Disconnect and Reconnect the Battery

Disconnecting the 12 V battery on your Hyundai i20 Mk3 (BC3, 2020–present) — to change it or to safely work on the electrics — is a simple engine-bay job. Owners reach for it when renewing a worn battery, isolating the electrics before wiring or bodywork, laying the car up for storage, or recovering after a flat. Done in the right order the i20 wakes up cleanly; you just re-teach it a couple of things it forgets while the power is off. If your i20 is the 48 V mild-hybrid (mHEV), the same job applies to the main 12 V battery — there’s no separate owner battery to hunt for, and the 48 V system is sealed and self-managing rather than something you disconnect.

Where the Battery Is

The 12 V battery is in the engine bay, under a cover. Even on the 48 V mild-hybrid i20 the main 12 V battery lives here — there’s no separate owner battery to find. Lift the cover and you’ll see the two terminals: negative (−) on the black lead and positive (+) on the red lead, usually under a protective flap. Identify both before you start so you’re certain which one you’re loosening.

Getting Ready

• Make sure the ignition is fully off and lights, climate, doors and boot are shut.
• Keep the key or smart key away from the car so it can’t re-wake a module mid-job.
• Note your radio presets and clock so they’re quick to restore.
• Wait a minute or two after switching off for the control units to power down.
• Have the right spanner (usually a 10 mm nut), gloves and eye protection ready.

Disconnecting

1. Make sure the ignition is off and the key/smart key is away from the car.
2. Loosen the negative (−, black) terminal first and lift the lead clear, then secure it so it can’t spring back onto the post.
3. If you’re removing the battery, loosen the positive (+, red) next, then undo the hold-down and lift the battery out.

Airbag/pretensioner safety: if you’ll be near airbags or seat-belt pretensioners, wait 3 minutes after disconnecting before continuing.

Reconnecting & Re-teaching

1. Refit the battery and hold-down, then reconnect positive (+) first and negative (−) last; tighten both and refit the cover.
2. Reset the date/time and check the trip computer.
3. Re-initialise the power windows — auto up/down may drop out. See our i20 Mk3 window guide.
4. If the service reminder needs clearing after the work, follow our i20 Mk3 service reset guide.

Hyundai doesn’t require a like-for-like 12 V battery to be coded on the i20 — fit it, reconnect in order, reset the clock and re-teach the windows.

If a Warning Light Stays On or the Car Feels Off

• Recheck both terminals are clean and properly tight — a loose clamp is the most common cause of trouble after a disconnect.
• Let the car relearn over a gentle 10–15 minute drive; several lights self-clear once systems see normal running.
• If one-touch windows still won’t work, run the window re-initialise again — it’s easy to miss a step.
• If it was hard to start or died again quickly, the battery itself may be flat or worn out.
• Any code that won’t clear on its own can be read and looked up so you know what you’re dealing with.
• Give it a few key cycles — some trip and computer data repopulates over the first drive rather than instantly.

Frequently Asked Questions

Where is the 12 V battery on the i20 Mk3?

In the engine bay, under a cover. On the 48 V mild-hybrid it’s the same main 12 V battery in the same place — there’s no separate owner battery elsewhere in the car.

Which terminal do I disconnect first?

Always the negative (−) first, and reconnect it last. That order stops a slipped spanner bridging the positive post to the bodywork and causing a short.

Will I lose my radio?

No — the i20’s audio isn’t code-locked, so it comes straight back. You’ll just need to reset the clock and re-enter your station presets after reconnecting.

Why won’t auto-up work afterwards?

Cutting power clears the windows’ learned top position, so one-touch and anti-pinch drop out. Run the quick re-initialise from our window guide and full auto up/down comes back.

Does a new battery need registering on the i20?

No — a like-for-like 12 V battery needs no coding or registration. Fit it, reconnect in the right order, reset the clock and re-teach the windows, and you’re done.

Can I jump-start it instead of disconnecting?

If the battery is just flat you can jump-start the i20 from the 12 V terminals rather than pull the battery. But a battery that keeps going flat is worn out, and disconnecting to renew it is the proper fix. After a jump-start, a decent drive lets the alternator recharge it; if it won’t hold that charge overnight, the battery has reached the end of its life.

For any warning light that stays on after the job, look up the code on autodtcs.com so you know exactly what it’s telling you.