EFB, AGM or conventional: which battery does your Start-Stop car need
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If your car has a Start-Stop system, you can't fit just any battery: it needs an EFB or an AGM, never a conventional one. In this guide we explain how they differ, which one your car needs depending on its equipment level, and what happens if you get it wrong — so you get it right first time.
Quick answer: if your car has Start-Stop it needs an EFB or AGM battery, never a conventional one. The golden rule is that you can move up in technology, but never down: if it came with AGM, fit AGM; if it came with EFB, you can fit EFB or AGM. Fitting a conventional battery where an AGM/EFB belonged drains the battery within a few months and deactivates the Start-Stop. In addition, on most of these cars you have to register the new battery with a diagnostic tool.
Cars with a Start-Stop system switch the engine off at every stop and start it again when you release the brake or the clutch. This multiplies the number of starts and forces the battery to power all the electronics (lights, air conditioning, multimedia) with the engine off. A conventional battery is not designed for that work and degrades very quickly. That is why EFB and AGM were created.
The three technologies at a glance
Conventional (SLI)
The traditional one, designed only to start the engine. It does not cope with repeated cycles. For cars without Start-Stop.
EFB
Reinforced flooded, ≈ twice the cycle life. For basic Start-Stop. More affordable than AGM.
AGM
Sealed, made of glass fibre. Maximum durability and fast charging. For Start-Stop with regenerative braking.
The golden rule: you can go up, never down
Comparison: conventional vs EFB vs AGM
| Conventional | EFB | AGM | |
|---|---|---|---|
| Technology | Flooded lead-acid | Reinforced flooded | Glass fibre (sealed) |
| Cycle life | Low | ≈ twice | The highest (≈ 3× or more) |
| Charge acceptance | Normal | Good | Excellent |
| Recommended use | Without Start-Stop | Basic Start-Stop | Start-Stop + regenerative braking |
| Price | £ | ££ | £££ |
What happens if I fit the wrong battery?
Fitting a battery "below" the original has consequences: it runs flat within a few months (it sulphates), the car deactivates the Start-Stop when it detects that the battery is not in good condition, and warning lights on the dashboard and charging problems may appear.
Important: you have to register the new battery
Most cars with Start-Stop have an intelligent battery sensor (IBS/EBS) and charge management controlled by the ECU. When you change the battery, you have to "register" it with a diagnostic tool (specifying type and capacity) so that the car resets its charging strategy and treats the battery as new.
If it is not registered, the system may undercharge (the battery does not last long) or overcharge. It is a step that many garages forget: make sure it is done, especially if you switch technology.
How to choose the right replacement battery
Choose a battery that matches or exceeds the details of the original:
- Technology equal or higher (golden rule).
- Capacity (Ah) equal or greater.
- Starting current (EN / CCA) equal or greater.
- Case dimensions and terminal layout identical (so it fits and the cables reach).
At baterias.com we work with the AGM and EFB ranges from VARTA, EXIDE and Tudor.
Choose your AGM or EFB by brand
These are our Start-Stop ranges from the three leading brands:
AGM range — for Start-Stop with regenerative braking
EFB range — for entry-level Start-Stop
Give us the registration plate or model of your car and we will confirm the exact battery it needs.
See Start-Stop batteriesFrequently asked questions
Can I fit an AGM in a car that had an EFB?
Yes. Moving up from EFB to AGM is correct and gives it more durability. What you cannot do is the opposite: fit an EFB or a conventional battery where an AGM belonged.
Can I fit a conventional battery in a Start-Stop car?
It is not advisable. It will run flat within a few months and the Start-Stop system will deactivate. Always use EFB or AGM according to what was fitted as standard.
Is it compulsory to register the new battery?
On most cars with Start-Stop and an intelligent battery sensor, yes. Without registration, the charge management does not adapt to the new battery and its life is shortened. It is especially important if you switch technology.
What is the difference between EFB and AGM?
EFB is a reinforced flooded battery for basic Start-Stop. AGM is sealed, made of glass fibre, with greater cycle life and better charge acceptance, for Start-Stop with regenerative braking. AGM is superior and more expensive.
How do I know whether my car has AGM or EFB?
It is stated on the original battery's label and usually appears in the vehicle handbook. If in doubt, tell us the registration plate or model and we will confirm it.