Wirebook › Bonus lesson
Why does a full battery die in the cold?
The tablet said 45% when you left the house. Ten minutes on a snowy hill and it switched itself off. Bring it indoors, wait a bit, and there's the 45% again. The battery didn't lose its juice or forget how much it had. In the cold, the tiny particles that carry the electricity inside it get slow, and a slow battery can't push hard enough for a hungry job. Here you can chill a pretend tablet, watch its push sag, and plan a snow day that keeps it working.
So does the battery get shivery and forget?
Nearly right. Inside a lithium-ion battery, tiny charged particles called lithium ions swim through a gooey jelly (the electrolyte) from one side to the other while the tablet runs. That swim is what lets electricity flow. When the battery is warm the ions swim easily. When it's cold, the jelly gets thicker and the chemical steps at each side happen more slowly, so the ions crawl. Engineers call that higher internal resistance.
A gentle job like reading barely notices. A hungry job like a big game or a camera flash asks for a lot of current at once, and a battery with high resistance can't keep its push (voltage) up while it delivers it. The voltage sags. Every gadget has a lowest voltage it is allowed to run at, and when it hits that line it shuts down straight away to protect the battery and its circuits. That's why it can say 30% or even 50% and still die.
The juice was never gone, which is why it comes back when the tablet warms up. The part that gets confused is the battery gauge, the little chip that works out the percentage. It counts the electricity going in and out and also watches the voltage. In the cold the sagging voltage can make it think the battery is emptier than it is, so the number can jump down and then creep back up later. So the battery doesn't forget. Its helper gets fooled.
Why you shouldn't charge a frozen battery
Using a cold battery doesn't hurt it: it just works badly until it warms up. Charging it below freezing is different. The ions arrive at the negative side faster than the cold material can tuck them away, so some pile up on the surface as a thin layer of metal. That's called lithium plating, and it permanently shrinks the battery and can make it unsafe. That's why phones, tablets and electric cars slow down or refuse charging when the battery is too cold, and why electric cars warm their batteries up before fast charging in winter.
Electric cars feel the cold too. In freezing weather they often go noticeably fewer miles on a charge, partly because the battery is sluggish and partly because the car is also heating the people inside.
Keeping a gadget going in the cold
- Keep it in an inside pocket, close to your body, and take it out only when you need it.
- Do hungry things (games, the camera flash, video calls) somewhere warm, or keep them short.
- Low power mode helps: the tablet asks for less current, so the voltage sags less.
- If it switches off, don't panic. Let it warm up to room temperature on its own. Never use a radiator, hairdryer or microwave.
- Bringing a very cold gadget into a warm, steamy room can make water droplets form inside. Leave it in a bag or pocket to warm slowly, and wait until it's at room temperature before charging.
- Older batteries already have more resistance, so they give up in the cold sooner. An old phone dying at 30% on a winter walk is normal, not a fault.
Grown-up how-tos on Tooltern: is it bad to charge a phone in the cold? · power bank won't charge my phone · how many charges a power bank really gives
Too hot is a problem too, in a different way: see why phones and computers get hot.