You have a car with start-stop, the battery died after less than a year, you went to the nearest shop and bought an "ordinary" battery of the same voltage and capacity. A few months later, same story. It's not your fault, you didn't have bad luck, you didn't get a bad batch. The issue is that a start-stop car needs a specific kind of battery, and there are two on the market: EFB and AGM. Here is how they differ, how to know which one belongs in your car, and the mistakes we see in the workshop every week.
Why an ordinary battery doesn't work for start-stop
A classic lead-acid battery (the one that has gone into every car for decades) is built to start the engine a few times a day, with the alternator slowly topping up what was used. A start-stop system asks something completely different of it. The engine shuts off at every traffic light, in queues, in front of the school, then starts again a few seconds later. That means dozens, even hundreds of starts during a single drive, and all of it while the battery is partially discharged because the AC, radio, heated seats and everything else keep drawing from it while the engine is off.
A classic battery tolerates that regime for six to twelve months and then gives up. Capacity drops, sulphates build up on the plates, and the driver assumes they got a bad battery. They didn't get a bad battery, they got the wrong battery for their type of car.
EFB battery, what it is and which vehicles it suits
EFB stands for Enhanced Flooded Battery. Structurally it is very similar to a classic one: it has a liquid electrolyte (sulphuric acid) and lead plates. The difference is that the plates are thicker, more firmly connected and wrapped in a special mesh that keeps the active material from shedding during the frequent charge and discharge cycles.
An EFB withstands roughly 75,000 to 85,000 starts, which is three to four times more than a classic battery. It is fitted to simpler start-stop systems, most often in smaller city cars and smaller petrol and diesel engines without advanced regenerative systems. If you have, say, a city hatchback with start-stop and no major extra electrical loads, there was probably an EFB in it from the factory.
AGM battery, what it is and when it is mandatory
AGM stands for Absorbent Glass Mat, a battery with the electrolyte absorbed into glass fibre mats. There is no liquid electrolyte sloshing around inside, everything is solid and sealed (which is why this type is also called VRLA, Valve Regulated Lead Acid). AGM tolerates much deeper discharges, accepts current from the alternator faster, and lasts for around 130,000 to 140,000 starts.
AGM is mandatory on vehicles with advanced start-stop systems, especially those with brake energy recuperation (the car charges the battery while you brake or coast). That includes many newer BMW, Mercedes and Audi models, plus plenty of Ford, Volvo, Opel and other models with higher trim levels. If you also have a lot of electrical loads in the boot or under the seat (subwoofer, extra lights, a heater), there is a strong chance the car came with AGM from the factory.
How to tell which type belongs in your car
The most reliable way is to look at the battery currently in the car. Look for the markings AGM, EFB, START-STOP or VRLA, usually printed in large letters on the top label and sometimes on the side. If it says AGM, the new one must be AGM. If it says EFB, you can fit EFB or upgrade to AGM. If it says none of those and the car has start-stop, that does not mean a classic battery is fine, it means someone before you already got it wrong.
Another reliable source is the service book or the catalogue at a serious supplier; by VIN or engine code you can clearly see what the factory fitted. A third option, if you are not sure, is to drop by the workshop and let us read the data from the car and check the original specification.
Important rule: EFB can be replaced with AGM as an upgrade, that is no problem. The other way around is not allowed. If the car came with AGM from the factory and you fit EFB or a classic battery, the car's electronics expect the AGM charging curve (different voltage, different regime) and will ruin the new battery within a few months.
Common mistakes when buying and replacing a battery
The first and biggest mistake is walking into a shop with the attitude "give me something 12 V, 70 Ah". Voltage and capacity are only half the story, the battery type and CCA (Cold Cranking Amperes) must match the factory specification. The second mistake is "upgrading" to higher Ah for no reason, for example fitting 90 Ah instead of the factory 70 Ah because "bigger is better". The alternator and voltage regulator are tuned to the original capacity, an oversized battery will not charge properly and will last shorter than it should.
The third mistake, which we see almost every week, is the driver disconnecting the terminal themselves, fitting the new battery and going home. On most newer vehicles that is not enough. The Battery Management System (BMS) remembers the age and state of the old battery and will keep behaving as if the new one is that same worn-out unit. That means it will not charge the new battery properly and will often disable the start-stop function altogether. That is why after replacement you need to do a BMS registration through a diagnostic tool, so the car "gets to know" the new battery.
What we do at the workshop when changing a battery
The procedure is always the same, no shortcuts. First we check what type the battery in the car is and what the manufacturer requires. Second, we pick a new battery of the same type (EFB or AGM), the same capacity and the same or very close CCA. Third, before removing the old battery we connect a backup power supply so the computer settings, radio codes and module data are not lost. Fourth, after fitting we run a BMS registration through diagnostics so the car charges and manages the new battery properly. Finally we measure the voltage and the alternator's charging output, because if the alternator is not charging correctly, no battery is going to last.
The battery is one of those parts where saving money in the wrong place usually costs you double. If you are not sure which type belongs in your car, or you have already fitted a new one and it is acting up, drop by the workshop and we will check it. Better to sort it out now than to be stuck in front of your building in a few months with a car that won't start.