Disconnect and Reconnect the Battery

Renewing a flat battery or disconnecting for other work on your MG HS (2019–present)? The 12 V battery is in the engine bay. Owners disconnect it for battery renewal, bodywork or electrical repairs, long winter storage, or to recover after a jump-start — and done in the right order the HS powers down and relearns its everyday settings cleanly. This applies to both the petrol HS and the HS PHEV (e-HS / Plug-in). On the plug-in hybrid, the only battery you touch is the 12 V one covered here; the high-voltage traction pack and its orange cabling are strictly off-limits and are a workshop task, never a driveway one.

Where the 12 V Battery Is

Open the bonnet and the 12 V battery sits in the engine compartment, usually under a plastic cover that lifts or unclips. Once the cover is off you’ll see the two posts: the negative (−) terminal on a black lead (the chassis earth) and the positive (+) terminal under a red flip-up cap. Identify both before touching anything. On the petrol HS there is no high-voltage system at all. On the HS PHEV the 12 V battery is still the owner-serviceable one that runs the lights, locks and electronics — but keep well clear of the orange high-voltage cables and connectors, which carry a lethal potential and must be left to a qualified workshop.

Before You Begin

• Make sure the car is fully off — on the PHEV, confirm it is not in READY mode — and take the key well away from the vehicle.
• Switch off the lights, climate fan, radio and accessories so nothing is drawing current when you break the connection.
• If the stereo uses a security code, have it (or your handbook) ready so you can re-enter it afterwards.
• Wait a couple of minutes after shutting down so the modules power down before you disturb the terminals.
• Have the right spanner to hand — typically 10 mm — plus gloves and eye protection, and take off any metal jewellery.

This is a common job.

Disconnecting

1. With everything off (and the PHEV confirmed out of READY), the key away and a couple of minutes elapsed, lift off the battery cover.
2. Slacken the negative (−, black) terminal first, lift the clamp clear of the post and tuck the lead aside so it can’t spring back.
3. Only if you’re removing the battery, flip up the red cap and release the positive (+, red) clamp. To simply isolate the car, taking off the negative alone is enough.

Reconnecting

1. Fit the positive (+) clamp first and seat it fully, then refit the negative (−) clamp.
2. Tighten both firmly by hand, refit the red cap and replace the cover.
3. Switch the ignition on, let the dash wake and run its checks, then start (or, on the PHEV, bring it up to READY).

What You’ll Need to Reset Afterwards

Removing power clears a handful of short-term memories that the HS rebuilds quickly:

• Clock and radio — reset the time and station presets, and re-enter the audio security code if prompted.
• Tyre pressures — re-store the TPMS baseline with the tyres at their correct cold pressures.
• Comfort settings — one-touch window behaviour and driver preferences may re-learn over the next few drives.
• Service reminder — if the maintenance message needs clearing after the work, follow the on-screen service menu or your handbook.
• Warning lamps that light straight after reconnection usually self-clear once the car has been driven a short distance.

If a Warning Light Stays On or the Car Feels Off

• Check both clamps are seated and tight — a loose earth is the usual culprit for strange electrical behaviour.
• Take a short drive so the powertrain adaptations can re-learn.
• Re-do the window one-touch relearn if the auto up/down has stopped working.
• If the battery kept going flat, rule out a genuinely weak 12 V battery rather than the disconnect itself.
• If a lamp stays lit, read the stored fault code to see what the car is flagging.

Frequently Asked Questions

Where is the battery on an MG HS?

In the engine bay, usually under a plastic cover, on both the petrol and PHEV. It’s a standard 12 V battery with the negative on a black lead and the positive under a red cap. On the PHEV this 12 V unit is the owner-serviceable one — the orange high-voltage system is separate.

Which terminal comes off first?

Take the negative (−, black) off first when disconnecting, and fit the positive (+, red) first when reconnecting. This order avoids shorting a spanner to earth while the battery is still live.

Is the PHEV safe to work on?

Working on the 12 V battery is fine on the HS PHEV — just confirm the car is out of READY first. Never touch the orange high-voltage cables or connectors; that pack carries a dangerous voltage and is strictly a workshop job.

Do I need to code a new battery on an MG HS?

For a like-for-like replacement, no special coding is needed for normal running. Fit the same type and size, reconnect in order, then re-set the clock, presets and tyre pressures.

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

It may clear some temporary codes and pending warnings, but it won’t fix an underlying fault — if the problem persists the light comes back. The service reminder is a separate counter, cleared through the car’s own service menu, not by pulling the battery.

Can I jump-start the HS instead of disconnecting?

Yes, if the 12 V battery is simply flat and you just want to get going, a jump-start avoids removing anything. Disconnecting is for renewal, electrical work or storage. On the PHEV you still jump the 12 V battery, never the high-voltage side.

Reconnected and still seeing a stubborn warning lamp? Read the stored code on autodtcs.com to see exactly what the car is reporting.