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I started getting low voltage codes all over the car. The rear battery, after a one hour trip, had 12.4 volts. Front battery showed 13.6. I replaced the rear battery but it is not charging. However , the front one is. Any ideas where to start?
 

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konigstiger, Thank you for that reply. I checked both batteries at rest and the voltage was good. I started the engine and the front battery dropped to 12.4v and stayed there for a few minutes. The rear battery appeared to be charging at 14.4v.( I charged the rear battery all night after it showed 12.4v last night after a short trip.) After a few minutes the front battery charged quickly to 14.4.
I measured the rear battery at the front terminal and also at the battery itself. Both readings were within 1/10th of a volt. I have a check engine light on that does not want to clear. However, different , unrelated codes come up each time I check the OBD. Does the alternator throw a code? If not, what procedure, other than the fact that is charging, should I check?

konigstier, I appreciate your info and I am sure many others do also. I read the 2 battery onboard pdf that you supplied, but not sure how and what to test further.

OF not, a friend of mine has a GL that exibited the same problem. He changed out the alternator and it still continued. He noticed a large voltage drop in the front battery ( which checked good ) after starting. He replaced the front batteyr and the problem went away. Could it be?
 

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You could replace front battery and may very well need to. Are you using MB diagnostics if not, I suggest having someone pull codes for definitive CEL answer.
 

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Konigstiger, I use an iCarsoft MB 2.0. A bit more tedious than a laptop with good software but it works for most items.

I picked up a new V4 front battery today. Advanced Auto for $135.00. ( many thanks to wampa007 ) I have it on the Ctek charger to top it off before I put it in the car tomorrow. Will report back.

Of interest, my indy shop said the starter relay can go bad and draw down the front battery? This is an interesting problem. But one thing I found is that it is worth figuring out. The S600 is one of the most enjoyable vehicles I have ever owned.

Much thanks.
 

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I replaced the Front V4 battery and all seems to be well. No codes and both batteries have good voltage. The check engine light was unrelated. It was due to unmetered air? I checked under the hood and had a worn/ chewed through 3/8" elbow with a hole in it. All is well with the S600 again. Thanks for the help.
 

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I replaced the Front V4 battery and all seems to be well. No codes and both batteries have good voltage. The check engine light was unrelated. It was due to unmetered air? I checked under the hood and had a worn/ chewed through 3/8" elbow with a hole in it. All is well with the S600 again. Thanks for the help.
There is a module responsible for distributing the charging current to each battery as they need them, and monitors the voltage for each one. As as it monitors batteries, it directs the alternator current based on the one that needs it, and is a percentage of the current and not all of it. If the problem repeats, you may need to have the BCM software updated or recalibrated to the new battery(s).
 
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