Disconnect and Reconnect the Battery

Taking the 12 V battery off a Peugeot 308 Mk3 (P5, 2021–present) is a simple under-bonnet task. You might do it to fit a fresh battery, before bodywork or electrical repairs, to prevent a slow drain over weeks of storage, or to recover the car after a flat. Done in the right order — negative off first, positive on first — the 308 wakes up cleanly and relearns the few settings it loses. This guide walks through the safe sequence, then lists exactly what the car needs re-taught once power is back.

Where the 12 V Battery Is

The 12 V battery lives in the engine bay under a cover. Lift the bonnet, prop it, and look for a plastic cover you can unclip or lift away to expose the two terminals. The negative (−) post carries the black earth lead and usually a minus symbol; the positive (+) post is under a red cap or flap and carries the main feed. Petrol (PureTech) and diesel (BlueHDi) 308s are conventional cars with no high-voltage system. The plug-in hybrid 308 also keeps its 12 V battery here, but leave its orange high-voltage components strictly to a specialist — nothing in this job asks you to go anywhere near them.

Getting Ready

• Switch the ignition fully off, take the key clear of the car, and make sure it hasn’t left the car in an awake state.
• Turn everything off — lights, climate, wipers, radio — so nothing is drawing current when you break the connection.
• Note anything you’ll want back: radio presets and any security code, plus your seat and mirror positions.
• Wait a minute or two after switching off so the modules go to sleep before you touch a terminal.
• Have the correct spanner ready (typically a 10 mm), and wear gloves and eye protection.

Disconnecting

1. Turn the ignition off, take the key clear and wait a minute.
2. Undo the negative (−) terminal first, slacken the clamp nut and lift the lead off, then move it aside so it can’t spring back onto the post.
3. To remove the battery, undo the positive (+) next, then release the hold-down clamp and lift the battery out keeping it upright.

Reconnecting

1. Connect positive (+) first, seat it fully and nip up the clamp, then fit the negative (−) last and tighten it. Refit the cover.
2. Switch on and reset the clock and date; the i-Cockpit screen reloads itself over the first minute or so.
3. Re-teach the one-touch windows — see our 308 window guide; re-initialise the sunroof too if fitted.

A matching like-for-like 12 V battery doesn’t need coding on the 308 — fit it, reconnect in order, and re-teach the windows.

What You’ll Need to Reset Afterwards

Cutting the power clears a handful of learned settings rather than anything permanent. On the 308 that usually means the clock and date, your radio presets, and the one-touch/anti-pinch windows, which stop auto-closing until re-taught. If your car has a sunroof, re-initialise it as well. Seat and mirror memory may need resetting, and a couple of driver-assist or tyre-pressure warnings can light briefly and then self-clear once the car has settled and been driven a short distance.

If the Dash Still Warns

• Check both clamps are properly tight — a loose earth is the most common cause of odd electrical behaviour afterwards.
• Take a short drive so the car can relearn and let any transient warnings clear.
• Re-run the window re-initialise if the one-touch or anti-pinch function still isn’t working.
• If a light persists, rule out a genuinely tired or flat battery, and read any stored fault code to see what the car is actually reporting.

Frequently Asked Questions

Where is the 12 V battery on a Peugeot 308 Mk3?

In the engine bay, under a plastic cover. Lift the bonnet, remove the cover and you’ll see the negative (−) and positive (+) terminals. The plug-in hybrid keeps its 12 V battery in the same place.

Negative or positive first on the 308?

Negative (−) comes off first when disconnecting and goes on last when reconnecting. On reconnection you fit the positive (+) first, then the negative last, and tighten both clamps.

Does a new 308 battery need coding?

A like-for-like replacement doesn’t need coding on the 308. Just reconnect in the right order, reset the clock and re-teach the windows.

Will disconnecting the battery clear fault codes or the service light?

It may clear some transient codes, but it isn’t a reliable way to reset the service reminder — use the proper procedure for that. A genuine fault will simply set its code again.

Why won’t my windows auto-close now?

The power cut wiped their learned travel, so the one-touch and anti-pinch feature drops out until you re-initialise the windows. It only takes a moment to restore.

Can I jump-start the 308 instead?

Yes — if the battery is just flat, a jump-start to the 12 V battery wakes the car and you avoid removing anything. Disconnecting is for renewal, storage or repair work rather than a one-off flat battery.

Any warning light that stays lit? Look it up on autodtcs.com. To clear the service message, see our 308 Mk3 service reset.