The battery needs 12.25V or more to charge. It also takes four hours for a full charge. I'd check first if the alternator is producing more than 12.25V. The backup battery shouldn't discharge much between flights so several flights should fully charge it.I've tried to perform the backup battery test several times, but each time at startup it is showing about 10.5 volts. Last time was right after a 2 hour flight. I'm assuming the battery is NG? Any other things to try?