two electrodes of the battery terminal is called the battery terminal voltage. This difference in terminal voltage acts as the driving force for the current. The battery terminal voltage is one of the most important parameters that determine the selection of the battery.

Appropriate battery terminal voltage must be chosen for the application or it might not work, sometimes it requires 3 V, sometimes 6 V, or sometimes even 12 V or higher. Usually, batteries with 6 V and 12 V are available for the solar PV system application.

Now each battery is made up of cells and depending on the material its terminal voltage of the cell is determined. To obtain the required 6 V or 12 V the terminal voltage of the cell is not large enough, many such cells are connected in series to add up and obtain the required voltage level. Six cells are connected to form a 12 V lead-acid battery.

The battery terminal voltage changes with several conditions of the battery, one being during the charging and the discharging. When the battery gets charged its terminal voltage increase and decreases when it is discharged.

Open Circuit Voltage and Terminal Voltage

When the battery is fully charged and no current is flowing then the terminal voltage of the battery is at its maximum and is equivalent to the open-circuit voltage. That is why it is referred to as battery open-circuit voltage V_O or electromotive force V_emf.

The terminal voltage of the battery and the open circuit of the battery is not the same. The terminal voltage of the battery is lower than the open-circuit voltage of the battery when its terminals are connected across the load and current starts flowing through it.

This happens because of the internal resistance of the battery. It is due to this resistance that the voltage drop occurs inside the battery. The voltage drop that occurs due to this internal resistance can be calculated by multiplying the current flowing through the battery by its internal resistance i.e. I × R_i.

Therefore, when the battery is connected to the load and current starts flowing then the actual terminal voltage of the battery will be the difference between the open-circuit voltage and the voltage drop due to internal resistance i.e. I × R_i. This can be written in the following way;

V_Batt = V_O – (I × R_i) ….. (1)

Where,

V_Batt= Battery terminal voltage in Volts.
I = current flowing through the battery in Amperes when it is connected to the load.
V_O = Open-circuit voltage of the battery in Volts.
R_i = Internal resistance of the battery in Ohms.

Let us take an example where a battery is having an internal resistance of 0.8 ohms and an open-circuit voltage of 15 V what is the voltage developed across its terminal when it delivers a current of 5 A, 12 A, and 18 A?

Open-circuit voltage (V_O)	Internal resistance (R_i)	Current (A)	V_Batt = V_O – (I × R_i)
15	0.8	5	11
15	0.8	12	5.4
15	0.8	18	0.6

From the above table, it can be seen that the voltage appearing across the battery V_Batt decreases with the increase in the current so to have a maximum voltage V_Battthe internal resistance of the battery should be as minimum as possible.

Terminologies for Battery Terminal Voltage

Now let us see briefly the various voltage terminologies associated with the battery;

Open-circuit Voltage (V_O): This is the voltage that appears across the battery when it is fully charged and no load is connected for the current to flow. It is the maximum possible voltage across the battery terminal.

Nominal Terminal Voltage: The nominal voltage also sometimes referred to as operation voltage is the voltage that appears across the terminals of the battery at which the load can operate. The standard battery operating/ nominal voltages are 3 V, 6 V, 12 V, and so on.

Cut-off Voltage: It is referred to as the voltage up to which the load can operate and below it, the battery should be disconnected from the load to avoid over-discharge of the battery.

Related Post: How to Design and Install a Solar PV System?

Charge Stored Capacity (Coulomb or Ampere-hour “Ah”)

The capacity to store the charge in the battery is called the charge stored capacity, it is measured in Coulomb or ampere-hour. It represents the amount of current (in amperes) it can deliver for a given time (an hour). When we say that the battery charge stored capacity is 1 Ah, it means charge delivered when a constant current of 1-ampere supplies the load for one hour. Mathematically it can be expressed as follows;

Capacity (C) = Current (A) × Hour (h) ….. (2)

Temperature plays a very crucial role in determining the capacity of a battery. It will have different capacities at different temperatures. The capacity provided by the manufacturer is at standard test condition i.e. at 25 ^oC. hence, one must consider the temperature of the location where the batteries are to be installed to obtain the correct value of capacity according to the location and application.

Now the question lies in how much current a battery can supply? Battery being an energy storage device can supply only a specified fixed amount of current. The amount of current that can be drawn from the battery can be easily determined if we know the duration of discharge time and the charge capacity which is available from the manufacturer. This can be understood from the following equation;

Current (A) = Capacity (Ah) / Discharge Duration (Hours) ….. (3)

It can be very well understood from the above equation that the longer the discharge duration smaller will be the current drawn from the battery and the shorter the discharge duration higher will be the current drawn from the battery.

This can be understood with an example of a storage water tank, if we draw water slowly, we can draw water for a longer period but if we draw water very fast the storage tank will be drained out very soon. So, if our discharge duration happened to be very small then the current drawn will be very high.

This should be avoided as the high current will cause a voltage drop inside the battery due to its internal resistance which will result in reduced voltage available at the battery terminal. Normally in a solar PV system, the discharge duration is high around 15 to 20 hours and so the current drawn will be small. Now let us understand this with an example;

In one case a 200 Ah battery is discharged in 40 hours while in other cases it is discharged in 10 hours. Let’s determine the current drawn in both cases.

Discharge time 40 hours:

Current (A) = Capacity (Ah) / Discharge Duration (Hours)

Current (A) = 200 / 40 = 5 A

Discharge time 10 hours:

Current (A) = Capacity (Ah) / Discharge Duration (Hours)

Current (A) = 200 / 10 = 20 A

Now from this example, it is very clear that during longer discharge time i.e. 40 hours the current drawn is low i.e. 5 A, and during shorter discharge time i.e. 10 hours the current drawn is high i.e. 20 A.

It is now easy to determine the electrical energy stored in the battery once the battery terminal voltage and its charge stored capacity are known. This electrical energy is a product of stored capacity and terminal voltage. It can be given mathematically as;

Energy (Watt-hour) = Capacity (Ah) × Voltage (V) ….. (4)

Thus, with large capacity and higher terminal voltage more electrical energy is stored in the battery. Now what will be the power of the battery, generally power in DC is defined as the product of voltage and current. For batteries, the product of terminal voltage and the current drawn from the battery is the power of the battery. Mathematically it can be written as;

Power (Watt) = Current drawn (A) × Terminal Voltage (V) ….. (5)

Let us now take an example to understand the idea of energy and power in the battery. If we have a 12 V battery with a charge capacity of 400 Ah. What will be its power and the stored energy?

Now let us first assume the discharge duration of the battery say 20 hours, now by using the equation below we can determine the current drawn from the battery.

Current (A) = Capacity (Ah) / Discharge Duration (Hours)

Current (A) = 400 / 20 = 20 A

Now that we know the current drawn for 20 hours, we can now easily determine the power of the battery as follows;

Power (Watt) = Current drawn (A) × Terminal Voltage (V)

Power (Watt) = 20 A × 12 V = 240 W

Also, the energy stored in the battery can be easily calculated as follows;

Energy (Watt-hour) = Capacity (Ah) × Voltage (V)

Energy (Watt-hour) = 400 Ah × 12 V = 2400 Wh or 2.4 KWh

State of Charge (SOC) and Depth of Discharge (DOD)

Practically the total charge stored in the battery cannot be utilized for the load application. Only a certain percentage of stored charge can be used to run the load, this is referred to as the Depth of Discharge (DOD) limit and is measured in percentage %.

If the battery has a DOD limit of 30 % then only 30 % of the stored charge can be utilized for running the load. If the DOD limit is 60 % only 60 % can be utilized if 70 % only 70 % can be utilized if 80 % only 80 % can be utilized for running the load.

Ideally, the DOD limit of the battery should be 100 % and practically it should be as high as possible. A high percentage of DOD limit is preferred for the solar PV application. The percentage of DOD limit varies for different batteries, for li-ion, it is around 85 to 90 % and for lead-acid batteries which are most popular for solar PV applications, it is about 50 %.

Other than the DOD limit the DOD simply can be defined as the level to which a stored charge is used to its total charge capacity of the battery. The terminal voltage of the battery reduces due to the use of its stored charge i.e. increase in DOD. The terminal voltage of a fully charged battery is higher than the fully discharged or partially discharged battery.

The manufacturers specify the maximum allowable DOD limit for their battery and it is recommended not to discharge the battery below the specified DOD limit as it would reduce the battery life quickly.

State of Charge (SOC): To determine the time of charge it is important to know the present charge level of the battery. This state of present charge level is indicated by the State of Charge (SOC) and is represented in percentage.

For example, a 70 % SOC indicates that 70 % of the total stored charge is present in the battery. The DOD can also be used to present the SOC, DOD is an invert of the SOC. If 70 % of SOC is subtracted from 100 % then we get 30 %, this 30 % represents the DOD of the battery. It can be mathematically expressed by using any of the following equation;

DOD in % = 100 % – SOC in % ….. (6)

DOD in % + SOC in % = 100 % ….. (7)

Let us understand with simple example, If the DOD is 80 % then by using any of the above, we can determine SOC in % as;

DOD in % + SOC in % = 100 %

80 % + SOC in % = 100 %

SOC in % = 20 %

As we continue to use the battery the SOC level decreases and the DOD level increases. It is important to keep in note that the battery should not be drained down below the DOD level specified by the manufacturer to avoid a reduction in the life cycle.

It is possible to measure the SOC of the battery by measuring the terminal voltage of the battery. As we know that the SOC of the battery decreases while it’s being used, the terminal voltage of the battery also decreases along with the SOC.

So higher the SOC higher will be its terminal voltage and the lower the SOC the lower will be its terminal voltage. The relationship between the open-circuit voltage and SOC (%) for a lead-acid battery is shown in figure 3 below.

C-rating

When current is drawn from the battery the terminal voltage of the battery is less as compared to when no current is drawn from the battery i.e. open-circuit voltage. This is because the battery voltage drops due to the internal resistance of the battery.

If the current drawn is larger then more will be the drop in the voltage due to its internal resistance and less will be the voltage that appears across the terminal of the battery. The charging and discharging of the battery at a high current rate is not safe.

The amount of the current discharged has an important role in the backup time of the battery. The battery discharges at a faster rate as we increase the load current, over-discharging the battery will result in a reduced life span and mechanical damage. The following figure 4 shows the discharge characteristics of the battery as a function of time.

The manufacturers provide the details of the maximum charging-discharging current and voltage for its long-lasting operation. A battery can be charged by constant voltage, constant current, or both constant voltage and current.

A typical lead-acid battery can be charged by any of the above methods. Proper charging and discharging of the battery will increase its life span. To charge them in an appropriate way a charge controller is used. Now to charge and discharge the battery properly using the charge controller the manufacturers provide the maximum charging and discharging current in terms of C-rating.

This C-rating specifies the amount of time that is to be taken to charge and discharge a given battery. The C-rating can be determined by dividing the storage capacity (Ah) by the number of hours taken to reach a 100 % SOC (i.e. fully charged battery) or time taken to reach the tolerable DOD limit. It can be mathematically expressed as follows;

C-rating (Amperes) = Storage Capacity (Ah) / Hours of fully charged or discharged time (t) ….. (8)

Now let us take some examples to understand the C-rating of the battery. If we have storage capacity C and the time for full charge or discharge is 10 hours then the C-rating can be written as C/10. Consider a battery of 200 Ah storage capacity. What would be its C-rating for 1, 2, 4, 6, 7, 8, 12, 50, and 100 hours of battery charging time?

Then using the equation of C-rating below;

For 1 hour of charging time: