When must batteries in an emergency locator transmitter (ELT) be replaced or recharged, if re-chargeable?

After one half of the battery's useful life.

Explanation:

Batteries of the emergency locator transmitter (ELT) must be replaced or recharged after one half of the battery's useful life because if it is exposed to the high temperature for a long period of time such as the air plane parked in the sun will result in the deterioration of battery which may makes the transmitter out of order before the expiry date of the battery. So it will be safe to do that after the use of one half of the battery's life.