Disconnect and Reconnect the Battery

Reviewed by the Service Reset editor · Updated August 27, 2026 · How we verify
Quick referenceTools10 mm spannerTime~15 minMethodDisconnect negative terminal first

These instructions apply to the Mercedes C-Class Mk4 (W205) 2014-2023.
For other models, please choose your vehicle here.

Disconnecting the 12 V battery on a Mercedes-Benz C-Class (W205, 2014–2023) is a routine job, but the W205 has intelligent battery management so a new battery must be registered, and the door mirrors may need re-initialising afterwards. On most W205s the battery is in the engine bay, but on some it is in the boot. This guide covers the safe order of work and what the car needs once the leads are back on.

Mercedes C-Class Mk4 — the vehicle covered in this guide.
Mercedes C-Class Mk4. Photo by Dinkun Chen via Wikimedia Commons, licensed CC BY-SA 4.0.

Where the Battery Is

On most W205 models the main battery is in the engine bay; some variants (and cars with the auxiliary battery system) have it under the boot floor. There is also a small auxiliary battery on many W205s — this guide covers the main starter battery. Replace like-for-like (AGM where fitted).

Before You Disconnect

  • Note your radio presets and COMAND preferences.
  • Turn the ignition off and ensure all electrical consumers are off; take the keyless fob away from the car so it powers down.
  • To keep settings alive, plug in a memory-saver. The steps below assume you are not.
  • If working near airbags or seat-belt pretensioners, disconnect the battery first.

Disconnecting — Correct Order

  1. Turn the ignition off and take the fob away.
  2. Expose the battery (lift the engine-bay cover, or the boot floor on boot-mounted cars).
  3. Slacken the negative (−, earth) terminal first and lift the lead clear, tucking it aside.
  4. Only undo the positive (+) terminal if you are removing the battery completely.
  5. If removing the battery, undo the clamp and vent hose and lift it out upright.

Reconnecting — Correct Order

  1. Refit the battery, reconnect the vent hose and secure the clamp.
  2. Connect the positive (+) lead first, then the negative (−) earth lead last. Tighten both firmly.
  3. Turn the ignition on for a few seconds before starting the engine.

What the W205 Needs Afterwards

  • Reset the date and time and re-enter your presets and COMAND preferences.
  • Re-initialise the door mirrors if they will not fold/adjust correctly — cycle the fold function and adjust them through their full range with the mirror switch.
  • Register a new battery (important). If you fitted a new battery, it must be registered with the W205’s power management using a Mercedes-capable diagnostic tool, or the car will mismanage charging and the ECO start/stop will not work.

If a Light Won’t Clear

One or two lights right after reconnecting is normal and should clear on a short drive. If an airbag, engine or chassis light remains, a fault code was logged. Reading it needs a diagnostic tool; you can look up what a Mercedes code means on our sister site autodtcs.com.

Tools You Need

  • Spanner/socket for the terminals and clamp.
  • Optional memory-saver to preserve settings.
  • A Mercedes-capable diagnostic tool to register a new battery.

Registering or Coding the New Battery

Yes. The energy-management system must be told a new battery has been fitted (and its type/capacity if it differs), or it will not charge it correctly. This teach-in is done through the OBD port with a suitable diagnostic tool at a specialist or dealer — there is no owner trick. An AGM battery must be replaced with AGM.

Frequently Asked Questions

Where is the battery on a W205 C-Class?

Usually in the engine bay; some variants have it under the boot floor, plus a separate small auxiliary battery. Check both locations on your car.

Do I have to register a new battery?

Yes — the power management needs the new battery’s parameters or it will charge it incorrectly and disable ECO start/stop. Registration is a quick step with a Mercedes-capable tool.

Why won’t my start/stop work after a battery change?

Because the new battery isn’t registered. Register it and the system returns.

How long should I leave it disconnected to reset the electronics?

Around 15 minutes with the doors shut lets the modules power down fully.

To clear the service reminder after maintenance, see our Mercedes C-Class W205 service indicator reset guide.

Independent guide — not affiliated with Mercedes. Always follow your official service manual and safety precautions. Read our full disclaimer.

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