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Old 06-09-07, 09:55 PM   #1
RoQuEr
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Charging car battery from cigarette lighter

Does this product do anything special? http://www.improvementscatalog.com/h...y-charger.html

I was wondering if any 12v DC battery plugged into my cigarette lighter could charge a car battery. I know this is basic physics, but I'd rather not find out the hard way. Could I plug in my jump start battery to the cigaretter lighter overnight and expect the car battery to be charged, and the jump-start battery to be drained?
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Old 06-10-07, 07:14 AM   #2
OldDude
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It obviously uses magic. Otherwise it is too small to hold enough charge to recharge a totally dead car battery.

If the batteries are the same technology, and number of cells, they will "share" their charge, and wind up with a partial charge on both. If the charging battery is a higher nominal voltage (different number of cells in series or a different technology), it will transfer "most" of its charge and be nearly discharged.

The question is how many amp-hours in the charging battery and how many amp-hours does the dead battery need. A typical car battery might be rated 50-100 Ah, and it needs about 20% more charge than it is missing to properly replace the charge. (Although you don't need a FULL charge to restart car)

Bottom line, if it is not as big as car battery (physical size), it is not as big as a car battery (electrical capacity). But maybe the charge fairy will step in and help.
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Old 06-10-07, 09:55 AM   #3
Nick Danger
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Why is it hard to build small, powerful batteries? It's easy to set up a chemical reaction that produces a lot of heat energy in a short time, like nitroglycerin. Why can't we set up a chemical reaction that produces a lot of charge in a short time?

Or, what's so cool about lead-acid batteries?
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Old 06-10-07, 10:51 AM   #4
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Lead-acid batteries are not state-of-the-heart in charge per pound by any means. They are so-so, but dirt cheap and rugged.

Lithium and nickel metal hydride store more energy per unit weight, but are expensive and a bit fussy on charge/discharge cycles.

I don't know if nitroglycerin is a good or bad example. It actually has less energy per unit weight than gasoline, diesel, etc., but is capable of much faster reaction times (explosive). Gasoline has to be vaporized and mixed to correct air/fuel ratio to approach that reaction speed

Edit: Wikipedia has a pretty good summary on different chemistry. Also links with more detail for each kind. Some newer chemistries are 3-4X better than lead acid, nothing is 10X better. Generally lead-acid costs less, but is bigger, heavier.
http://en.wikipedia.org/wiki/Rechargeable_battery
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Last edited by OldDude; 06-10-07 at 11:10 AM.
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Old 06-10-07, 10:58 AM   #5
krkuhl
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didnt work on my full size chevy truck in college but worked fine on my little saturn. no idea how/why. both had their lights left on overnight causing their battery to die. then again, my chevy was quite a bit older than my saturn at the time.
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Old 06-10-07, 11:56 AM   #6
OldDude
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Quote:
Originally Posted by krkuhl
didnt work on my full size chevy truck in college but worked fine on my little saturn. no idea how/why. both had their lights left on overnight causing their battery to die. then again, my chevy was quite a bit older than my saturn at the time.
I'm guessing the truck had a much larger engine and therefore a larger battery, starter motor needed more current, etc. The little booster battery was less able to make a difference than on the scaled down system in the Saturn. But it's a YMMV situation, at best.
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Old 06-10-07, 02:16 PM   #7
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Interesting fact: A lithium ion battry, such as the one in your cell phone has half the potential energy as an equal amount of dynamite.

edit: Since the portable battery is 12v, and the car battery is really 14.8 or so, then I can expect it to only partially charge the car, if at all, correct?
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