AGM and start-stop battery replacement

Why start-stop cars need a different battery

A conventional battery starts the engine once per journey. A start-stop car restarts it at every red light, and keeps the AC fan, screens and lights running off the battery while the engine is off. That is many times the workload, and a standard battery is not built for it.

AGM and EFB batteries are designed to be discharged and recharged repeatedly. AGM also accepts charge quickly, which cars with brake-energy recovery rely on.

AGM or EFB: which one

The rule is to match what the car was built with, or go one step up.

How to tell what your car has

The label on the existing battery says AGM or EFB if it is one. If the engine switches off by itself at a standstill, the car has start-stop. On many BMW and Mercedes models the battery is in the boot or under a seat rather than under the bonnet.

If you cannot get to it, send us the make, model and year. We confirm the type and size before the technician leaves.

Battery registration on European cars

Many European cars track the age of their battery and adjust charging to suit. When a new battery is fitted, the car needs to be told, or it goes on treating the new battery like the old one. Tell us your model and year when you message, and we confirm whether your car needs registration and how it will be handled before we set off.

Signs an AGM battery is failing

AGM batteries tend to fade rather than die suddenly, and the car often gives the first warning itself:

• Start-stop stops working — the engine no longer cuts out at traffic lights.
• A battery or energy-management message appears on the dashboard.
• Comfort features such as seat heating or the rear screen heater switch off on their own.
• The engine cranks slowly after the car has stood for a day or two.