Why do Car Batteries Die in Cold Weather?

cars covered in winter snow

Imagine this. It’s the coldest day all year.  You go out to start your car so it can warm up.  When you turn the key, maybe the starter runs for a couple seconds and quits.  Maybe it doesn’t even try at all.  If you live in a cold climate, this has probably happened to you.  Why do batteries go dead in cold weather?  Is it just bad luck?  Is the electricity frozen?  What’s going on?

As it turns out there are a number of factors that make winter especially tough on your car battery.  Batteries don’t discharge as well in cold weather, they don’t charge as well in the cold, and you probably put more demand on the battery in the winter.

The Science Behind Batteries Dying in the Cold

A car battery works through a chemical reaction.  There are plates of lead dioxide and plates of lead, immersed in sulphuric acid that’s diluted with water.  The sulphuric acid breaks up into hydrogen, with a positive charge (basically, it’s short an electron), and sulfate with a negative charge (it has extra electrons).  The hydrogen reacts with the lead dioxide and more sulphuric acid to form lead sulfate and water.  In that process it has to take on an electron to make up for the one the hydrogen was short of.

The lead reacts with the sulfate to form lead sulfate.  It gives up the sulfate’s extra electrons in the process.  Remembering the exact details of the chemical reaction isn’t really important here.  The important part is that one set of plates is giving up electrons and one set is taking them on.  That’s what creates the electric current.  That’s how a battery discharges or puts out electricity.

When the battery receives electricity from the alternator, those chemical reactions are reversed.  Lead sulfate is turned back to lead and sulfate, taking on electrons in the process, and lead sulfate and water are converted back into lead oxide, sulphuric acid and hydrogen, losing an electron in the process.  That’s what recharging a battery actually does, it returns the chemicals to the state where they’re ready to react and create a flow of electrons.

What does all this have to do with cold weather starting?  Well, all the separating and combining that the chemicals in your battery have to do to discharge electricity requires some starting energy.  Heat is energy, so colder temperatures mean less available energy.  The colder it is, the more slowly these reactions take place.

What Other Batteries Die in the Cold?

Cold weather makes all sorts of batteries slow down.  I once left an iPod in my car overnight in freezing weather.  In the morning the iPod wouldn’t turn on.  I left it in my pocket for a couple hours, and once it warmed up, it worked fine.

You also may have heard of people putting household batteries in the freezer.  Because cold batteries don’t discharge as easily, a disconnected battery in the freezer won’t “leak” power as quickly as one at room temperature.

Back to Your Car. Cold Weather Killed Your Battery. Now What?

When your car won’t start on a cold morning, it means your battery can’t support enough chemical reactions to make the power needed to turn your starter motor.  So, you jump start your car and it should be good to go, right?

Not so fast.  You’ll need to run the engine for a while to recharge the battery, which should be no surprise, but the bad news is that a cold battery also recharges more slowly.  Just like the discharging reaction, the recharging reaction goes more slowly in the cold.

There’s also usually a higher draw on the system during winter.  You might be running your heater blower, defroster, heated seats and other accessories more in the cold.  That means you’re using more power and charging the battery less.  The starter even needs more power in the cold, because cold oil means more friction in the engine.

What can you do to have easier cold weather starts?  First of all, a new battery will work better than an old one, so if your battery is quite old, you should consider replacing it.  You can also try to reduce your draw on the battery to help it charge better.  If you’re leaving the car idling, turn off the radio, heated seats, and heater blowers.

Finally, you can try to keep your battery warm over night.  Parking in a garage helps.  It’s also possible to purchase electric heaters designed for keeping your battery warm.  Even throwing a blanket over your hood might help a little bit.  We’ve even heard of people bringing their batteries inside over night, but that seems like it adds a lot of extra effort to your morning commute, but if you’re willing to start off every day reinstalling your battery, give it a shot.

 

Written by Dan Smolinsky.

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