7.Batteries in Solar PV System

Type of Batteries

Factors affecting Battery capacity

Temperature

Cold battery - Chemical reaction will be slow and will not be efficient

Hot Battery - Heat aids chemical process but the life of the battery will come down drastically.

Batteries are like Human being the battery should keep in an ideal temp of 25"C like a person's comfortable temperature.Usually the battery will keep in a temperature controlled environment.

Age

As a battery ages, the chemical reactions won't work properly. The active material in the electrolyte will become layered in the plates of the battery

Adding new battery in a battery bank may not be a good idea, old battery will drag the performance of the new one also.If the battery is new and well maintained, you can add a new battery to the bank.

If you add an old battery in a new battery bank, the old one will affect the entire performance of the system.

Discharge Rate of Battery

Theoretical No of hrs a battery could work is the discharge rate of the bettery

Eg 100Ah battery capacity means - 1Amp for 100Hrs (1 Amp X 100Hrs = 100Ah)before its completely discharged.

*

*

*

*

*

High voltage means high state of charge.

A 100Ah battery will deliver 

• More power delivered if 1Amp (will be more than 100Hrs)
• Less power delivered if 100Amp (less than 1Hour)

Usually battery manufacturers will specify C/20 rate 

Specifying Batteries

What is the right battery solution for the system?

For Grid Interactive system - VRLA or sealed maintenance less battery will be  recommended, because the no of charging and deep discharging will be very less. 

For Stand alone system - Flooded led-acid batteries will be specified, because that will last longer and will be deliver more energy in the environment.

How Much my Client will maintain the system?

If the client will not use the maintenance properly the VRLA will be recommended for both the systems.

Specifying Battery Bank Size

This is like buying fuel for one year for your car.

You must think how much miles you drive and what will be the fuel consumption of the vehicle.

Same as like we need to know - Electrical Load in the system we connected, No of Hour of Battery Backup

A stand alone system - You need to measure all the loads
A utility-interactive system (for Inverter and UPS) - only back up loads you need to consider

Sizing the Charging source of the battery system

If the Battery bank is too small compare to system, wont allow the system to charge fully
If the Battery banks is big compare to the charging system, battery bank will not be fully charged, and will suffer in a long run.
Sizing the right batter bank is like selecting the faucet and container for water.

Considering other charging sources

If the secondary source is utility grid, the charger in inverter will be too small, may take more time to charge the system.
If the charging source another renewable energy source like micro wind turbine or micro hydro electric system that will take a lot of time to recharge the battery.
If Secondary source is engine generator, recharging the battery bank is limited to size of generator.

Days of Autonomy

The number of days the the client want to go without needing his battery bank recharged.
Eg.Two days Autonomy - will works without charging for two days.

Try to optimize the capacity of battery bank based on the requirements.

The Depth of discharge of the battery bank

While you are sizing the battery bank you need to consider the DOD
If you consider greater DOD, the battery bank size will be smaller, and lower cycles of charging/discharging
If you consider lower DOD, the battery bank size will be bigger, and higher cycles of charging/discharging
for many batteries you may consider the DOD as 50 to 75%

Surveying no of strings

If you have only one string, the electrons will pass through the same way and make sure that the batteries will charged and discharged in the same way.
If you have more number of strings, properly maintain equal resistance in each path is difficult.
Its better to reduce the number of strings or consider string monitor (will come with UPS and inverter) as an option.

Examining Environmental conditions

If the battery temp is less, the power delivered by the battery will be less than that of a normal battery.

Temperature will play a role in the battery capacity, so its better to maintain the battery in a proper environment. also you may consider a temp sensor connected with the charge controller, this meter will measure the temperature and based on the feedback, the charge controller will deliver proper voltage and current.