How to Maintain Your Car Battery in Cold Weather

Why Cold Weather Is So Hard on a Car Battery

A car battery is a chemical device, and chemicals do not like the cold. As the temperature drops, the reactions inside the battery slow down, so it holds less usable charge and delivers less power to the starter. A battery that is fully charged at 20°C might only deliver around two-thirds of its cranking power at freezing point, and closer to half in a hard frost. Meanwhile, your engine is asking for more: cold oil is thicker, so it takes more effort to turn over.

Add up those two things and you can see why the first really cold morning of the year is when a marginal battery finally gives up. The good news is that a little routine attention in autumn and through the winter will keep you starting reliably.

Warning Signs Your Battery Is Struggling

Batteries rarely fail without warning. Pay attention to these clues:

• The engine turns over more slowly than it used to, especially on the first start of the day.
• Lights dim noticeably when you crank the engine, or flicker at idle.
• The battery warning light stays on after the engine has started.
• You need more than one attempt to start on cold mornings.
• Dashboard clocks and radio presets reset themselves, a classic sign of a voltage drop.
• Swollen casing, a sulphur or rotten-egg smell, or crusty deposits around the terminals.

If your battery is more than four or five years old, treat it as a candidate for replacement rather than waiting for a no-start morning.

Check and Clean the Terminals and Connections

Poor connections cause as many starting problems as a tired battery. Take a look at the terminals, and if you see a white, green or blue crusty build-up, that is corrosion getting between the cable and the post.

• Make sure the engine is off and the keys are out of the ignition before you touch anything.
• Wear eye protection, and never lean over a battery with a naked flame or a lit cigarette. Charging batteries give off hydrogen, which is explosive.
• Disconnect the negative (black) terminal first, then the positive. That order matters, because it stops a stray spanner touching the bodywork and causing a short.
• Clean the posts and the inside of the clamps with a wire brush or a bit of emery cloth. Pouring a little hot water over the terminals will dissolve most of the crusty deposit, then dry everything properly.
• Reconnect the positive first, then the negative, and tighten the clamps firmly without going mad. They should not move by hand, but you should not be crushing the lead either.
• Smear a thin film of petroleum jelly or silicone grease over the terminals to keep moisture and corrosion at bay.

While you are there, check the battery is held down properly. A battery that rattles around can suffer internal damage, and a loose clamp is an MOT advisory waiting to happen.

Test the Voltage Yourself in Five Minutes

A basic digital multimeter costs very little and tells you a great deal. With the engine off and the headlights having been off for at least an hour, touch the probes to the battery posts.

• 12.6 volts or above — fully charged and healthy.
• 12.4 volts — around three-quarters charged. Charge it soon.
• 12.2 volts or below — roughly half charged or less. This battery needs charging and probably testing.

Now start the engine and check again. You should see somewhere between 13.8 and 14.4 volts, which shows the alternator is charging. If the reading stays around 12.5 volts with the engine running, you have an alternator or belt problem rather than a battery problem. Many garages and motor factors will test a battery and its charging system free of charge if you would rather have it done properly.

Why Short Journeys Are the Real Battery Killer

Starting an engine takes a big gulp of current, often several hundred amps for a couple of seconds. The alternator then has to replace that. It does not happen instantly. A run to the shops and back, ten minutes each way, may only just cover what the starter used, and in winter, with lights, heater blower, rear screen demister and heated seats all running, you can easily end a short journey with less charge than you started with.

Do that five days a week and your battery slowly drains, even though you are driving every day. If your motoring is mostly short trips, charge the battery properly once a fortnight with a smart charger, or take the car for a decent 30 to 45 minute run at a steady speed with as many electrical items switched off as is safe and comfortable.

Winter Habits That Keep You Starting First Time

• Park in a garage or under cover if you can, and if you must park outside, keep the car out of the prevailing wind.
• Switch off the heater blower, heated seats and rear demister before you turn the key, so all the current goes to the starter.
• If the car sits unused for weeks, use a trickle charger or smart maintainer rather than letting it flatten. A battery left discharged in the cold can freeze and be ruined.
• Check the alternator belt for glazing, cracks or slack, and have it adjusted or replaced if needed.
• Keep a set of jump leads in the boot, and know where your earth point is for a jump start. Connect positive to positive, then negative to a metal earth point on the dead car, never to its battery post.

None of this takes long, and half an hour spent on the battery in October can save you an hour standing in a frozen car park in January.