First, verify that the backup battery is empty (i.e. the voltage of the backup battery is < 1 V). During this measurement the main battery must be disconnected and the phone must be switched OFF!
Then connect the main battery and switch the phone ON. Monitor the voltage of the backup battery in order to verify that the voltage will rise. After a few minutes switch the phone off and disconnect the main battery and measure that the backup battery is charged. Typical behaviour of the backup battery voltage during charge and discharge is described in the following pictures as a function of time.
If the voltage rises and falls quickly, battery is dead. If the voltage stays 0V, check resistance VBACK<-> GND: If there is no short circuit, Retu is faulty. Replace Retu.
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You can now recondition your old batteries at home and bring them back to 100 percent of their working condition. This guide will enable you to revive All NiCd batteries regardless of brand and battery volt. It will give you the required information on how to re-energize and revive your NiCd batteries through the RVD process, charging method and charging guidelines.