Battery Maintains Weak Charge - Why?
 

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

Hi

It could be that your battery is on its last legs.

But you are right also to suspect the charging circuit and it will pay to check out said charging circuit is performing to spec even if you are going to replace the battery.

John, remove the voltage regulator and check the condition of the brushes.

A quick voltmeter test with the engine running can confirm the soundness of your charging system. 13.5 - 14.4V is the nominal adequate range of voltage with the engine running (accessories turned off). Lower than that, and your alternator is not producing sufficient current to keep your battery charged.

That said, I just replaced my 3 1/2 year old battery on my 300E last winter. The previous one lasted over 5 years...so there is no absolute rule of thumb for battery longevity.

You could swap out your battery with your wife's as a test...;)

when i bought my car 6 months agao i had a battery that would run down every few weeks. i charged it at a local parts store a coupla times, thinking i had an open circuit draining current. thne one day after being stranded by a too weak battery, i decided to check its water level. couldnt see any.

replaced battery, no problems since.