Mercedes-Benz Auxiliary Battery Malfunction & How to Replace It
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“Auxiliary battery malfunction” means a small second battery in your Mercedes, separate from the main starter battery, has failed a self test or is not being charged. Depending on the model it backs up the start stop system, the transmission park lock, the ignition lock, or on older E Class cars the electrohydraulic brakes. The usual fix is a new auxiliary battery, but there are three quite different kinds of auxiliary battery across the range, and on some cars the main battery is the real cause. This guide covers which one you have, where it is, and what to check before buying anything.
At a Glance
- Most common cause: the auxiliary battery has simply worn out. Mercedes service bulletins on several models call for replacing it when the car logs a resistance fault.
- There are three types. A small 1.2 Ah cell, a larger roughly 12 Ah AGM battery, and on the W205 C Class family a capacitor based “voltage converter” module. They are not interchangeable.
- Location depends heavily on the model. Footwell, under the dash, under a seat, in the trunk or in the engine bay. See the table below.
- On the W205 C Class, check the main battery too. A Mercedes bulletin blames premature ageing of the main battery for start stop failures and says to replace it, not the sensor.
- Do not rely on a quick battery tester. Mercedes bulletins for this fault specifically tell technicians not to use a Midtronics test result to decide.
- Cost: about $18 to $107 for the small cell, about $138 to $171 for the 12 Ah battery, and about $148 for the W205 family module, plus labor if you are not doing it yourself.
What the Auxiliary Battery Does
- ECO start stop backup. Keeps the electronics alive while the engine restarts. Mercedes calls it the ECO start stop backup battery on models such as the W222 S Class and W166 ML.
- Transmission park lock. On newer models such as the A Class (177) it powers the park pawl so the car can be put in Park if the main supply fails.
- Ignition lock backup. On the 177, 118 and 247 series it supports the electronic ignition lock.
- Brake backup on the W211 E Class and C219 CLS. It keeps the SBC electrohydraulic brakes working if the charging system or main battery fails.
Which Type Does Your Mercedes Use?
| Type and part number | Models |
|---|---|
| Small 12 V 1.2 Ah cell (N000000004039) | W221 S Class and C216 CL, W164 ML and X164 GL, W251 R Class, A Class 177, CLA 118, GLA and GLB 247 |
| About 12 Ah AGM (A2115410001, A0009829308 superseded by 000982960864) | W211 E Class, R172 SLK, W222 S Class, W166 ML, X166 GL, W463 G Class |
| Capacitor based voltage converter (A2059053414, Hella 205905341480) | W205 C Class (C300, C400, C43, C450, C63), W213 E Class (E300, E43), GLC (GLC300, GLC43), some 2015 to 2018 S Class models |
These are not interchangeable, and fitment can change with build date. Confirm the part number against your VIN with a Mercedes parts counter. Some models have more than one auxiliary unit; owners of the W222 report two in the trunk.
Where It Is, by Model
| Model | Location |
|---|---|
| W211 E Class (2003 to 2009) | Engine bay, right rear corner under the cabin filter housing |
| W221 S Class, C216 CL | Driver side under the dashboard, behind the fuse box. Early 2006 to 2007 cars reportedly do not have one. Replacement guide. |
| W164 ML, X164 GL | Under the front passenger seat, next to the main battery. Replacement guide. |
| W251 R Class | Under the passenger side carpet |
| W212 E Class | Behind the driver side dashboard panel. Some start stop equipped cars also have a unit in the trunk. |
| W205 C Class | Front passenger footwell, under the carpet beneath a black plastic panel. One Mercedes parts site places the C300 unit under the cabin air filter housing instead, so check both. |
| R172 SLK | Behind the driver’s seat. Replacement guide. |
| W166 ML | Rear cargo area under the floor panels (owner reports) |
| W222 S Class | Trunk (owner reports) |
| W213 E Class, X253 GLC, X166 GL | Not confirmed. One W213 owner reports it below the glovebox. |
A note on model names: the GLC SUV is the X253 and the coupe is the C253. There is no “W253”.
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What Mercedes Says: Service Bulletins
| Bulletin | Models | What it says |
|---|---|---|
| LI54.10-P-058326 (2014) | W222 S Class (non AMG, early VINs) | “Additional Battery, Service Required” with code B11C11B (resistance limit exceeded). Replace the ECO start stop backup battery, A 000 982 93 08. Do not run a Midtronics test. |
| LI54.10-P-058943 (2014) | W166 ML, X166 GL, early VINs; G Class model year 2013 | Same fault and remedy, part A0009829308. On the W166 the bracket lug has to be bent up 90 degrees. |
| LI54.10-P-064158 (2016) | A, B, CLA, GLA, GLE (166 facelift) and others | Code B11C11B, but Mercedes said do not replace any parts while the cause was investigated. |
| LI54.10-P-069719 (2019) | A Class 177, CLA 118, GLA and GLB 247 | “Back up battery, service required” after the car sat more than 10 days. Codes B21DC01, B11C113, B11C116. Part N 000000 004039. |
| LI54.10-P-074072 (2021) | W205 C Class built up to June 2020 | Start stop not working. Mercedes blames premature ageing of the main battery and says replace the main battery, not the battery sensor. |
Sources: LI54.10-P-058326, LI54.10-P-058943, LI54.10-P-064158, LI54.10-P-069719, LI54.10-P-074072.
Two practical lessons come out of those bulletins. First, the fault codes on newer cars are stored in the SAM module, not the engine computer, so you need a scan tool that reads the SAM. Second, on some cars the message appears simply because the car sat unused for a long time and the small battery discharged.
Other Causes Besides a Worn Auxiliary Battery
- The main battery. On the W205 C Class in particular, Mercedes’ own bulletin points at the main battery for start stop failures. A tired main battery also drags down the auxiliary battery’s charging.
- The relay or control module that charges it. On the W211 family a battery control module in the trunk switches a relay that charges the auxiliary battery. One documented W211 case had a corroded sensing pin on that module reading the auxiliary battery at about 6.2 volts; a new battery did not clear the fault. Our auxiliary battery relay guide covers the relay.
- A fuse. On the W205 family, if the old unit looks burnt, check fuse 448 (red, 10 amp) in the rear SAM behind the trunk floor.
- Wiring. Recall 16V-211 on 223 cars of the 2016 SLK corrected a backup battery supply wire to the ignition switch that was connected incorrectly at the factory.
Does It Need Coding or a Reset After Replacement?
We found no Mercedes document requiring coding or registration after fitting a new auxiliary battery, and none of the bulletins above include a coding step. On the W211 the warnings typically stop after the next key cycle, and on the W221 the message clears straight away or within a few drives if the new battery arrives partly charged. Stored fault codes may remain in the SAM module’s history until they are cleared with a scan tool.
If the message comes back within days of a new auxiliary battery, the battery was not the problem. Look at the main battery and the charging relay or module.
Can You Drive With the Warning On?
Usually, for a short time, but it depends on what the auxiliary battery does on your car. On models where it backs up the electronic park lock or the ignition lock, a combined main battery problem could leave you unable to put the car in Park or switch it off normally. On the W211 and C219 it backs up the brakes. Get it diagnosed promptly rather than driving on it for weeks.
48 Volt EQ Boost Models Are Different
Cars with the 48 volt mild hybrid system (EQ Boost) use an integrated starter generator in place of the starter and alternator, a small lithium ion 48 volt battery, and a converter feeding the 12 volt system. Start stop restarts are handled by the starter generator. Mercedes has issued software updates for no restart after ECO stop on several of these models, for example LI15.30-P-078376 and LI07.10-P-077327. The 48 volt system must not be worked on live. Do not assume a conventional auxiliary battery diagnosis applies to these cars.
What It Costs
| Part | Price |
|---|---|
| Small 1.2 Ah cell, aftermarket (N000000004039 replacement) | $17.65 to $70.67 |
| Small 1.2 Ah cell, genuine | $106.99 |
| 12 Ah AGM, genuine 000982960864 | $138 to $171.35 |
| Bosch AUX14 12 Ah AGM equivalent | $153.99 |
| W205 family voltage converter, Hella | $147.99 |
Prices from FCP Euro, MBPartsGiant, Home Depot and Interlight in October 2026. Very cheap aftermarket versions of the W205 module exist; their quality is unknown, and on a part that backs up the park lock that matters.
Where a Scan Tool Fits In
The codes that separate a worn auxiliary battery from a main battery or charging fault are stored in the SAM and battery management modules, which a generic OBD2 reader does not reach. Reading them before you buy a battery is the quickest way to avoid replacing the wrong one.
The YOUCANIC UCAN-II Pro reads Mercedes SAM and body module codes, shows battery voltage, and clears stored faults after the repair.
When to Hand It Over
- The car has the 48 volt EQ Boost system.
- The message returns after a new auxiliary battery and a healthy main battery.
- You cannot reach the battery without removing dashboard or seat components you are not comfortable with.
- The car will not go into Park or will not switch off.
Frequently Asked Questions
Is the auxiliary battery the same as the main battery?
No. It is a small separate battery that backs up certain systems so they keep working if main power drops. It is much smaller and is usually in a different location.
Can I replace the auxiliary battery myself?
On most models, yes, with basic hand tools. Check the location for your model above and disconnect the main battery first.
Why does the warning come back with a new battery?
The charging circuit for the auxiliary battery, such as a relay, converter or wiring, may be the real fault. Read the codes before buying another battery.
Related guides: Mercedes-Benz SLK 300 R172 Auxiliary Battery Replacement · Mercedes-Benz Check / Test Battery Voltage with Multimeter · Understanding the Central Gateway Control Unit in a Mercedes-Benz Vehicle · Check Mercedes-Benz Battery Alternator No Tools or Voltmeter
I own a 2016/17 GLA 180 (manual transmission) purchased from the sole agent. During my last visit to the agent for servicing, they informed me that the auxiliary battery was low and that a warning would appear after some time. I noticed that the “Auto Engine Stop” function was not activating as often as before, since the main function of the Aux. battery in my model is to run the feature comforts while the engine is off. I easily located the battery (at the front passenger footwell) and recharged it using a smart charger. Now, the Auto Engine Stop function activates more often, indicating that the Aux. battery has been given a new life. You can try this too! Also, in my model (and similar ones), the frequency of the Auto Stop function activation can serve as a tell-tale sign of the Aux. battery’s health.
I have a right-hand drive 2009 cl coupe and we get the message abs system, not functioning. The car works fine but thought we should replace the auxiliary battery. We have ripped the car apart and cannot locate it! we have tried under the dash on both sides, the boot on all sides, the passenger seat back seat, you name it.
It is a Japanese model exported to New Zealand, and has done 70,000 kms. All batteries have been tested and are OK, but we can’t find the auxiliary battery Location. I have purchased 000000004039 replacement from the USA a genuine Mercedes part.