My battery sometimes holds a charge, sometimes doesn't. The charging system works fine - plenty of voltage to charge the battery, and it does.

I checked the battery voltage tonight - 10 volts. It couldn't even snap the solenoid, let alone crank the engine. I put the charger on it for about a minute before (for grins), I take it off and check the voltage - it read 12.5 volts. Mind you, the charger didn't indicate full charge.

I'm thinking I've got a bad cell in the old (5 years old) battery. After fully charging it another day, I held my meter on it while the voltage steadily dropped several volts in a few minutes (it seemed).

I've got the charger on it now. I'll let it charge for an hour or so and check it again.

 

I would say bad cell.

 

I replaced the battery with a new one, and no more problems.