General Choosing the ampere-hour (Ah) capacity of a battery involves several trade-offs. If size, weight, and cost are of no concern, then bigger is always better. However, most applications require a minimum size battery. The main factors affecting the final decision are: current demands on the battery, allowable recharging time, and charger capabilities.

Battery Capacity Calculations Following these four steps yields a minimum battery size that's right for the application.

Step 1 Calculate the total ampere-hour draw-down from the battery by the load during the time between battery charging cycles.

(Ah = average current draw X total time)

Step 2 Most battery manufacturers consider a battery to be at the end of its service life when its present capacity has decreased to 80% of its original capacity. Therefore, multiply the Ah value calculated in step 1 by 1.25.

Step 3 Most sealed lead-acid batteries lose 50% of their capacity at -40C. If a sealed battery is to be discharged while cold, multiply the Ah value calculated in step 2 using the following formula. If the ambient temperature is always 23C or higher, skip this step.

DeltaTemp = 23C - Cold Ambient Temperature(C)
Note: If the cold temperature is negative, you would add the temperature's value to 23C. So, 23C -(-40C) is the same as 23C +40C = a DeltaTemp of 63C

Ah rating = (Ah step 2) X ((DeltaTemp X .0159) + 1)

If the charger to be used is a standard or enhanced 3-mode type, skip step 4, as the battery will be 100% charged when the charger enters the float mode.

Step 4 When charging a battery with a 2-mode charger, the battery is not necessarily 100% charged when the charger switches from constant-current (bulk) mode to constant-voltage (float) mode. The level of charge is normally in the range of 85% to 95%. The remaining charge will be delivered to the battery slowly.

Because the state of charge is from 85% to 95% at mode switchover, the battery should be oversized by 5% to 18% to compensate for this. This works because the same number of ampere-hours can be drawn from an 18% oversized battery that is charged to 85%, as can be drawn from a non-oversized battery that is charged to 100%.

Based on the battery charge rate, obtain the %recharged number from the following table and plug it into the formula to determine the exact size needed.

At C/3 charge rate --> 75% recharged (use .75)
At C/5 rate --> 85% (use .85)
At C/10 rate --> 90% (use .90)
At C/20 rate --> 95% (use .95)
At C/100 rate --> 100% (use 1.0)

Ah rating = (Ah step 3) divided by (%charged at switchover point)

Conclusion It may seem from the preceding four steps that the required Ah rating of a battery can quickly become unreasonable. However, going through the numbers shows that, at room temperature, a 50% increase above the initially calculated (step 1) Ah rating is usually sufficient.