dave2001auto

Hi, what should the range of the voltage read when read though the FSS?
What is the voltage for a fresh battery?
What is the voltage for a battery on its last leg?
I'm getting 12.1V from the FSS (door open)
and 12.3V when measured using a Fluke meter from the battery terminals in the trunk (trunk light is on).

dave
2009 e320

dave2001auto

I retested with battery charger on it. Both on FSS and battery are the same now at 12.5V. FSS voltage is reading true.

Is my battery on its last leg?
The charging voltage is 14.2 Volts with 800 to 2000 rpm headlights on.

LemonSqueezy

14.2 is perfectly within range for a healthy battery. If the car is off, low to mid 12s should be good.

jandmtv

Those seem like healthy numbers to me. But I
Make a note to replace the batteries on my mercedes and bmw every 4 years. They just don't seem to last any longer than that.

dave2001auto

Thanks for the reassurances on the battery.
It seems like with the AGM battery, with a low drain it has a fairly high internal resistance, but can build up a large surface charge to fire up the glow plugs and starter.

Anybody know the current draw when getting the info for the FSS? Turning off the FSS and closing the door increased the voltage by 0.2 Volt. Seems like higher amp, high internal restance and/or surface charge.

dave2001auto

After 100 miles of highway driving and parking overnight, the FSS voltage was 12.3 V with the door closed and door lights off. The battery is less than 2 years old but was in AZ with over 100 degrees F temperature that destroys batteries. The AZ MBZ dealer said that batteries usually last on 3 to 4 year in AZ due to the high tempertures. But they still use 5W-30 instead of 5W-40 oil.

mtrevelino

Attached is a interesting chart on the battery voltage. I think someone posted it either on this site or another forum.
Mike T.