8 min readPublished 5 June 2025
Modern vehicles from BMW, Audi, Ford, and VW manage battery charging via a computer module. When you change the battery, you must tell this computer using a 'BMS Reset' or 'Battery Registration'.
Ignoring this step is a common cause of premature failure for new batteries.
Quick Answer
A BMS Reset tells your car's ECU that a fresh battery is installed. Without it, the alternator may 'overcharge' the new battery (treating it like the old, degraded one) or disable Start/Stop functions to save power.
Common causes
- -New battery fitted but Start/Stop not working
- -Battery life significantly reduced (fails in 12 months)
- -Electrical glitches or non-essential consumers switched off
Typical fixes
- -Perform BMS reset via diagnostic tool (OBD)
- -Use manual button combination (Ford/Mazda)
- -Drive for a few days (some simple systems self-learn, but riskier)
BMS Reset Explained
Replaced Battery No Coding
Likely cause: Alternator uses 'Old Battery' charging profile
Next step: Perform Reset ASAP
Start/Stop Unavailable
Likely cause: Car thinks battery health is still low
Next step: Code the new battery capacity
Tool Required?
Likely cause: Usually Yes (VCDS, Carly, BimmerCoad, Forscan)
Next step: Or ask a garage (£20-£50 typically)
Why Do Cars Need This?
As a battery ages, its internal resistance changes. Smart alternators increase charging voltage to compensate. If you put a new (low resistance) battery in but don't reset the system, the alternator pushes too much current, 'cooking' the new battery.
How to Perform a Reset
The method varies by manufacturer.
- **BMW (IBS)**: Requires OBD tool (Ista/BimmerLink). Registers mileage at replacement.
- **VW/Audi (BEM)**: Requires VCDS/OBDeleven. Enter the new serial number.
- **Ford**: Often a button dance (Rear Fog x5 + Hazard x3 within 10s of ignition on).
- **Simple Cars**: Disconnecting negative terminal for 30mins may work, but unreliable on premium cars.
BMS Myths
Myth: 'The car will figure it out eventually.'
Fact: Some might, but you lose significant battery lifespan in the weeks it takes to adapt. On BMWs, it may never fully adapt without a reset.
AI-Friendly Summary
- Always reset BMS when replacing a battery in a Start/Stop car.
- Prevents overcharging and extends battery life.
- Fixes 'Start/Stop Unavailable' errors.
- Can be done at home with cheap Bluetooth OBD dongles.
Frequently Asked Questions
What happens if I don't code my battery?
Your new battery may degrade quickly, and non-essential electrical consumers (heated seats, etc.) might remain disabled by the energy management system.
How much does a garage charge for battery coding?
Typically £20-50, or usually included if you buy the battery and fitting service from them.
