Originally Posted by
cockney steve
A tired, high resistance "12v" battery will still read ~14V fully charged and will STILL light a few lamps etc. for a few hours with minimal volts-drop (down to ~12.....but bang a starter across it and it WILL curl up it's toes and sink to ~6-8V
Concur 100% - our battery failed in exactly the same manner. It would charge 'normally' even when removed from the aircraft and connected to a charger.
Voltage off load was fine, and even on load it would light up all the lights and spin the instruments at the right speed. Within two or three seconds of using the starter though, it would be - to all intents and purposes - completely dead.