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Amp hours explained: what drains a car battery

Amp hours explained: what drains a car battery

The power and longevity of a battery is noted by its Amp Hour (Ah) rating. Basically, this denotes how long a battery will last if it isn’t recharged.

For example if a battery is rated as, say 8Ah, it will be able to provide 4 amps for 2 hours. If a battery is rated as 100Ah, it will produce 100 Amps for 1 hour.

This does not necessarily mean a battery will last only one hour, because it will last 2 hours if it’s asked to produce only 50 Amps, 4 hours at 25 Amps, and so on.

Find out how to test and charge a car's battery here.

Batteries are also rated according to their Cold Cranking Amps (CCA), which denotes the power they have to turn over the engine and get it started in cold weather.

This is why it’s worth taking note of these two figures, because there’s little point in having a vastly powerful battery to power a 1.0-litre hatchback, while at the same time a diesel engine will require a much higher CCA rating than that 1.0 petrol engine.

Various situations and conditions can cause you vehicle’s battery to become low or discharged. In winter the engine is harder to turn over in the first place, plus the fact that systems such as the ventilation, heated rear window and (if your car has them) heated seats, are all in more frequent use.

It pays to garage your vehicle if you can, and to use a charger to give your battery a couple of hours’ of top-up charge each month.

Also, make sure you turn off the heated seats and windows once they’ve done their jobs, because if they’re all switched on when you turn on the ignition they can instantly deplete your battery to the point that it hasn’t enough power to start the engine.

Winter tends to be the time when people do lots of short journeys in poor weather, which means the battery rarely gets a chance to recover the charge it has used getting the car going and demisted.

That’s why it’s a good idea to take your vehicle for a slightly longer journey (say 10 miles or so) once a week just to give the battery a much-needed chance to recharge.

Amp hours explained: what drains a car battery