Dr Chill

2016 C450 1 month out of warranty with Auxilliary Battery Malfunction Warning that is very annoying. Tried disconnecting plug from negative battery terminal without success in eliminating warning. Before doing that the auto start stop was still working even with the warning present. Car battery was replaced 6 months ago and car is running fine. How can I get rid of the warning without investing in a new voltage converter? Thx in advance

whoover

Lots of systems can start failing as the aux voltage drops. Not a good idea to just ignore it.

Andy74

Do you really want to risk damaging your car to save the cost of an aux battery. You might end paying more if some system went down.

Dr Chill

From what I have read from the forums, the voltage controller mainly retains power for the auto start stop system. What other known functions does it have?

whoover

The division of labor between the main battery and aux "battery" is complicated, but under normal operations the aux battery runs everything but the starter. The logic behind auto start is owned by the aux battery, but the actual turning over of the engine is the main, starter, battery. That said, when the aux battery fails, a relay hands the responsibilities to the main battery for a while, but as the system degrades non-essential consumers are disconnected in priority order. First to go are windshield defrosters and seat warmers. Then things like infotainment. When safety is threatened, the car goes into a sort of limp mode.

Andy74

fabbrisd1

1. With your VIN# call your MB dealer's Part's Dept and get part# and quote for the genuine MB replacement

2. With that Part# - go to Amazon and do a product search - frankly there are China non-OE and non-MB rebuilt's as low as $56 - and on other internet websites new MB as low as $110. Note - there are a LOT of China knock-off, and a LOT of 3rd party supposed "re-builts" in the market, and if you order a "genuine" new MB part make sure you order from a reputable web site

3. There are YouTude videos for the replacement procedure under the passenger front carpet - takes 15min really

Go forward, never go back... keep the beat !

Dr Chill

Bought one from MB dealer wholesale parts website for $126 including core. Thx for making me think straight.

Andy74

Well done. Next step is installation. What is your plan.

Dr Chill

I'll put it in my mother in law's car this weekend.

Andy74

So the car is not yours. Your mother in law is a lucky woman to have you. Digging how to fix her car and save her money.

fabbrisd1

There are YouTube video's for the W205... easy to work that part thru:

3min Step-By-Step video:
Real time 17min replacement video:
$11 Amazon MultiMeter:

Dr Chill

Not saving her money, saving me money. Do I need to disconnect the battery first?

Andy74

See this video it doesn't look he disconnected the main battery.

You Tube

GringoBling

the aux battery is not the stop-start battery

GringoBling

none really, you can disconnect it from the battery ground its a 2 pin harness, and that will defeat the stop-start thing and everything else stays the same.

stockbmw

Also have this disconnected for past year and so far so good. The way I see it, my main battery will be charged without interruption, and starter will be worked way less for $1 a week more on gas.

GringoBling

yup I calculated my MPG "savings" and it was like .5 mpg. Not worth.