Auxiliary Battery for Start-Stop System: Location Guide for Popular Vehicles

Modern vehicles equipped with automatic start-stop systems often have a secondary (auxiliary) battery in addition to the main battery. This auxiliary battery is critical for operating start-stop features, powering accessories when the engine is off, and maintaining smooth operation of electronics during frequent restarts. Knowing its location is important for diagnosis, replacement, or troubleshooting start-stop system warnings. Below is a detailed guide to auxiliary battery locations in common vehicles, with a special focus on Mercedes-Benz and other popular brands.

Why Is There an Auxiliary Battery?

The main reason is that the start-stop system requires extra power during engine-off periods and restarts. The main battery cranks the engine, while the auxiliary battery:

• Keeps electronic systems (lights, radio, sensors, computers) powered when the engine stops
• Supports relays and control units to prevent voltage drops during frequent start-stop events
• Acts as a backup if the main battery voltage is too low for start-stop operation

If your car displays messages like “Start-Stop Not Available — Battery Low” or “Auxiliary Battery Malfunction,” you may need to check or replace the auxiliary battery.

Mercedes-Benz: Auxiliary Battery Locations by Model

C-Class (W205, 2015–2021)

• The auxiliary battery is located in the passenger-side footwell, underneath the carpet, behind a plastic access panel. To reach it:
  1. Move the passenger seat fully back and up.
  2. Lift the floor mat and pull back the carpet from the firewall.
  3. Remove the small plastic cover to reveal the battery (usually a small black or gray AGM unit).

E-Class (W213, 2017+)

• On most E-Class models, the auxiliary battery is located in the trunk (boot), behind a side trim panel, or in a compartment under the trunk floor next to the main battery. Check the right side of the trunk or under the floor cover for a small battery (smaller than the main one).

GLC, GLE, GLS SUVs

• GLC (X253): Usually in the passenger-side footwell, same as C-Class.
• GLE/GLS (W166, W167): Typically located under the front passenger seat, beneath a plastic cover. Slide the seat all the way back and look for an access panel.

S-Class (W222, W223)

• The auxiliary battery is often found in the trunk, usually next to or near the main battery under the trunk floor cover.

Older Mercedes (E-Class W212, ML/GLE W166, etc.)

• Many have the auxiliary battery in the engine bay, on the passenger side near the firewall, or inside the cabin (footwell or under seat). Always consult your owner’s manual for the exact location.

Other Brands: Auxiliary Battery Locations

• Jeep Grand Cherokee: Most modern models with start-stop (2016+) have a small auxiliary battery next to the main battery under the hood, sometimes beneath a black cover or near the passenger-side fender.
• Ford F-150: Typically uses a large AGM main battery; most models do not have a separate auxiliary battery for start-stop, but some hybrids do (often under rear seats or in the bed).
• BMW: Many models place the auxiliary battery in the trunk, right side, behind a trim panel or with the main battery.
• Audi: On some models, in the trunk or under the rear seat.

How to Identify and Replace the Auxiliary Battery

1. Consult your owner’s manual for the exact location and battery type.
2. Look for a small, sealed AGM battery (often 7–12Ah capacity, much smaller than the main battery).
3. Replacement usually requires basic hand tools—disconnect the negative terminal first, then the positive, and reverse the process for installation.
4. On some Mercedes and Jeep models, after replacing the battery, the start-stop or auxiliary battery warning may clear automatically, or after a few drive cycles. If not, you may need to clear codes with a diagnostic tool.

Common Symptoms of Auxiliary Battery Failure

• “Auxiliary Battery Malfunction” or “Start-Stop Not Available” warnings
• Start-stop system not working
• Radio, lights, or electronics flicker when the engine shuts off at stops
• Persistent low battery warnings even after main battery replacement

Summary

The auxiliary battery is a small but crucial component in vehicles with start-stop systems, usually found in the passenger footwell, trunk, or under the hood depending on your car’s make and model. If you have a start-stop or battery warning, check both the main and auxiliary batteries and replace as needed. For exact replacement steps and battery types, consult your owner’s manual or let me know your specific vehicle model and year for tailored instructions.