These instructions apply to the Mercedes A-Class Mk4 (W177) 2018-Present. For other models, please choose your vehicle here.
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Disconnecting the 12 V battery on a Mercedes A-Class Mk4 (W177, 2018–present) is a straightforward engine-bay job. This guide covers safe disconnection and the note that matters most on a modern Mercedes — registering a new AGM battery so the energy-management system charges it correctly.
Where the Battery Is
The 12 V battery is in the engine compartment, under a cover (most A-Class W177 use an AGM battery). Check your handbook to confirm the exact position for your car. The plug-in A250e also has a high-voltage battery — that’s workshop-only; you only ever work on the 12 V.
Disconnecting
- Turn the engine off, switch everything off, and remove the key (or keep a keyless fob out of range).
- Loosen the negative (−, black) terminal first and lift the lead clear.
- If removing the battery, loosen the positive (+, red) next, then the hold-down.
Airbag/pretensioner safety: if you’ll be near airbags or seat-belt pretensioners, leave the system to discharge for a couple of minutes before going further.
Reconnecting
- Reconnect positive (+) first, then negative (−) last; tighten and refit the cover.
- Reset the clock and check your MBUX/driver settings.
Fitting a New Battery? Register It
Modern Mercedes track battery condition for smart charging and start/stop. A new AGM battery should be registered/coded with a Mercedes-capable diagnostic tool (dealer or specialist) so it charges correctly and ECO start/stop works. A like-for-like AGM will run without it, but registering is recommended; a simple disconnect/reconnect of the same battery doesn’t need it. Always replace AGM with AGM — don’t fit a standard battery.
Frequently Asked Questions
Which terminal first on the A-Class?
Always the negative (−) first, and reconnect it last.
Do I have to code a new battery?
It’s recommended — registering the new AGM with a Mercedes tool keeps the charging and start/stop correct.
Can I fit a normal (non-AGM) battery?
No — replace AGM with AGM. A standard battery won’t cope with the charging strategy and start/stop.
Will I lose my radio?
No — MBUX isn’t code-locked; just reset the clock.
For any warning light that stays on, look up the code on autodtcs.com. To clear the service reminder, see our A-Class W177 service reset guide.
Disclaimer: The information on this page is provided for general guidance only. Always follow your official service manual and safety precautions when working on your vehicle. We are not responsible for errors, omissions, or any damage resulting from the use of this information.
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