Why does a battery fail in winter?

Roadside-assistance statistics are merciless: most flat batteries happen with the first frosts of each season. It’s no coincidence — winter hits the battery from several sides at once, combining its lower performance with the engine’s greater demands.

In this article we explain why frost lowers the battery’s capacity, why a cold engine demands more of it, and how short trips finish the battery off. We also explain why winter doesn’t so much “damage” the battery as reveal its earlier wear, and how to prepare the car to avoid a morning surprise in the car park.

Frost lowers the battery’s capacity

Chemical reactions in the battery slow down at low temperature. In severe frost the real available capacity can drop by tens of percent — a battery that worked flawlessly in summer may no longer cope in winter. The lower the temperature, the greater this drop, which is why the first real frosts are the hardest for the battery.

It’s worth understanding where this drop comes from. A battery produces current thanks to a chemical reaction between the plates and the electrolyte. At low temperature these reactions run more slowly, and the thickening electrolyte hinders the flow of ions. The result is that the battery physically can’t deliver as much current as in the warmth — even though it’s still charged. It’s reversible: once warmed, performance returns, but on a frosty morning it’s exactly this momentary drop that decides whether the car starts.

A cold engine puts up more resistance

At the same time the engine oil thickens, and the starter has to overcome greater resistance. So the battery has a harder job to do with less power available. That combination most often ends in a refusal to start.

This is the crux of the winter problem: two unfavourable phenomena overlap at exactly the same moment. On one hand the battery has less power, on the other the engine needs more of it, because the thick oil puts up more resistance and cold seals and pistons move worse. In summer both these things are comfortable, in winter they’re on the edge. It only takes the battery being a little worn for that edge to be crossed.

Which battery copes better with winter

The ability to start in frost is described above all by the starting current, marked as CCA. It tells you how much current the battery can deliver at low temperature in a short time. That’s why, when choosing a battery, it’s never worth going below the factory CCA value, and a small surplus gives a valuable reserve for winter and for the battery’s natural ageing.

The battery type matters too. Modern AGM batteries, fitted in start-stop cars, cope with tough conditions and deeper discharges better than plain flooded ones. Regardless of type, though, condition is key: even the best battery, if it’s already old and undercharged, will fail sooner in frost than an average but healthy and fully charged one. In other words, starting in frost is decided not so much by the battery’s price as by its real condition and charge.

Short trips finish the battery off

• More loads in winter: window and seat heating, the blower, lights
• Short city trips don’t manage to recharge the battery
• Stops in the frost deepen the battery’s discharge
• An old battery has no reserve left for these tough conditions
• Moisture and salt encourage terminal corrosion, worsening contact

Short trips are a separate enemy of the battery in winter. Starting draws a considerable charge from the battery, and a drive of a few minutes doesn’t give the alternator time to top it up. On top of that, in winter plenty of loads are running: rear-window, seat and mirror heating, the blower on full and the lights on almost all the time. The alternator has to serve them all, so little is left for recharging the battery. As a result the battery runs in a state of constant undercharge and gets weaker by the day. That’s why a car used in winter only for short city distances is the most exposed to a morning refusal to start.

Winter doesn’t damage the battery — it merely mercilessly reveals the wear that was invisible in summer.

Why exactly the first frosts

Many drivers are surprised the battery fails at the first bigger cold snap, even though everything worked the day before. It’s a typical scenario: through the warm part of the year the battery slowly loses condition but has enough reserve for it not to show. Only a sudden temperature drop takes away that reserve and exposes the wear that had been building for months. The car still started in the evening, and in the morning it didn’t, even though “nothing changed” but the temperature outside.

That’s why the first frosts act like an endurance test. Batteries on the edge of health fail en masse within a few days as soon as the temperature drops below zero. It’s also why a battery check is best done in autumn, before nature runs this test itself at the least convenient moment. A few-minute measurement in autumn can save a stressful morning when the car won’t start and you’re in a hurry to work.

How to prepare

Before the season it’s worth checking the battery’s condition, and if it shows signs of wear — replacing it in good time. Far cheaper and more convenient than resorting to an emergency jump-start in the frost, in a hurry and with no certainty the car will start at all.

• Check the battery voltage in autumn, before the winter season
• Take care of clean, firmly tightened battery terminals
• From time to time cover a longer route to fully recharge the battery
• For longer standstills use a maintenance charger
• Don’t leave loads switched on with the engine off
• Consider a portable jump-starter in the boot just in case

Simple habits on a frosty morning help too: before starting it’s worth switching off unnecessary loads such as window heating, seat heating or the radio for a moment, so all the current goes to the starter. In a car with a manual gearbox, pressing the clutch during starting relieves the engine, because the starter doesn’t also have to turn the gearbox components. And if the car sits idle for a longer time, it’s worth connecting it to a maintenance charger, which keeps the battery fully charged.

Frequently asked questions about the battery in winter

Can a new battery also fail in frost?

A new, healthy battery copes with frost far better, but it too has limits. If it’s undercharged — for example by nothing but short trips or by parasitic drain — even a fresh battery can fail. So buying a new battery alone doesn’t exempt you from looking after its charging and checking its condition. If a brand-new battery goes flat in winter after a few days’ standing, it’s actually a sign of an excessive current draw in the car that needs diagnosing.

Is it worth bringing the battery indoors overnight into a warm room?

In extreme frost and with a weaker battery it can help as a stopgap, because a warm battery delivers more starting current. It’s an emergency measure, though, not a fix for a lasting problem. If the car only starts after “warming” the battery, it’s a clear sign the battery is already worn and it’s time to replace it. A healthy, well-charged battery should cope with typical frost without such tricks.

Is jump-starting in frost safe?

Yes, as long as we do it correctly and keep to the right order of connecting the leads. Jump-starting is an effective way to start a car with a flat battery, but you have to remember it doesn’t fix the cause. After starting it’s worth covering a longer route or charging the battery with a charger, and if the problem recurs, checking the battery and the whole charging system.

How many years does a battery last before it fails in winter?

On average 4–6 years, but it’s winter that most often marks the end of its life. A battery that already has a few seasons behind it is most exposed to a refusal in frost. So after the fourth year it’s worth checking its condition before each winter and not delaying replacement when signs of wear appear. Treat the autumn battery check as a fixed point of preparing the car for winter, on a par with tyres.

Can frost damage the battery permanently?

The cold itself mainly limits momentary performance, but a deeply discharged battery in heavy frost can literally freeze, because the diluted electrolyte has a higher freezing point. Freezing can permanently damage the battery and even split its casing. That’s why in winter it’s especially important not to allow a deep discharge.

What to do when the car no longer starts

If on a frosty morning you find a dead car, you have a few options. The quickest is a jump-start from another car or a portable jump-starter (a booster), worth keeping in the boot for winter. You can also remove the battery, warm it and charge it with a charger, then go back to the car. Each of these methods will start the car, but none replaces the diagnosis of why the battery failed — and that’s the most important thing if the problem isn’t to return.

If the battery is already a few years old and refuses in frost, the most sensible thing is to replace it with a new one, chosen for the car. Rescuing yourself with a jump-start every morning is a road to nowhere — the problem will keep coming back, and the whole electrical system, including the starter and alternator, suffers in the process. A planned replacement is cheaper and less stressful than a daily battle with a flat battery and looking for someone to “lend some power”.

We’ll replace the battery at your address — including in winter, even when the car no longer starts. We’ll come with the right battery, chosen for your car, register it in the computer if needed, and take the used battery away for disposal. You don’t have to tow the immobilised car anywhere or look for help in the frost.