CAPACITY

A battery’s capacity determines roughly how long a battery will last at a specific voltage given a specific discharge rate. For example, if you choose a 12V, 2Ah (2000mAh) battery pack (regardless of chemistry), the battery should be able to run a 12V motor consuming 2A continuously for 1 hour. Alternatively, it can run a 12V motor consuming 1A for 2 hours, or a 12V motor consuming 0.5A for 4 hours. The rule of thumb is to divide the capacity (assuming you are running an actuator at the same voltage) by the actuator’s current under normal load to get the time the motor will last.

Example 1 2x Drive Motors: 6V nominal, 1A each under normal load

1x 6V NiMh Battery Pack, 2200mAh (equivalent to 2.2Ah)

Note that the battery was chosen based on the motor’s nominal voltage.Should you instead operate 6V motors from a 7.2V battery, the calculations become more difficult (use the total watt-hours divided by the total watts per hour to get an idea).

Therefore the 6V battery pack will last:

2.2Ah battery / (2 motors x 1A per motor) = 1.1 hours

Example 2 18 servos used for a hexapod robot which operate at 6V nominal and consume 250mA under normal load*

1x 6V NiMh battery pack at 5Ah.

First, we will assume that all motors are under load at all times (i.e. worst case scenario) and therefore all 18 will be consuming a total of 4.5A

5Ah battery / 4.5A = 1.1 hours

Note again that the battery was chosen based on the motor’s nominal voltage.

[feature] This tutorial has been borrowed from Coleman Benson's post at Robot Shop [/feature]