i HAVE A 69 CHEVELLE CONV THAT WON'T START OFTEN.NO CRANK, IDIOT LIGHTS COME ON BUT THATS IT I HAVE A NEW STARTER, BATTERY AND CABLES AND IGNITION SWITCH.IF I HOOK UP A CHARGER ONLY FOR A SECOND IT WILL START RIGHT AWAY WITH NOT EVEN A COMPLETE TURN ON THE ENGINE. ALMOST LIKE A LOOSE CABLE OR DIRTY BATTERY CONNECTION

try cleaning your grounds. almost sounds like you have too much resistance for the battery and the extra boost from the charger gets it to turn over.

yeah my 72 did the same thing. It was the grounds