The Sears Die-Hard battery in my '88 260e seems to not be charging fully. If the car sits for a few days the battery will be too weak to start the car. I checked for stray loads but found nothing unusual connected other than the clock (which is normal).

I put a charger on the battery and checked the gravity in each cell thinking maybe a cell was bad but they all measured a weak 1200 (or discharged) on my tester. Checking the battery in my wife's 300e, I got a healthy 1300 on the scale so I know the tester is pretty accurate.

The battery is almost 4 years old and I suppose it could be getting weak but before I replace it, is there anything else that could be the cause, like a bad voltage regulator?

Thanks,

John