RED light with battery visit workshop on 2003 Mercedes-Benz E500

Recently every time I start my car for the first 15 minutes on the dash in red(picture of the battery) underneath visit workshop. Stop at the mechanics and they check voltage of battery and alternator both check out fine...At 15 minutes of driving light goes out...Any ideas all comments are welcome I really want this fixed. Also, if you know of any reccommended repair shops in delaware county, pa...Thanks

by in Lansdowne, PA on July 11, 2010
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4 answers 2 comments
ANSWER by , August 24, 2010
It's not the alternator! The E500 has an auxillary battery that is dedicated to the SBC system, unfortunately it goes out at about every 2 years. Average cost is ~$189.00 unfortunately it is only sold at the MB dealer. The auxillary battery is located in the Engine compartment whereas the regular battery is in the trunk.
COMMENT by , November 30, 2010
ANSWER by , July 11, 2010
It sounds like an internal alternator problem. Older Bosch alternators used to make the "Battery warning light" glow or remain on when the regulator (part of the alternator) was starting to fail. Perhaps check Mercedes specific forums on line and see if anyone else has experienced this problem in case it is a common fault with the instrument cluster or other such problem.
ANSWER by , June 19, 2011
The small battery under the cabin filter needs to be replaced it is not holding the charge. After 15 min of driving the battery has enough charge to make the warning go away, until it loses the charge again when the engine if off and not charging .
COMMENT by , January 28, 2012
So is the axillary battery located in the engine (under the hood) or is it on the passenger side under the floor heat vent where the cabin air filter is? thank you
ANSWER by , November 08, 2015
I have an 2003 E500 and it did this once after being parked for several days it also was sitting on the garage floor when the key was turned dash lit up red said vehicle to low do not drive then red battery symbol and see repair shop. I charged the battery for a day and checked the air bag fittings with soapy water and found a slow leak under the rear of the car at the differential on a block of 3 air lines one loose fitting. No problems in either area it's been 8 months now