The starter battery: when it fades and what matters when replacing it

A lead-acid battery loses some capacity with every charge and discharge cycle, and it loses it faster when it often sits half empty. Short trips, long periods standing and many consumers running with the engine cold are the everyday life that hurts it most: the starter takes a lot, and the short drive gives little back.

Cold exposes what was already there. Chemically, a cold battery delivers less current while a cold engine with thick oil demands more. That is why batteries fail on the first cold mornings of the year, even though they have been getting weaker for months.

Most drivers only notice a weak battery when the engine will not start. Yet the car usually shows beforehand that the reserve is running low — not with a warning light, but with small things that are easily blamed on the weather.

These signs usually appear weeks before the failure:

A simple voltmeter across the terminals of a parked car says something about the state of charge but little about health: even a worn-out battery can look respectable at rest. What tells you more is a test under load, which workshops and many parts sellers do in a few minutes with a battery tester.

If the battery is only flat rather than worn out, a charger helps — together with the question of why it went flat. A consumer that does not switch off, or an alternator that does not deliver enough, will flatten the new battery too. A good workshop therefore checks the charging voltage as part of the battery test.

Starter batteries come in three common types. The classic flooded battery with liquid acid is found in cars without stop-start. The EFB, an enhanced flooded battery, tolerates more frequent starts and sits in many cars with a simple stop-start system. The AGM battery, where the acid is held in a glass-fibre mat, is intended for cars with stop-start and brake energy recovery, which cycle the battery particularly often.

The rule when replacing is simple: the same type as from the factory, or the one the manufacturer approves for the car. A flooded battery in place of an AGM is a step backwards that takes its revenge in a short life and error messages. The opposite direction is not permitted everywhere either — that too is in the manufacturer's documentation.

Every battery carries the figures it is chosen by: capacity in ampere-hours, cold-cranking current in amps, plus type, dimensions and terminal layout. Capacity and cold-cranking current should not be below those of the original battery; a little above usually does no harm, as long as the type is right and the battery fits its tray.

Terminal layout is where wrong purchases happen most often. The same dimensions exist with the positive terminal on the left and on the right, and the cables rarely reach the other side. Put a photo of the fitted battery next to the offer before you order.

Many newer cars have battery management with a sensor on the negative terminal. It tracks the battery's state of charge and age and adapts the alternator's charging strategy accordingly. If a new battery is fitted without the system learning about it, it keeps charging it like an old one — with the result that the new battery is overcharged or permanently undercharged.

Registration is done with the diagnostic tool and takes a few minutes. Whether a given car needs it is in its documentation, or the workshop will know. Anyone replacing the battery themselves should find out first — otherwise the cheap DIY job becomes the reason for the next battery.

A jump-start is the quickest way out with a flat battery, and on modern cars it is more delicate than it used to be. The owner's handbook names the connection points: under the bonnet there are often dedicated jump-start terminals, and the negative lead belongs on the earth point the manufacturer specifies, not necessarily on the negative terminal of the flat battery. The order of the leads, and removing them in reverse order, are described there too.

A swollen, leaking or frozen battery does not get a jump-start but a replacement. And once the car starts, the battery is far from full: a short drive barely charges it. A longer drive or a charger should follow, and if the car soon needs another jump-start, it is time for the battery test.

The battery is not always under the bonnet; on many cars it sits in the boot, under a seat or behind a panel. When disconnecting, the negative terminal comes off first; when reconnecting, it goes on last — so no short circuit occurs if a tool touches the body. Acid and charging gases call for gloves, eye protection and no naked flame.

After disconnection some cars forget settings: electric windows that have to relearn their end positions, steering angle sensors, a code on older radios. A memory saver that supplies power during the swap avoids most of that. Workshops have one as standard.

A starter battery does not belong in household waste. Most countries place take-back obligations on retailers and provide collection points; exactly where and how is regulated by each country. If you have the battery changed at a workshop, the old one stays there.

Electric cars have a 12-volt battery as well. It powers control units, locks and the circuit that connects the big battery — if it is flat, the car stays put even though the traction battery is full. The same applies to it as in a combustion car: type, ratings and, where needed, registration.