Disconnect and Reconnect the Battery

Reviewed by the Service Reset editor · Updated August 22, 2026 · How we verify
Quick referenceTools10 mm spannerTime~15 minMethodDisconnect negative terminal first

These instructions apply to the Ford Fiesta Mk8 (2017-2023).
For other models, please choose your vehicle here.

Disconnecting a battery on the Ford Fiesta Mk8 (2017–2023) is a normal 12 V engine-bay job — but the later 1.0 EcoBoost mHEV versions add a 48 V mild-hybrid battery in a different place, and that one isn’t an owner part. This guide keeps the two apart so you work on the right battery safely.

Ford Fiesta Mk8 — the vehicle covered in this guide.
Ford Fiesta Mk8. Photo by Vauxford via Wikimedia Commons, licensed CC BY-SA 4.0.

Two Batteries on the mHEV — Know Which Is Which

  • 12 V starter battery (every Fiesta) — in the engine bay under a cover. This is the one you disconnect or replace for normal electrical work, flat-battery problems and jump-starts.
  • 48 V lithium battery (mHEV only) — under the front passenger seat, under a carpet flap. Leave it alone: the 48 V system has to be deactivated first, which is workshop territory.

Below, “the battery” means the 12 V in the engine bay.

Disconnecting the 12V Battery

  1. Switch the ignition off and make sure all consumers (lights, climate, radio) are off.
  2. Wait 2 minutes for the modules to power down.
  3. Disconnect the negative/earth (−, black) terminal first and lift the lead clear.
  4. If removing the battery, disconnect the positive (+, red) next, then the hold-down.

Reconnecting and What to Re-set

  1. Reconnect positive (+) first, then earth (−) last; tighten and refit the cover.
  2. Reset the clock/date and re-enter radio presets.
  3. Re-initialise the power windows — one-touch drops out after a power cut. See our Fiesta window initialise guide.

Fitted a New Battery? Reset the BMS

The Fiesta uses a Battery Monitoring System (BMS) for smart charging and Auto Start-Stop. A new 12 V battery should be registered with Ford diagnostic software (dealer or independent with FDRS/Forscan) so it charges correctly and re-enables start/stop. A like-for-like swap runs without it, but start/stop and charging behave better once it’s reset; a simple disconnect/reconnect of the same battery doesn’t need it.

Frequently Asked Questions

Is the Fiesta a 48V mild hybrid?

Only the later 1.0 EcoBoost mHEV versions — they have a 48 V system and a 48 V battery under the passenger seat. Other Fiestas use only the 12 V battery.

Where is the 12V battery?

In the engine bay under a cover — the same place on every Fiesta, mHEV or not.

Which terminal first?

Disconnect negative/earth (−) first and reconnect it last.

Why won’t Auto Start-Stop work after a new battery?

The BMS needs resetting so it trusts the new battery’s charge state; until then (or until it’s fully charged) start/stop may stay off.

For any warning light that stays on, look up the code on autodtcs.com. To clear the oil-service reminder, see our Fiesta Mk8 service reset guide.

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

Related Articles