Mercedes W205 & W213 Auxiliary Battery Location (with DIY Access Guide)

Many modern Mercedes-Benz models—including the W205 (C-Class 2015–2021) and W213 (E-Class 2017–present)—use an auxiliary battery in addition to the main starter battery. If you see warnings like “Auxiliary Battery Malfunction,” knowing the exact location and how to access this battery is key for fast diagnosis and DIY replacement. This guide is specifically targeted for the Mercedes-Benz W205 and W213 chassis (and similar models).

What is the Auxiliary Battery?

• Purpose: The auxiliary battery provides backup power to electronic systems such as Start/Stop, emergency gear selector, power steering lock, and other “convenience” features if the main battery voltage drops or during engine stop/start events.
• Type: On W205/W213, it may look like a small 12V battery (AGM or gel cell), or it may be a “voltage converter” (capacitor-based), but both serve the same function.

Where is the Auxiliary Battery Located? (W205 & W213)

• W205 (C-Class 2015–2021):
  • The auxiliary battery is located inside the cabin—specifically, in the front passenger footwell, behind a plastic panel under the glove box and behind the carpet trim.
  • It is not in the trunk or engine bay (unlike main batteries on some Mercedes models).
• W213 (E-Class 2017–present):
  • Uses a very similar setup—auxiliary battery is located under the passenger-side footwell panel, just like the W205.
  • For both chassis, you’ll need to remove interior trim to access it.

Step-by-Step: How to Access & Replace the Auxiliary Battery (W205/W213)

1. Move Passenger Seat Back:
   
   Fully slide the front passenger seat backwards for maximum access to the footwell.
2. Remove Floor Mat & Plastic Footwell Panel:
   
   Take out the passenger-side floor mat. Carefully pry up the plastic door-sill trim (runs along the bottom edge of the door opening)—use a trim tool or hands.
3. Pull Back Carpet:
   
   Gently pull the carpet in the footwell upward to reveal the lower side of the dashboard and firewall.
4. Unscrew the Under-Glovebox Panel:
   
   Under the glove box, find a plastic trim panel held by a few screws (often Torx or Phillips). Remove these screws. On some cars, you may also need to gently unplug the footwell light or temperature sensor connector attached to this panel.
5. Locate the Auxiliary Battery Unit:
   
   With the panel removed, you’ll see a small black/silver battery (or voltage converter) mounted near the firewall or above the passenger footwell fuse box. It is held in place by a small metal bracket or bolt.
6. Disconnect & Replace the Auxiliary Battery:
   
   • Unclip or unscrew the battery connector(s). Remove the retaining bracket.
   • Swap in the new auxiliary battery (make sure it matches the original type and voltage).
   • Reconnect the wiring and bracket. Ensure everything is secure and nothing is pinched by the trim.
7. Reinstall All Panels:
   
   Reverse the steps above: attach the under-glovebox trim, press the carpet back in place, snap on the door sill trim, and reinstall the floor mat.
8. Start the Car:
   
   Start your Mercedes. The “Auxiliary Battery Malfunction” warning should clear automatically after a short drive or several ignition cycles.

Tips and Additional Notes

• Always disconnect the main (starter) battery before touching any battery wiring to avoid accidental short-circuits or error codes.
• Wear gloves and eye protection—car batteries can vent acid if damaged.
• Don’t forget to plug in the temperature sensor or light if you disconnect any wiring in the footwell panel.
• The auxiliary battery is a common wear item—replacement interval is usually 4–7 years, depending on driving and climate.
• If the warning light persists after replacement, scan the car for error codes or perform a “hard reset” by disconnecting both main and auxiliary batteries for 10–15 minutes.

Summary Table: Auxiliary Battery Location

Chassis Code	Auxiliary Battery Location
W205 (C-Class)	Passenger footwell, behind lower dash/carpet panel under glove box
W213 (E-Class)	Passenger footwell, behind lower dash/carpet panel under glove box

Conclusion

On Mercedes W205 and W213 models, the auxiliary battery is easy to find and replace—located behind the front passenger footwell panel under the glove box. With basic tools and patience, you can access and swap this small but important battery yourself, clearing error messages and restoring all comfort and safety features to your Mercedes.