C350e auxiliary battery malfunction

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JoeD007

New Member
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Mar 1, 2021
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Location
Leeds
Car
C350e

C350e Auxiliary Battery location

Hi I have 2016 c350e that has just shown the message auxiliary battery malfunction. It came on the following day but today it hasn't appeared? As this is the hybrid version does it have an actual battery or is it a voltage converter? Mercedes garage want to charge £140 to diagnosis. Any help...
 
It has a large AGM 12V battery at the back of the car, left side, under the boot floor.

Here you go, a video on how to replace it:

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It also has another, smaller, 12V lithium battery lurking somewhere in the passenger compartment which could be the one the error referred to?. ePC calls this smaller one the Auxiliary battery, so that sounds favourite.

Item 200 on this page:

 
It has a large AGM 12V battery at the back of the car, left side, under the boot floor.

Here you go, a video on how to replace it:

To view this content we will need your consent to set third party cookies.
For more detailed information, see our cookies page.

It also has another, smaller, 12V lithium battery lurking somewhere in the passenger compartment which could be the one the error referred to?. ePC calls this smaller one the Auxiliary battery, so that sounds favourite.

Item 200 on this page:

Thanks for this. Do you know if the auxiliary battery recharges itself as the message didn't appear this morning? I have driven the car the past 2 days
 
I don't know...but I can't imagine that the 12V batteries wouldn't be maintained from the HV battery under all normal circumstances. Bear in mind the car doesn't have a conventional alternator and relies on the HV battery to run the inverter that generates the 12V. If you left the car with a low HV state of charge, it might have decided to let the 12V take it's chances for a bit?? Really don't know, just guessing.

I'd be inclined to charge the HV battery then just keep an eye on it for a bit. Or just drive it a while. Not so easy these days though :(
 
I don't know...but I can't imagine that the 12V batteries wouldn't be maintained from the HV battery under all normal circumstances. Bear in mind the car doesn't have a conventional alternator and relies on the HV battery to run the inverter that generates the 12V. If you left the car with a low HV state of charge, it might have decided to let the 12V take it's chances for a bit?? Really don't know, just guessing.

I'd be inclined to charge the HV battery then just keep an eye on it for a bit. Or just drive it a while. Not so easy these days though :(

All Mercedes no full-electric cars have AUX battery these days. You can expect them last 4-8 years. When it starts to weaken the first thing you will see is the AUX Battery malfunction warning at the start. In the beginning, it goes away with longer drives and a turn off - on cycle.

The final solution is to have this battery replaced. It will cust you 200-300 at a MB dealer, which you can bring down to 50-70 if you are handy enough to replace it yourself (very easy fix).
 
Even full EV cars have a 12v system supported by a conventional 12v battery. In time, most manufacturers will move to 48v I think, but for now, as all the systems are based on 12v I think it’ll be a while.

I can’t see even EVs ever doing away with a low voltage system of some sort, with battery, as there is always a requirement to be able to isolate the. HV battery while maintaining the operation of safety systems.
 
All Mercedes no full-electric cars have AUX battery these days. You can expect them last 4-8 years. When it starts to weaken the first thing you will see is the AUX Battery malfunction warning at the start. In the beginning, it goes away with longer drives and a turn off - on cycle.

The final solution is to have this battery replaced. It will cust you 200-300 at a MB dealer, which you can bring down to 50-70 if you are handy enough to replace it yourself (very easy fix).

Hm. I had bought a 350e over 6 weeks ago, I had the AUX warning and got it fixed by the trader. I don't the warning anymore, however the battery charges fully and it says it car can go up to 14 miles by using electric.
 
Hm. I had bought a 350e over 6 weeks ago, I had the AUX warning and got it fixed by the trader. I don't the warning anymore, however the battery charges fully and it says it car can go up to 14 miles by using electric.
The AUX battery and your main hybrid battery has very little to do with each other. AUX is a small (perhaps 2 litre sized) one while your main (most possible 48V) battery is the one below and behind the rear seats in the trunk.
Your range is defined by the health of this large battery and the driving conditions: AC on, other comfort stuff is on, (recent) travel speed, etc. The 14 miles sounds quite low. So you may have to have the large (HV) battery checked. If the total charge capacity is below the acceptable value (say 80% or so) you may try to go for a replacement at your seller. Be prepared for an uneasy discussion as it costs way above 5k.
 
14 miles on a full HV charge from a C350e is pretty much the best you’ll get in the real,world. I can’t recall the guess-O-meter on mine ever offering 14 miles even from new. I see 13 a lot though.

Again, specific to the C350e, the ‘main’ system battery is 12V and is indeed under the boot floor. I recently discovered that the aux battery isn’t a battery at all, it’s a super capacitor apparently, and is found lurking behind the carpets etc up under the dash, passenger side.
 
The AUX battery and your main hybrid battery has very little to do with each other. AUX is a small (perhaps 2 litre sized) one while your main (most possible 48V) battery is the one below and behind the rear seats in the trunk.
Your range is defined by the health of this large battery and the driving conditions: AC on, other comfort stuff is on, (recent) travel speed, etc. The 14 miles sounds quite low. So you may have to have the large (HV) battery checked. If the total charge capacity is below the acceptable value (say 80% or so) you may try to go for a replacement at your seller. Be prepared for an uneasy discussion as it costs way above 5k.
Interesting, I've read a few forums and most of them said that they get around 14 miles of electric.
 
Interesting, I've read a few forums and most of them said that they get around 14 miles of electric.
Many people indeed on the forum say 14miles is pretty much good on second C350e since when the car is brand new gives max 19miles.
 

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