Choosing Battery Capacity

[SphinX]

Salam,

For Sealed Lead Acid Battery

Specifying battery capacity involves a bit more than multiplying the load current by the backup time in hours. You must first de-rate the battery for capacity tolerance, temperature, and discharge rate.


First, multiply the average load current by the backup hours of operation you need.

Next, add 15% to cover loss of capacity from tolerance and UN-cycled batteries.

For every 10C (16F) below room temperature (72F) your worst case low temperature is add 10%.

If your back-up time is less than 20 hours, add 10% for every time you have to double your back-up time to equal more than 20 hours.

An example may help:
20 minutes would have to be doubled 6 times to equal more than 20 hours. Thus you would have to add 60% on to your required capacity.
Finally, add 40% to provide for an economic life cycle. A battery with 60% of its capacity left is considered worn out.


Example
10 Hours @ 200 MA, average current, worst case temp is 0C
10 hour back-up time at .2 Amperes 2.0 AH
15% from step 2 .3 AH
0C add 20% from step 3 .4 AH
Add 10% from step 4 .2 AH
add 40% from step 5 .8 AH

Total 3.7 AH

Bye

 

[otuzsubat]

There is a battery bank sizing worksheet at
h**p://www.nooutage.com/battery.htm

Also, similar worksheets are present.