How to Stop Your Car Battery Going Flat in Winter

A flat battery is the most common reason for a winter breakdown call-out, and cold weather is when it happens. Battery problems are the reason for more RAC call-outs in winter than anything else¹. Flat or dead batteries can happen to any car regardless of age and at any time, but in cold weather such problems are far more likely to occur².

The good news is that most of this is preventable. Have the battery tested before the cold sets in, keep it charged through a cold spell, and know how to jump-start it safely if it does go flat. Council winter driving guidance puts the average life of a car battery at five years³, so if yours is around that age, or starting has become sluggish, get it checked now rather than waiting for the first hard frost.

This page covers why cold drains a battery, the signs it is weakening, how to keep it charged, how to jump-start safely, and what to keep in the car if you do get stuck. For the wider picture, see Driving in Snow and Ice: Staying Safe on the Road and Car Checks Before Winter.

Why cold weather drains a car battery

Cold affects a battery in two ways. First, the battery itself works less well in low temperatures, so it holds and delivers less charge just when the engine needs more of it to turn over. Second, winter driving loads the battery harder: during winter months the battery will drain more quickly with the additional use of electrical items such as heaters, lights and heated rear windscreen³. Every one of those draws on the same battery that has to start the car.

That combination is why flat or dead batteries can happen to any car regardless of age and at any time, but in cold weather such problems are far more likely to occur². A battery that coped all summer can fail on the first really cold morning.

Age matters too. Council guidance puts the average life of a car battery at five years³, and the Met Office says car batteries need replacing every few years, according to usage, so make sure you get yours checked at your car service¹. A battery near the end of its life has less margin when the temperature drops.

It is worth knowing that winter is not the only season that stresses a car. In summer there are often breakdowns due to cooling-related faults, whether faulty cooling fans or a lack of coolant⁶. The difference is that a cooling fault usually gives warning, while a battery can simply fail to start the car one morning.

Signs your battery is weakening before winter

The simplest check is to have the battery tested. Cold temperatures can decrease your car battery's performance, so have it tested to ensure it holds a sufficient charge⁷. Many garages will do this at a service, and the Met Office advises getting the battery checked at the car service¹.

Watch for the everyday signs as well. A slow, laboured turn of the engine on a cold morning, lights that dim when you start up, or needing to jump-start more than once all point to a battery on its way out. If in doubt, replace it: official guidance is to make sure your battery is fully charged and replace it if it is no longer reliable⁴. North Wales Fire and Rescue Service puts it plainly: is the battery fully charged? Replace it if it's not completely reliable⁸.

Use this checklist before the cold arrives:

• Test the battery and replace it if it is not holding charge⁷.
• Check it is fully charged and top it up if needed⁹.
• Look at the age: around five years is the average life³.
• Check the terminals are clean and tight, and the battery is secure.
• Test the lights, heaters and heated screen so you know what is drawing on the battery³.

Keeping the battery charged through cold spells

The core advice from councils across the UK is consistent: keep the battery fully charged and topped up¹⁰, and check that your vehicle battery is in a good condition and fully charged⁹. Cornwall Council's winter driving guidance is simply to keep your battery fully charged¹¹. Traffic Wales advises making sure your battery is in good condition¹².

How you do that depends on how you use the car:

• Regular longer runs give the alternator time to replace the charge used to start the engine.
• Short journeys in cold weather, with heaters and lights on, can gradually run the battery down.
• A battery maintainer or trickle charger keeps a healthy battery topped up through a cold spell, but it cannot rescue a failing battery. Have the battery tested first⁷.
• Park under cover where you can: official advice for heavy snow is to park vehicles in a garage if available¹³.

If you use a charger or maintainer at home, follow the fire and rescue service's advice: charge on a hard, flat, heat-resistant surface, away from extreme heat, cold, direct sunlight, and flammable materials¹⁴. If a battery has been left in freezing conditions, allow it to warm up slowly indoors before using or charging it¹⁵. Never charge a battery in a confined space where flammable vapour could build up.

Jump-starting safely when the battery goes flat

If the battery does go flat, jump leads are the standard remedy. Before you connect anything, read your vehicle handbook: the maker's instructions for your own car take priority, and some modern cars should not be jump-started in the usual way.

The safe order, following the fire and rescue service's battery advice, is:

1. Make sure both vehicles are off and the handbrakes are on.
2. Connect the red lead to the positive terminal of the flat battery, then to the positive terminal of the good battery.
3. Connect the black lead to the negative terminal of the good battery, then to a metal earth point on the car with the flat battery, away from the battery itself.
4. Start the good car, let it run, then start the flat car.
5. Remove the leads in reverse order, and let the engine run to recharge.

If you are stuck at the roadside, make the vehicle and yourself as visible as possible to other road users and rescuers, keep warm, and if you use a mobile phone, give accurate details of your location⁵. On a motorway, switch on your sidelights in low-light conditions, but turn them off if your battery starts to run down¹⁷.

What to keep in the car for a winter breakdown

Getting stuck in your car in winter could be dangerous, so get a winter car kit ready¹⁸. The Met Office's list is a good starting point: an ice scraper and de-icer, a torch, a paper road atlas, an in-car phone charger, warm clothes and blankets, high-visibility clothing, jump leads, an empty fuel can, food and drink, a shovel, warning triangles and sunglasses².

Different official sources add useful items. Transport Scotland's list includes a torch and spare batteries, a pair of boots, a first-aid kit, a tow rope, a hot drink in a flask, a fully charged mobile phone and a map for unplanned diversions¹⁹. The NHS adds warm clothing, a phone charger and drinking water²⁰. National Highways suggests warm clothes, a hi-vis jacket, a torch and your breakdown cover details²¹.

Pack a few essential items in your car's boot to make sure you are ready in case of getting stuck in cold and wintry weather²². Keep the kit in the car all winter, not just for long journeys, because a breakdown can happen on a short trip too.