Lost power with charged batteries. Charger keeps you going on and off

While driving our golf cart, that had 3/4 power according to the indicator, it suddenly stopped and lost power. The indicator light went up to full charge but we had no power to move. The light stayed on full and occasionally dimmed. After pushing it home, we went to plug in the charger and the charger kept going on and off in fast succession. We unplugged it. Our batteries are due for changing but we never had this happen. It is still dead. What would cause this? Thank you.