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Comment by greenyoda | Hacker News Reader

Starting the car takes a lot of power from the battery (the battery has to run the starter motor to crank the engine), so if you don't drive the car long enough to recharge the battery to at least where it was before the trip, you'll have less charge than you had before. If you repeat that cycle enough times, the battery will eventually die.

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My fire department bought a new ambulance.

Emergency lights use a lot of power, as you can imagine. Even the LED ones. But we also don't want to idle on scene for a while.

So new ambulances (and engines, too) have a nice feature. Leave the lights running. Turn the engine off.

When the battery voltage gets low enough, the vehicle will start the engine, let it run til the voltage is better, then shut itself back off.

Nice, but a bit deadly in an enclosed space due to fumes.

> so if you don't drive the car long enough to recharge the battery to at least where it was before the trip, you'll have less charge than you had before. If you repeat that cycle enough times, the battery will eventually die.

I've not tested the low threshold, but a 20 minute drive seems to be more than sufficient.

I'm surprised why cars still don't include a charge controller that counts the amount of energy in/out of the battery and gives you a battery gauge, or even a simple voltage gauge.

We can do it just fine in cheap laptops (with lithium batteries which require more complex charge management), why can't we do it for cars that cost tens of thousands?

Cars have had voltage gauges for decades. The really aren’t very useful because essentially by the time the battery reads less than its nominal 12v it’s basically dead.

True but it will at least confirm that the battery is dead, as opposed to something else. I remember back when I was a kid my parents' car would not crank (you hear the solenoid engage but the engine doesn't turn) and nobody had any idea what it was (if you're not a car person and this never happens to you it might not be obvious especially considering all the other electricals still work fine on the slightly lower voltage). A voltmeter showing below 12V or a "low battery - <12.2V" light would've quickly cleared that up.

Basically all cars monitor battery voltage. Many models even have a dashboard gauge for it. But what is that information useful for, other than diagnostics? Why give it dashboard real estate? Besides, you can always pop open the hood and check the battery with a $12 multimeter.

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