At Least It's a Dry Heat

Dear Car Talk

Dear Car Talk | Jul 17, 2018

Dear Car Talk:

I know it's hot out, but I have a winter question. All my life I've heard that if you get stuck in the snow and don't know when you'll be rescued, in order to keep from freezing to death you should run your engine for 15 minutes per hour and make sure the tailpipe is cleared of snow.

What about cars equipped with seat warmers? Can I run them all the time without running down the battery, or only while the engine is running? What about if I have a passenger or just want to keep my pumpkin spice latte and cranberry scone warm for later? Can I run both of them? Thanks. -- Ralph

Your seat warmers are powered by electricity, Ralph. If the engine is running, the alternator is producing so much extra electricity that the seat heaters can run indefinitely. But if the engine is not running, the seat heaters will run off the battery.

And eventually, that would drain the battery. How long will that take? Well, I'd take an educated guess that the average seat heater draws about 4 amps. That's about as much as your car radio uses. And seat heaters don't draw 4 amps continuously; they cycle on and off at a frequency that depends upon whether you set them on low, medium or high.

But let's take the worst-case scenario and assume that your seat heater runs continuously for an hour. That would use about 4 amp hours. Your battery probably is rated at 600 amp hours or more when fully charged. So there's almost no way it's going drain the battery to the point that you can't restart the car in 45 minutes.

And if you run the engine for 15 minutes every hour, the alternator -- which makes 80 or 100 amps -- is going to recharge the battery and more than make up for the power that the heated seat uses. In fact, if you cycle the engine as you describe -- 15 minutes on, 45 minutes off -- you probably could safely use both seat heaters indefinitely. Or until you run out of gas.

Now, I can't guarantee this. Lower temperatures reduce battery life. An old battery or a faulty charging system could alter the equation. And it's always wise to exercise caution in a situation that could result in your passing through the digestive system of a grizzly bear. But my guess is you'd probably be fine using both seat heaters until the mounted police arrived with their St. Bernards and flasks of Bartles and Jaymes.

If you make it through this imagined Armageddon, write to me and let me know I was right, Ralph.

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