Start-stop systems arrived on ordinary cars around fifteen years ago, driven by tightening emissions and fuel consumption rules. The idea is simple. The engine shuts down when the car comes to a halt and restarts the moment you lift off the brake, so no fuel is burned while you sit at a junction.<\/p>\n
What that did to the battery was anything but simple. A conventional starter battery was built to deliver one big burst of current in the morning, then spend the day being topped up by the alternator. A start-stop car asks it to do that job dozens of times a trip, and to run the air conditioning, lights and electronics on its own during every engine-off pause.<\/p>\n
Ordinary flooded batteries do not survive that for long. Two battery types were developed for the job, EFB and AGM<\/strong>, and fitting the wrong one causes problems well beyond a short battery life<\/strong>.<\/p>\n Three things happen in a start-stop car that barely happened in older vehicles.<\/p>\n Far more engine starts.<\/strong> A car in heavy town traffic can stop and restart its engine twenty or thirty times in a single journey. Each restart is a heavy current draw, and that number of deep hits wears conventional plates out quickly.<\/p>\n The battery runs the car alone, repeatedly.<\/strong> During every engine-off pause the alternator is not turning. The blower, lights, infotainment, cluster and every control module draw from the battery. Then the engine restarts and the battery gives up a large burst on top of that.<\/p>\n The battery lives in a partial state of charge.<\/strong> This kills ordinary batteries fastest. A start-stop vehicle deliberately holds the battery below full charge so there is room to absorb energy recovered during braking, and a conventional flooded battery left part charged starts forming hard sulfate crystals on its plates<\/a>.<\/p>\n So a start-stop battery is not simply a stronger version of a normal one. It has to handle many shallow cycles, accept charge quickly, and sit below full charge without degrading. That is what EFB and AGM were built to do.<\/p>\n EFB stands for Enhanced Flooded Battery, and the name tells you most of it. The design is still a flooded lead acid battery with liquid electrolyte, reinforced for cycling work.<\/p>\n The plates are thicker and carry a polyester scrim on the positive plate surface, which holds the active material in place and stops it shedding under repeated cycling. Internal resistance is lower, so the battery accepts charge faster and gives up current more readily. An EFB typically handles roughly double the charging cycles<\/strong> of a standard starter battery.<\/p>\n EFB is the normal fitment for entry level start-stop systems: basic stop and start, usually without regenerative braking, and without the heavier electrical loads of a premium model.<\/p>\n It is also a sensible upgrade for cars with no start-stop that work hard electrically. A car doing constant short urban journeys, or carrying a dashcam<\/a> and upgraded audio, benefits from the extra cycling strength even though it never shuts down at a junction.<\/p>\n AGM stands for Absorbent Glass Mat. The chemistry is still lead acid, but the electrolyte is no longer free liquid. It is held in a glass fibre mat packed tightly between the plates.<\/p>\n That changes the battery’s behaviour in several useful ways. The plates are held under compression, so active material cannot shed as easily. Internal resistance is lower again than EFB, giving faster charge acceptance and a stronger cranking burst. Because the acid is absorbed rather than loose, the battery is sealed, cannot spill<\/strong>, and does not suffer acid stratification the way a flooded battery does.<\/p>\n The result is a battery that copes with deep, repeated cycling far better than anything flooded. That is why AGM is the fitment for cars with full start-stop plus regenerative braking, and for premium models carrying heavy electrical equipment such as large infotainment systems, electric seats and driver assistance hardware. The sealed construction is also why some models mount the battery in the boot or under a seat.<\/p>\n There is a hierarchy here, and getting it the right way round matters.<\/p>\n Check the battery that is in the car now.<\/strong> The label on the top or front will usually say AGM or EFB outright, and our guide on reading car battery labels<\/a> explains the other codes printed there. This is the fastest and most reliable check, provided the existing battery is the original type and was not already fitted wrongly by someone else.<\/p>\n Check the owner’s manual.<\/strong> Most manufacturers state the required battery technology and capacity in the specifications section.<\/p>\n Look for the start-stop system type.<\/strong> A car with full start-stop and regenerative braking, particularly a European model, is usually an AGM vehicle. Entry level start-stop more often takes EFB.<\/p>\n Then apply the rule that catches people out:<\/p>\n In short, you can move up the ladder but never down it<\/strong>.<\/p>\nWhy start-stop is so hard on a battery<\/h2>\n
What an EFB battery actually is<\/h2>\n
What an AGM battery actually is<\/h2>\n
How to tell which one your car needs<\/h2>\n
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AGM and EFB side by side<\/h2>\n