Disconnect and Reconnect the Battery

Disconnecting the 12 V battery on your Smart Fortwo (W451, 2007–2015) — the second-generation city two-seater with its rear-mounted engine — is a job you will meet for a new battery, winter storage, an electrical repair, or before working near the airbags. It is a straightforward task once you know where the battery lives, and taking the terminals off and refitting them in the right order lets the car power back up and relearn its settings cleanly. On the rear-engined Fortwo the 12 V battery is at the front of the car.

Where the 12 V Battery Is

Because the engine is in the rear, the 12 V battery sits at the front, under the front service flap behind the front trim. Open that panel and the battery will be under a cover or clamp. Identify the terminals before you reach in: the negative (−) post is the earth, usually with a black cap or lead, and the positive (+) post is the live one, marked red and normally hidden under an insulating cover.

Some mhd (micro-hybrid) Start/Stop versions also have an additional small battery — the main 12 V starter battery is the one you disconnect for normal work.

First, a Few Checks

• Make sure the ignition is fully off with the key out of the car and away from it.
• Switch everything off: lights, heater, heated screen, radio.
• Note your radio code if the head unit uses one, along with your presets.
• Give the electronics a couple of minutes to power down after switching off before you loosen a terminal.
• Have the correct spanner ready and wear gloves and eye protection.

Disconnecting Safely

1. Switch everything off, key to OFF, and remove it.
2. Close all the doors and wait until the running lights go out, so the car has gone to sleep.
3. Disconnect the negative (−) terminal first, then move the lead clear of the post.
4. Now disconnect the positive (+) terminal and cover or move it clear.
5. Working near airbags or pretensioners? Wait a few minutes after disconnecting before you start.

Reconnecting

Refit in the reverse order — live post first, earth last.

1. Refit the positive (+) terminal first and snug the clamp.
2. Refit the negative (−) terminal last and tighten it.
3. Check both clamps are firm, refit any cover, and give the car a minute before switching on so the modules boot in sequence.

What You’ll Need to Reset Afterwards

Cutting the power clears a few short-term memories. On the W451 Fortwo that typically means:

• Clock and radio — re-enter the time, your presets, and the security code if the head unit asks.
• Power windows — re-initialise the one-touch function by holding the switch fully up at the limit, then fully down and back.
• Service display — this settles after a short drive; note that disconnecting does not reset a due service interval.
• Warning lights — a passing ABS or stability light usually clears itself once you have driven a short way.

If Something Isn’t Right Afterwards

• Recheck that both clamps are tight and squarely seated.
• Take a short, steady drive so the electronics can relearn and self-clear.
• Re-do the window one-touch setup if auto-up or auto-down has stopped working.
• Rule out a genuinely flat or worn-out battery that will not hold charge.
• A persistent light stores a code — look it up on autodtcs.com.

Frequently Asked Questions

Where is the battery on a Smart Fortwo W451?

At the front of the car, under the front service flap behind the front trim — the engine sits in the rear, so the battery is up front.

Which terminal do I take off first?

The negative (−) terminal comes off first and goes back on last; the positive (+) goes on first when reconnecting. That order avoids shorting the live post to the body.

Does the Start/Stop version have two batteries?

The mhd micro-hybrid has an additional small battery, but you disconnect the main 12 V starter battery for normal work — that is the one at the front under the service flap.

What will I need to reset afterwards?

The clock and radio presets (and any code), the one-touch window function, and the service display, which settles on a short drive. Any passing warning light usually self-clears too.

Do I need to recode a new battery?

A like-for-like 12 V battery of the correct size and rating normally just fits with no coding. Match the type the car was built for and let it settle a few minutes before starting.

Can I jump-start it instead of disconnecting?

If you only need to get a flat car going, a jump-start keeps your settings and is fine. Disconnect properly only when the battery genuinely needs isolating — for renewal, storage, or airbag work.

If a warning light stays on after reconnecting, look up the code on autodtcs.com.