2010+ Mercedes-Benz S CL Class W221 Auxiliary Battery Replacement

The most common symptom of a faulty auxiliary battery is a warning message on the dashboard displaying: Auxiliary Battery Malfunction. The early 2006 and 2007 S-Class models have two batteries but no auxiliary battery under the dash. One battery is behind the rear seats; the second is in the engine bay.

This guide also applies to CL-Class W216 from 2007 to 2013. Applicable to models such as CL550 CL63 AMG.

How to Replace the Auxiliary Battery

What you will need

• N000000004039
  • Specs: 12Volt 1.2 Ah Lead Acid Battery, Non-Spillable
• 10mm wrench
• Torx Socket Set
• Dashboard Trim Removal Tool

Procedure

1. Disconnect the negative battery from the main battery. The main battery will be on the engine bay, passenger side, or behind the rear seat (accessible through the trunk).
2. Remove the plastic cover on the driver’s side of the dashboard using a dash removal trim.
3. Remove two Torx screws that hold the fuse box in place. Pull the fuse box out, but do not unplug the electrical connector.
4. Remove the 10mm bolts that hold the auxiliary battery bracket in place.
5. Remove the auxiliary battery from the bracket. Next, unplug the negative and positive wires from the auxiliary battery.
6. Install the auxiliary battery in reverse order.

There is very little space in the dash, so removing the auxiliary battery may be difficult. If that is the case, remove the panel from under the dashboard. This will allow you to access the auxiliary battery from under the dashboard.

Once you replace the auxiliary battery, the Malfunction on the dashboard should turn off immediately. If the new battery was not fully charged, the message may take a few drives to clear.

If the Message Stays After a New Battery

Charge the main battery and test it too; a weak main battery can trigger the same message. If both batteries are good and the warning returns, the auxiliary battery relay is the next suspect. See our auxiliary battery relay K57/2 guide. Reading the stored codes tells you whether the car sees the battery or the relay circuit as the fault; a scanner with Mercedes coverage, such as the YOUCANIC UCAN-II Pro, reads them on supported models.

Main battery and auxiliary battery are not the same job

Worth being clear about which one you are replacing. The auxiliary battery keeps key systems such as the shifter and ignition powered when main battery voltage dips, including during engine stop start on models that have it, and on most models it does not need registering. The main battery does. Mercedes records the age and type of the main battery and adjusts charging voltage accordingly, so a car that has not been told about a replacement keeps charging a new battery on the old one’s profile, which leaves it chronically undercharged and shortens its life by years.

A failing auxiliary battery also produces warning messages that look like main battery faults, which is why people replace the wrong one. Reading the stored codes before buying tells you which battery the car is actually complaining about. A tool that reaches every module, such as YOUCANIC UCAN-II Pro, shows that and performs the battery registration when you come to the main battery.

Frequently Asked Questions

How long does a Mercedes auxiliary battery last?

Usually around four to six years. Because it is small, it often fails before the main battery does.

Can I drive with Auxiliary Battery Malfunction showing?

For a short time, yes, but the backup supply for systems such as the transmission park function is not available. Replace it soon.

Do I need to register the auxiliary battery?

No. The auxiliary battery normally needs no registration, but clear the stored codes after the replacement.

Check these troubleshooting and repair guides for more help on your Mercedes-Benz.

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