In this post, we are going to learn about what is battery and its primary functions and also other important aspects of batteries in solar applications. 

What is Battery?

In simple terms, a battery is a device that can be used to power electronics and electrical devices.

In scientific terms, a battery is an electrochemical device that converts chemical energy into electrical energy.

The battery is made up of one or more electrochemical cells with external connections, positive and negative terminal provided to power electrical devices such as watches, mobile phones, laptops, and even electric vehicles.

Types of Battery

Batteries can be classified mainly into 2 types:

  1. Primary batteries
  2. Secondary batteries

Primary batteries

Primary batteries are commonly called non-rechargeable batteries. They can store and deliver electrical energy but cannot be recharged. Primary batteries are not used in Solar PV applications as they cannot be recharged.

Primary Battery - Alkaline Battery

Typical Alkaline batteries, carbon-zinc, and Lithium batteries are primary batteries.

Secondary batteries

Secondary batteries are commonly called rechargeable batteries. They can store, deliver electric energy and can be recharged again by passing an electric current.

Secondary Batteries- Li-ion batteries

Secondary batteries are best suited for Solar PV applications as they can be recharged again and again until the life cycle of the batteries comes to an end.

The commonly used secondary batteries are lead-acid, Nickel-cadmium (NiCd), Nickel Metal Hydride(NiMH), Lithium-ion (Li-ion).

We can discuss the differences, advantages, and disadvantages of the different types of secondary batteries in another post. 

3 Most important functions of the batteries in solar applications

Note that the most important, but poorly understood component next to the PV module in the solar power system is the battery. It’s time that we learn about the primary functions of batteries used in the Solar PV(Standalone) System.

3 Main functions of batteries are:

  • Energy Storage Capacity and Autonomy.
  • Voltage and Current stabilization
  • Supply Surge Currents

Energy storage Capacity and Autonomy

What is Battery capacity?

It is the capacity to store electrical energy produced by the PV array and supply to the electrical loads as needed or on-demand. 

The important function of batteries in stand-alone PV systems is to allow the electrical loads to operate when there is not enough electrical output supply from the PV array.

Another important aspect of a stand-alone PV System is to have autonomy.

The capacity of the stand-alone system to operate the electric loads directly from the battery without any energy supply from the PV module. 

This autonomy can be attained by proper design of the battery storage capacity for solar PV applications.

Storage of electrical energy in batteries is required to operate electrical loads at night time or during extended periods of cloudy or overcast weather.

During the design and sizing of the stand-alone PV system, the battery storage capacity is generally calculated and sized to meet the average daily electrical load for a specified number of days(Autonomy days) without the energy supply from PV modules.

The more the number of Autonomy period, the larger the battery capacity required for a given demand.

  • Less critical PV applications: 2 to 6 days of Autonomy.
  • Critical loads such as public safety: 10 to 15 days of Autonomy.

Voltage and Current stabilization

The ability of the electric power storage system to supply power to electrical loads at stable voltages and currents, by suppressing transients that may occur in the Power System.

A PV array can operate at an infinite number of operating points between the short-circuit current and open-circuit voltage.

When electrical loads are directly connected to a PV Array( for e.g: Solar Water Pump ), the load impedance decides the operational voltage of the PV array, which may not be suitable to operate the load at its prescribed conditions.

With the help of batteries, we can ensure that the PV array is operated near its maximum power voltage.

In Solar PV systems, Batteries acts as a buffer between the PV array and electrical loads in which the load power oscillates or changes to time.

Supply Surge Currents

The Surge current means the current that is 5 to 10 times the normal running or operating level of the load.

Supply Surge Currents means, it is the ability of a power storage system to deliver high peak operating currents to electrical loads for a short duration of time.

While the PV array may be large enough to supply the total energy needed by a load over a day, it may not be large enough to meet the momentary surge power demand by a load at any particular time.

A battery can supply hundreds of amperes for a short duration of surge requirements of the load and the discharged battery will be recharged again by the Solar PV array at lower currents during the day.

Examples of common loads that require momentary surge currents are electric motors. These motors can be found in Compressors, Power tools and other motor loads.

Battery specifications of Solar Photovoltaic Applications

Please note since automobile batteries are meant for delivering high current for a short time and the battery is then quickly recharged, these types of batteries should be avoided in Solar PV applications.

On the other hand, PV batteries operate differently and they should deliver power over a long period and be recharged slowly. So never use Automobile batteries for Solar Photovoltaic applications. 

In this context, we must know the various battery specifications for Solar PV applications.

Battery capacity

It is a measure of a battery’s ability to store or deliver electrical energy, commonly expressed in units of ampere-hours(Ah).

An Ampere-hour is equal to the transfer of one ampere of current over one hour. For example, a battery that delivers 5 amps of current for 20hrs is said to have delivered 100 ampere-hours.

Capacity is generally specified at a discharge rate, or over a certain period.

In a Lead-acid battery, the capacity of a battery depends on several constructional factors like the quantity of active material, the number and physical dimensions of the plates, and the electrolyte specific gravity.

We should also know that battery capacity also depends on the operational factors like the discharge rate, depth of discharge, cut-off voltage, temperature and cycle history of the battery.

Sometimes a battery’s capacity is also indicated as energy storage capacity. It is expressed in Kilowatt-hours(KWh), which can be approximated by multiplying the rated capacity in ampere-hours by the nominal battery voltage and dividing the product by 1000.

For Example :

A nominal 12V, 100Ah battery has an energy storage capacity of (12 * 100)/1000 = 1.2 Kilowatt-hours.

Cold temperature reduces the Capacity:

Battery capacity goes down as the temperature goes down. This is because the chemical reactions at cold temperatures are slower and less active materials can be accessed and converted when cold.

Fast Discharging Reduces Capacity:

As batteries are discharged and reaction products like water are produced. During fast discharges,  the reaction products get in the way of fresh electrolyte, so capacity is limited. 

In contrast, if the battery discharges slowly, the fresh electrolyte can more efficiently penetrate the plates and more capacity is made available for discharging.

Battery Cycle

It refers to the process of charging and discharging a battery. A discharge to a given Depth of Discharge followed by a complete recharge is considered as one cycle.

Battery discharge is the process that occurs when a battery delivers current, quantified by the discharge current rate. 

Charging is the process when a battery receives or accepts current, quantified by the charge current or rate.

The discharge can be very small or shallow, or it can be very severe or deep. All batteries can be cycled, but the question is how deeply and how many times before a permanent loss of capacity occurs.

Battery life depends upon how deep and how many times the battery is cycled. Solar PV systems need deep cycling batteries!

One common use of the battery is called “float service”. In an application like UPS, the battery storage is not regularly used but is available on a standby basis for emergencies like loss of main utility power. 

Here batteries are trickle charged at a small rate. When they are needed, they are discharged relatively quickly, but then gradually and fully recharged from utility power over many days. Then they float again for weeks or months. 

Battery manufacturers often claim 20 years of usable life for batteries in float service. This, however, is NOT the way, the battery in PV applications will be used!

You should know that the batteries used in photovoltaic applications will definitely be subjected to cycling daily, and perhaps also deeply cycled. So useful service life in PV “cycling service” will be less(5-10 years).

Charge/Discharge rates:

The rate of charge or discharge of a battery is expressed as a ratio of the nominal battery capacity to the charge or discharge time in hours.

 For example, a 100 Ah battery is discharging at the rate of 2 Amps. The time to completely discharge a fully charged battery at this rate would be the capacity divided by the current.

100Ah/2 Amps = 50 hrs. 

So you can say that the battery is discharging at the “50-hour rate” or “C/50”.

Discharge rate in Amperes = C/T    (C= Capacity in Amp/hours, T= Time in hours)

The above notation is helpful because it allows you to express relatives’ rates of battery charge and discharge without referring to the exact size of a battery.

For example, most manufacturers recommend charging their batteries no faster than the C/5 rate to limit gassing and overcharge. This means 20 amps for a 100Ah battery, and 100 Amps for a 500 Ah battery. 

Moderate charge rates are around C/20 or C/30, while trickle charging at C/100 will hardly produce any gassing at all in most batteries.

Batteries used in typical Solar PV systems experience very low rates of charge and discharge compared to industrial applications.

 For example, the maximum charge rates from the PV array to the battery are commonly about C/40, and typically discharge rates supplied to the load may be as low as C/100 to C/200. 

In contrast to this, a typically industrial forklift battery might be discharged in one 8-hour shift.

Depth of discharge(DOD)

The depth of discharge(DOD) of a battery is defined as the percentage of capacity that has been withdrawn from a battery compared to the fully charged capacity.

 By definition, the depth of discharge and state of charge(SOC) of a battery add to 100 percent. 

In PV systems it is usually mentioned either as the allowable or maximum DOD or the average daily DOD.

Maintaining a state of charge(SOC) at 80% is allowing a 20% depth of discharge(DOD).

Don’t get confused between the two!

Maximum DOD:

It is the maximum percentage of full-rated capacity that can be withdrawn from a battery. It is a maximum discharge limit for a battery, generally dictated by the cut off voltage and discharge rate. 

In stand-alone PV systems, the low voltage load disconnect(LVD) set point of the battery charge controller dictates the maximum DOD limit at a given discharge rate.

Average daily DOD:

It is the percentage of the full-rated capacity that is withdrawn from a battery with the average load profile.

 If the load varies seasonally, for example in an SPV lighting system, the average daily DOD will be greater in the winter months due to the long operation period of nightly loads.

Factors Affecting Battery Capacity in Solar PV Systems

Let’s see what factors influence the choice and performance of a solar battery in PV Systems. 

Fact: The Battery bank is typically the second most expensive component in most photovoltaic systems, next to the PV Array.

For a life cycle of twenty to thirty years of PV systems, the replacement and maintenance costs for batteries may be the highest.

For the above reasons, PV plant designers should have a good understanding of the factors affecting battery capacity.

As we know Capacity is a measure of a battery’s ability to store or deliver electrical energy, commonly expressed in units of Ah(ampere-hour).

The factors affecting the battery capacity are classified into 2 main factors

    • Design factors
    • Operational factors

Design factors 

The design factors that affect battery capacity are:

    • Quantity of active material.
    • Number, design and physical dimensions of the plate.
    • Electrolyte Specific gravity

Operational factors:

Operational factors affecting battery capacity :

    • Discharge rate
    • Depth of discharge.
    • Cut-off voltage
    • Temperature
    • Age
    • Charge cycle

 Let’s see more in details :

Effects of Temperature on  Battery Capacity:

Battery manufacturers usually rate capacity at a temperature of 25 degrees Celsius.

Cold temperature decreases the total capacity available from the battery. Conversely, a battery-operated at temperatures greater than 25 degrees Celsius will deliver more than the rated capacity.

So let’s take a closer look:

In cold temperatures during discharge,  the electrolyte does not penetrate as deeply into the active material on the plates, and the cut-off voltage is reached sooner.

PV system designers must be aware of the effects of temperature on battery capacity.

If the battery size in a PV system is calculated based on the expected capacity at 25 degrees Celsius, the battery may be too small to provide the necessary autonomy period during cold temperatures.

As a result, the battery could be severely discharged and thus making the system not provide the required power to the electrical loads.

Therefore additional battery capacity must be installed in off-grid/stand-alone PV power systems to compensate for the expected reduction in capacity at lower temperatures.

As we said, the battery-operated at a temperature greater than 25 degrees Celsius will deliver more than the rated capacity.

 However, under no circumstances should the battery be heated or operated at elevated temperatures to increase the available capacity.

Most battery manufacturers recommend their batteries be operated in temperature ranges of between 20 degrees and 30 degrees Celsius.

We need to apply the above data during our battery sizing calculations to ensure that even during the coldest times, the PV systems have the required capacity to supply the loads.

When we calculate the amount of capacity we need to give the days of autonomy desired, the calculated value must be increased by an appropriate factor if the battery is operated below 20 degrees Celsius.

Cut off Voltage Affects Battery Capacity:

The cut-off voltage is the lowest voltage in which a battery system is allowed to reach in operation, and defines the battery capacity at a specific discharge rate.

The battery manufacturers often rate capacity to a specific cut off, or end of discharge voltage at a defined discharge rate.

For Lead-acid batteries, the cut off voltage used to rate capacity is generally 1.75 volts per cell or 10.5 volts for a nominal 12V battery. The cut-off voltage for nickel-cadmium cell is typically 1.0 volt.

Note: 

The cut off voltage defined by a battery manufacturer most often represents a fully discharged battery.

Batteries used in the Solar PV system should never be allowed to reach low of a cut off voltage and should be generally limited to no more than 80% depth of Discharge as determined by the low voltage disconnect point of the battery charge controller.

Effects of Self Discharge rate on Battery Capacity

In open-circuit mode without any load or charging, a battery undergoes a natural reduction in the state of charge, due to internal mechanisms and losses within a battery.

Different battery types have different rates, the most significant factor being the active materials and grid alloying elements used in the design.

Higher temperatures result in results in higher discharge rates, particularly for lead-antimony deigns.

Discharge Rate Affect Battery Capacity

In a lead-acid battery, when a battery is discharged, the lead sulfate and water reaction products are formed. During fast discharge, the reaction is confined to the layers of the active material that are in immediate contact with the free electrolyte, limiting the cell capacity.

Due to insufficient time for the electrolyte to diffuse into the pores of the plates and the sulfate molecules forming at the surface,  clogs the pores, preventing full use of all active material. This effect is amplified as the rate of discharge increases.

During fast discharges, the final cut off voltage is reached sooner and less total capacity is usable. So battery capacity is not a fixed value but depends on the rate of discharge.

Discharging a battery slowly delivers more capacity from the battery, but discharging it quickly delivers a less total capacity of the battery.

And get this :

The above effect is not permanent. If a battery is fully discharged at a fast rate, it only delivers a fraction of its rated capacity. 

If it is then fully recharged and it is discharged at a slower rate, then more capacity will be available. However, a few cycles may need to be performed on the battery to achieve stable capacities at the new rate of discharges.

Details of on Lead acid-acid battery and other battery technologies will be discussed in other blog posts.

 

References: Wikipedia.org