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.
