Advice · updated 2026-10-03
AGM versus EFB battery
An AGM battery is replaced with AGM. An EFB can stay an EFB or move up to AGM. An AGM must not be swapped for a weaker battery. We'll fit the right one at the car in London, Birmingham or Manchester.
How the two are built
An EFB is still a flooded battery, with thicker plates so it can cope with stop-start cycling. An AGM holds the acid in a glass mat inside a sealed case. It is built for harder cycling and is common when the battery lives in the boot or under a seat.
Both are stop-start batteries. They are not interchangeable in both directions. The label on the old unit is the starting point, then the vehicle data. A photo of that label is the most useful thing you can add to a quote.
Which way the replacement may go
If the car has an EFB, fit an EFB or upgrade to AGM when the car accepts it. If the car has an AGM, fit an AGM. Fitting an EFB or a flooded battery into an AGM car is a downgrade, and it is refused.
A flooded battery in either car is the wrong part. The engine may still start. Stop-start often will not, and the charging profile can be wrong. Registration, where the car needs it, records the type you actually fitted.
Charging after the choice is made
A healthy 12V battery rests around 12.6V. With the engine running, charging voltage is often about 13.8 to 14.7V, but smart charging can sit lower once the battery is reported full. That behaviour depends on the car knowing which battery is fitted.
The alternator is not a bulk charger. Choosing AGM or EFB and then hoping a drive will finish the job is backwards. The fitter comes to the car in London, Birmingham or Manchester and completes the fit, including registration when required.
Questions before you send a quote
Can I put an EFB in an AGM car?
No. An AGM must not be downgraded.
Can I put an AGM in an EFB car?
Often yes, as an upgrade, when the car accepts it. The callback confirms that.
How do I know which I have?
Read the label. If it is unclear, photograph it and say so on the quote form.
A worked example
A Mini that left the factory on AGM, with the battery in the boot, gets another AGM and registration if that Mini needs it. A small Ford on EFB can stay on EFB or move up to AGM. Neither car gets a plain flooded battery. The choice is confirmed on the callback before the visit.
Safety
Do not jump a cracked, leaking, swollen or frozen battery while you compare types. Leave the leads off and describe the case on the quote form.