Cold: Cold and snowy.
Courtesy jpmatthJK! It’s science, of course.

Usually science loves us, and we love science, but when the temperature drops (or, here in Minnesota, when the temperature drops and drops and drops) science starts to hate us just a little bit.

How do I know this? Because, like so many other lost and lonely souls, when I went out to start my car this morning… it did nothing. And I think I heard it mutter an awful, awful word at me from one of the dash vents.

So what gives, science? Yes, I understand that I would die if I were left out all night in -30 degree weather, but my car is a robot, and robots can’t even comprehend the weaknesses of humans, much less experience them. Why did my car die?

The car died, of course, because the battery died, and the engine couldn’t be started.

Why do batteries die in the cold?

It boils down to my old acquaintance, Chemistry. (I’m Science now. Pretend I’m Science.) Batteries can work in the first place thanks to a chemical reaction taking place between the positive and negative terminals. In a car battery, the terminals (to which you clamp jumper cables) are made of lead and lead dioxide (which is a lead atom with two oxygen atoms). Between the terminals is sulfuric acid (which is a sulfur atom with four oxygen atoms and two hydrogen atoms). The lead terminal wants to react with the sulfuric acid, and so it does—it kicks the hydrogen atoms off the sulfuric acid, and combines with what’s left to create lead sulfate (which is a lead atom a sulfur atom, and those four oxygen atoms). When the hydrogen is kicked out of the sulfuric acid, an electron is also released. On the lead dioxide side, hydrogen is getting kicked off the acid, and oxygen is getting kicked off the lead dioxide. Lead sulfate is formed again, and, with the help of the free electron from the lead terminal side, that spare oxygen and hydrogen combines to form water (which we all know is two hydrogen atoms and one oxygen atom).

All of this is only going to happen, however, if there’s a wire connecting the lead dioxide and lead plates outside the battery, so electrons can flow from the negative (lead dioxide) terminal to the positive (lead) terminal. If there’s something in the middle of that wire, like the starter for an engine, those electrons can do some work.

Unfortunately, this chemical reaction also depends on temperature. The colder it is, the less willing all these molecules will be to mess around with each other, and fewer electrons will be tossed around. If it’s really cold, there may not be enough of a reaction to start your car. Also, because the reaction produces water, there’s a chance that the water could freeze if it gets cold enough, cracking the battery case altogether. Then you’re really up Brown Creek.

If you’re battery is just low, and the cold has made it weaker, you might try jump-starting it (remember, positive terminal to positive terminal, negative terminal on the live car to a metal spot on the dead car). With the help of a fresh battery, your weak battery could build up enough charge to start your engine, which would warm the battery and start to recharge it. If your battery is frozen, however, don’t try to jump it—it could explode. Now, an explosion would be kind of awesome, but flying battery acid is scary, and it doesn’t matter if it’s science’s fault or not if your face gets burned off.

So that’s why our cars didn’t start this morning. Feel better? No? Me neither.

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