KT Battery

Battery Capacity

Battery capacity is the total amount of energy stored within a battery. The measurement of capacity is in Ampere-hours (AH). Ampere-hours is the product of the current times the number of hours to total discharge.

Ampere-hours = current X the number of hours total discharge

The capacity is normally compared with a time of 20 hours and a temperature of 20C. There are five factors that influence the capacity of a battery:

• Size - The volume and plate area of a battery increases with capacity
• Temperature - As a battery gets colder, its capacity decreases.
• Rate of discharge - This is the discharge rate, measured in amperes. As the rate goes up, the capacity goes down.
• History - Deep discharging, excessive cycling, age, over charging, under charging, all reduce capacity.

Choosing Battery Capacity

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.

1. First, multiply the average load current by the backup hours of operation you need.
2. Next, add 15% to cover loss of capacity from tolerance and UN-cycled batteries.
3. For every 10C below room temperature (72F) your worst case low temperature is add 10%.
4. 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.
5. Finally, add 40% to provide for an economic life cycle. A battery with 60% of its capacity left is considered worn out.

Here is an Example -- 10 Hours @ 200 MA, average current, worst case temp is 0C

1. 10 hour back-up time at 0.2 Amperes - 2.0 AH
2. 15% from step 2 - 0.3 AH
3. 0C add 20% from step 3 - 0.4 AH
4. Add 10% from step 4 - 0.2 AH
5. add 40% from step 5 - 0.8 AH

Total = 2.0 + 0.3 + 0.4 + 0.2 + 0.8 = 3.7 AH