PeachParts Mercedes-Benz Forum - View Single Post

JHZR2 · #12 12-22-2009, 02:07 PM

All good ideas. Make sure everything is very clean.

most car batteries are 40-80Ah, so you are likely 50-100% charged after 21 hours.

The best way to tell is to apply a small load for a while (chargers are over 12.7V, and thus a "surface charge will form - indicating that your battery is >13V when it is not). Either install the battery to the car and turn on the dome light for a bit or try to run something 12V off the battery for 30 seconds.

At steady state, OCV for a fully charged battery is 12.72V or so. 12V is roughly "dead" if the battery has no load on it.

Id take a battery that shows full charge, and put it someplace where the temperature is consistent. Let it sit for a day or two, and check the voltage. It should not drop - if it is, you may have a slight internal short.

Id also put a multimeter inline with your positive cable (negative hooked to the battery) and check current. This will indicate if there is some sort of small short in the car. The reading shouldnt likely be more than 40mA, but verify that number as I may be incorrect.

Older MB alternators (good ones) are pretty low. With little load, it is easy to be far below 14V. Some battery chemistries want a charge at 14+V. Everything may be "fine" but mismatched enough that with time, degradation and now cold weather, it just does not work right...