AGM vs EFB: the differences, which one your Start-Stop car needs and whether you can swap

What is an EFB battery?

EFB stands for Enhanced Flooded Battery: an improved battery with liquid electrolyte. Inside it is similar to a conventional battery, but with reinforced plates and additives that let it cope with the constant restarts of Start-Stop. It withstands roughly twice as many cycles as a conventional battery. In the Varta line-up it is the Dynamic EFB range.

What is an AGM battery?

AGM stands for Absorbent Glass Mat: the electrolyte is not liquid but absorbed into a glass-fibre mat between the plates. It is sealed, spill-proof and handles constant discharging and recharging far better: it can withstand about three times the cycles of a conventional battery. That's why it is used in the cars that demand the most from their battery. In the Varta line-up it is the Dynamic AGM range.

Differences between AGM, EFB and conventional batteries

For guide prices for each type with fitting included, see car battery replacement in Málaga.

Why does a Start-Stop car need a special battery?

A car with Start-Stop switches the engine off at every stop and starts it again, so in town it can restart dozens of times on a single journey. While the engine is off, the battery keeps powering the lights, air conditioning and electronics. And cars with energy recovery charge the battery in sharp bursts every time you brake. A conventional battery isn't built for that kind of work: it loses capacity very quickly and the car ends up disabling the Start-Stop. That's what EFB and AGM batteries are for.

Can I swap one for the other?

• From EFB to AGM: yes. AGM handles more cycles, so it's an upgrade. On cars with battery management, the change of technology must be registered in the ECU so the car charges it as an AGM.
• From AGM to EFB: no. The car is designed for an AGM: an EFB will last considerably less and the Start-Stop may fail.
• From EFB or AGM to conventional: no. It's the "saving" that ends up costing the most: the battery may not even last a couple of years and the Start-Stop stops working.

How to tell if your car has an AGM or EFB battery

1. Check the label on the current battery. It almost always says "AGM" or "EFB". If it says neither and the car has no Start-Stop, it's most likely a conventional battery.
2. Look at where the battery is. If it's in the boot or under a seat, it's very likely to be AGM: many manufacturers place it away from engine heat.
3. Check the owner's manual under maintenance or battery.
4. Give us your registration number. With it we can tell the exact technology and size for your car, free and with no obligation.

EFB or AGM in the most common models

The rule depends on the system, not the make: basic Start-Stop, EFB; energy recovery or mild hybrid, AGM. The same model can take one or the other depending on the engine, version and year, so treat this list as a guide and always confirm with the battery label or your registration number:

• Usually EFB in their basic Start-Stop versions: Dacia Sandero, Renault Clio, Seat Ibiza, Peugeot 208, Citroën C3 and Opel Corsa.
• Can be EFB or AGM depending on engine and version, with AGM in the energy-recovery or mild-hybrid versions: Volkswagen Golf, Seat León, Nissan Qashqai, Kia Sportage and Ford Puma.
• Hybrids: on the Toyota C-HR and Yaris Cross the 12 V battery is usually AGM. It's the one that one day stops the car switching on even though the big battery is charged.

The Málaga heat and these batteries

Heat shortens the life of all three technologies. Because an AGM is sealed it doesn't lose water through evaporation, but it doesn't like working very hot either, which is why many cars carry it outside the engine bay. In practice, what extends the life of an EFB or AGM most in Málaga is fitting the right technology, registering it when the car requires it and having it tested every autumn from the fourth year on. If you want to know how long each one typically lasts, we explain it in how long does a car battery last.

Registering the new battery

Many Start-Stop cars have a battery management system that adjusts charging according to the age and type of battery. When a new one is fitted it must be registered in the ECU; otherwise the car charges it as if it were the old one and it wears out sooner. We go through it step by step in registering the battery on a Start-Stop car. When we replace the battery at our shop on C/ La Orotava, 100, registration is included for cars that need it.