Question about capacitors

Question

I just put a capacitor in my car for my stereo and I went out to my car and the battery was dead. I do not know if the battery was dead because I left a light on(i couldn't find one on) or what, but it was dead. My question is, is this because of the capacitor? If the capacitor has less than 12 Volts when I leave my car, and turn it off, will the capacitor drain the car battery?

Answer 1

I'd check the lights first.  Then I'd check that you didn't some how create a short to ground when you installed this capacitor.  Next I'd chack that the capacitor is good - it could have an internal short that makes it look like a resistor between the battry terminals.  I'd also check that any bleed off resistor is just bleeding off charge from the capacitor and not providing the battery a discharge path.

It shouldn't matter how much voltage is on the capacitor when you shut the car off.  

Answer 2

This must be a super capacitor.

They come in various sizes, with 10 Farad one of the largest, so I'll assume this is the value you have.

energy in a cap is E = ½CV²

E = ½(10)12² = 7200 J

energy stored in a small car battery is about 500 watt-hrs, which converts to 1800000 J

the ratio is 250 to one, so your car battery should be able to charge the cap hundreds of times before it becomes discharged.

Perhaps the capacitor is damaged and leaking? Or your car battery is damaged and can't store much charge?

BTW, if your battery was totally drained, it was damaged, and has less capacity than it did the day before. You might have to replace it.

.

Answer 3

It's not likely.

Usually, stereos (and any associated capacitors) are *supposed* to be wired to the accessory position of the ignition, with the exception of a wire to maintain the clock / radio station presets.

So, unless you left the key in the accessory position, the cap would be disconnected.

The only chance of a capacitor discharging the battery would be if the capacitor was faulty (shorted) - which would probably be pretty phenominal to watch when powered up (sparks / smoke / rupture) - or if it was at least half the size of the battery or larger.

Check for faulty wiring first.