Battery System

• Battery cells are the basic building blocks of the battery system.
• Amount Stored in a Battery depends up on the size of the system.
• Battery is grouped with negative and positive plates immersed in an electrolyte(liquid or gel)

Battery Banks:-

• Connect in series to create strings of Battery which allows increase in voltage.(Current remains the same)
• For additional capacity, strings connects in parallel.

Depth of Discharge:-
Amount of Energy drawn from the battery. Mostly its 80% (Means you can remove 80% of the charge from the battery)

State of Charge:-
Opposite of D.O.D. if SOC is 20%, Only 20% charge left with the battery.

SOC + DOD = 100%

The Charging and Discharging cycle depends on
Temperature
How Fast it discharge
Typically the battery manufacturer relate the number with No of cycle withstand against DOD.


Chemistry of Charging and Discharging:-

Pb + PbO2 + 2H2SO4   <--------------------->   2PbSO4 +  H2O

Fully Charged Battery - LED, Led oxide, and Sulfuric acid


Discharged Battery   -Led Sulfate and water.


Charging cycle,
LED sulfate breaks up and lead acid reforms,
water become strong Sulfuric acid
hydrogen gas generated, amount of gas varies,

Theory:- 
In recharging period, the lead sulphate accumulated, on the plate return to Sulfuric acid, but in real life the plates will be corroded.

Best way to prevent corrosion is to follow the charging cycle specified by the manufacturer.
Make sure the charging voltage and current  is proper, these values depends on temperature, battery Technology, and overall battery bank size.



ELECTRICAL USAGE

The first thing you’ll need to know is the amount of energy you’ll be consuming per day. It’s worth the time to do a careful evaluation of exactly what loads (appliances, electronics, etc.) you plan to use and for what lengths of time. Keep track of this information on a loads list; you’ll refer to this list often for sizing other components as well. Your final tally should be expressed in Watt-hours (Wh) per day. If you know the kilowatt hours (kWh) per day just multiply that number by 1,000 to determine the Watt-hours per day. (Example: 1.2 kWh = 1,200 Wh)

DAYS OF AUTONOMY

 Next, you must determine the number of days of battery back-up that you want to have on hand. In other words, if you are unable to charge your deep cycle batteries by any means, and you still need to draw power, you must provide this additional storage by increasing the size of your battery bank. For solar panel powered systems (PV), Days of Autonomy represents the number of cloudy days in a row that might occur and for which you intend to store energy. Consult a weather website, local meteorologist or even long-term area residents.

If you conclude that you need more then five days of battery backup, you may want to explore multiple sources of electricity generation or backup generator options (like a fossil-fueled generator). If your primary electricity source is wind power, determine the number of days when there is little or no wind. This information can be found in the data you’ve collected using your data-logging anemometer. Hydroelectric turbine systems are unique because they usually operate continuously, and therefore do not require extensive storage. If you’re sizing a battery bank to be used in conjunction with an on-demand fuel-powered generator, the number of days of backup will represent the number of days you wish to go without using your generator.

DEPTH OF DISCHARGE

Another factor to consider is the planned (DoD) of your deep cycle battery bank. Flooded lead acid batteries (FLA), sealed AGM batteries and sealed gel batteries are all rated in terms of charge cycles. A single cycle takes a battery from its fully charged state, through discharge (use), then back to full charge via recharging. The depth of discharge is the limit of energy withdrawal to which you will subject the deep cycle battery (or battery bank). DoD is expressed as a percent of total capacity. The further you discharge a battery, the fewer cycles that battery will be capable of completing. Simply stated, deeper discharge shortens battery life.

CALCULATIONS

Once you’ve pinpointed all these variables, it’s time to calculate the size of your battery bank! Let’s go through the steps below, using the following example system: 

• A system load of 6,000 Watt-hours per day
• Three Days of Autonomy (back up) needed
• Planned Depth of Discharge (DoD): 40%
• Battery bank ambient average low temperature 60° F.
• A 48V system