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What happens if your car battery runs flat?

In a car with a petrol or diesel engine, the battery has a life of around 5-6 years, assuming it’s being used reasonably regularly, but there are ways in which you can reduce this, one of them being letting your car battery run completely flat.

Car batteries are designed to be kept fully charged, not discharged and recharged like a mobile phone battery. Discharging the battery until it’s flat, or deep cycling, shortens the life of the battery.

Why would your car battery run flat?

Parasitic drain

There are systems in your car that need to have power all the time, such as the clock and anti-theft alarm. While these take almost no power at all, they do drain the battery and, if left several weeks, will draw down a lot of the battery’s charge.

Auxiliary devices

Some devices such as mobile phone chargers can still draw power when plugged in, even if they’re not charging and the car’s ignition is off

Leaving something on

Leaving headlights lights on after leaving the car is the most common cause of a flat battery, followed by leaving an internal light on (this can happen if you don’t close the door properly). Most new cars have a warning if you leave the lights on and take the key out of the ignition

Faulty alternator

If the alternator is not charging the battery, it will eventually run down as you use the lights, heater, radio, GPS, indicators and other electrical functions in your car.

Corroded terminals or faulty cables

If power can’t get to the battery, then it can’t charge. Cleaning the battery terminals periodically will prevent corrosion building up.

Low water levels

In batteries that require topping up with water, if the water level gets too low, the battery cells become too acidic and you’ll get sulphation on the battery plates, where crystals of lead sulphate form on the battery plates.

Sulphation can also occur when a battery is deprived of a full charge. It impedes the chemical to electrical conversion leading to longer charging times, shorter run time before it’s flat, shorter battery life, excess heat build-up and eventually battery failure.

How can you tell if your battery is starting to fail?

The first sign is that the engine will turn over slowly. This is called slow cranking. If your car is one that has trouble starting under normal circumstances, slow cranking might mean that it doesn’t fire up. Eventually the battery will lose enough power that it won’t be able to turn the engine at all and you’ll hear a clicking noise instead.

If the battery light illuminates, then there’s either a problem with charging (usually the alternator) or the battery isn’t providing any power to the car.

How can you keep your battery from running flat?

Most new batteries are so-called ‘zero maintenance’ batteries. This doesn’t mean that you don’t need to look after them, though. Check the terminals every month and clean off any corrosion that’s forming.

Zero-maintenance battery

A cheap option for this is a tablespoon of baking soda with a cup of hot water and an old toothbrush. Disconnect the negative battery cable then the positive battery cable before doing this. Make sure that the terminals are dry before you reconnect the cables.

If your battery needs topping up with water, check monthly and fill it with distilled water.

Checking the water level in a battery
A battery box helps keep the battery clean and corrosion-free

Taking the car for a drive of at least 15 minutes every two weeks should stave off battery damage due to it being left too long. If this isn’t possible, you can purchase a trickle charger or battery maintainer to keep it topped up. Turning the car on and letting it idle does not charge the battery quickly.

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Darren is a member of the Institute of Advanced Motorists and the NZ Motoring Writers' Guild

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