Disconnect and Reconnect the Battery

Disconnecting the 12 V battery on an Audi A4 (B9/8W, 2015–present) is a job most owners can do on the driveway with a single spanner. You’ll want the battery isolated when you renew it, before any bodywork or electrical work near the wiring, when the car is going into long storage, or to reset an electrical gremlin that a full power-down often clears. The important part isn’t the loosening — it’s doing it in the right order and knowing what the car will ask you to re-teach afterwards. Done properly, the A4 relearns almost everything on its own within a drive or two.

Where the 12 V Battery Is

The 12 V battery on the B9 A4 lives in the engine bay, under a cover near the bulkhead. Lift the cover to expose the two terminals: the negative (−) post carries the earth strap and usually wears a plain black clamp, while the positive (+) post sits under a red or coloured cap. Take a moment to be certain which is which before you touch a spanner — on a 12 V car the danger isn’t a shock, it’s bridging positive to bare metal with the wrench and raising a spark. The mild-hybrid TFSI variants still start from this ordinary 12 V battery; anything to do with a separate high-voltage system is a specialist job, but the 12 V work described here is owner-safe.

Getting Ready

• Make sure the ignition is fully off and the car is not in an accessory state — wait for the dash to go dark.
• Carry the smart key several metres away so the car can’t wake and re-energise the circuits.
• Switch off the lights, climate and any accessories, and shut the doors and boot to stop the car staying awake.
• Note down your radio/security code if you keep one, though the MMI usually reloads without it.
• Give it a couple of minutes for the control modules to power down before you loosen anything.
• Have the correct spanner (typically a 10 mm), plus gloves and eye protection.

Disconnecting

1. Turn the ignition off, take the key clear of the car and wait a minute for the electronics to settle.
2. Slacken and lift the negative (−) terminal first, then tuck the lead well aside so it can’t spring back onto the post.
3. If you’re removing the battery, lift off the positive cap, undo the positive (+) clamp, then release the hold-down bracket and lift the battery out level.

Reconnecting

1. Seat the battery, refit the hold-down, then connect the positive (+) first and tighten it.
2. Connect the negative (−) last, nip both clamps up firmly (not over-tight), and refit the cover.
3. Switch on and reset the clock — the MMI reloads itself over the next minute or so.

A matching, like-for-like battery doesn’t strictly need coding. If you fit a different type or capacity, it should be registered to the battery-management system with an OBD tool so the car charges it correctly — a routine step at most workshops.

What You’ll Need to Reset Afterwards

Cutting power wipes a few learned settings. On the A4 you’ll typically need to:

• Reset the clock and any radio presets; the MMI itself comes back on its own.
• Re-teach the one-touch/anti-pinch windows — see our A4 window guide.
• Expect stop/start to behave conservatively for a drive or two while the battery monitor relearns the state of charge.
• Reset seat/mirror memory presets if they were stored, and let any minor warning lights self-clear over a short drive.

If a Warning Light Stays On or the Car Feels Off

• Check both clamps are tight and clean — a loose earth causes the oddest faults.
• Drive gently for a few miles so the modules can relearn their idle and charge values.
• If the windows still won’t one-touch, run the re-initialise procedure again from a fully closed position.
• Rule out a genuinely tired or flat battery — a weak battery throws random electrical warnings that no reset will fix.
• Read any stored code to see what actually tripped before you chase it further.

Frequently Asked Questions

Where is the 12 V battery on the Audi A4 B9?

It’s in the engine bay, under a cover near the bulkhead. Lift the cover and you’ll find the negative and positive terminals; the positive one usually sits under a coloured cap.

Negative or positive first on the A4?

Disconnect the negative (−) first and reconnect it last. On refit, positive goes on first, negative last. This keeps the spanner away from an easy short to the bodywork.

Does a new Audi A4 battery need coding?

A like-for-like battery generally doesn’t. A different type or capacity should be registered to the battery-management system with an OBD tool so the charging profile matches the new battery.

Will I lose my MMI or radio?

No code is normally needed — the MMI reloads on its own. You’ll just reset the clock and re-enter any radio presets you had saved.

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

It can clear some transient codes, but it won’t reset the service interval — that has its own procedure. See our A4 B9 service reset. A genuine fault will simply return until it’s fixed.

Can I jump-start it instead of disconnecting?

If the goal is just to recover a flat battery, yes — a jump-start avoids the whole disconnect. Only take the battery off when you’re renewing it or doing electrical work.

Any warning light that stays lit after all this? Look the code up on autodtcs.com to see what it means before you spend money on it.