Battery Capacity (Ah)
It is the current-time product a battery can deliver at a given discharge rate, expressed in ampere-hours. The run time is roughly found by dividing the usable capacity by the average current. The usable portion is limited by the depth of discharge the battery chemistry permits.
Example: a 100 Ah lead-acid battery and an average consumption of 10 A. In lead-acid, half the capacity is generally used to avoid loss of life, that is 50 Ah. From this, about 5 hours of operation follows. If the same battery were of lithium chemistry, the usable share would be far higher.
Three factors that spoil the calculation:
- Discharge rate: in a lead-acid battery the capacity is usually given for a 20-hour discharge. At a fast draw, the real capacity stays below that value.
- Temperature: in the cold, the usable capacity falls noticeably.
- Duty time: if the motor does not run continuously, the average current is far lower than the peak current. Find the real average by measuring.
For an energy calculation it is safer to think in Wh: the product of voltage and ampere-hours gives energy directly and makes it easier to compare systems of different voltages.
Context where this term is used: DC Motor Driver Selection Guide