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I have a couple of these incredibly powerful 12V battery packs and I got thinking about how feasible it would be to MacGyver an engine starter out of them.

These things are rated on the package to deliver 30C and even higher: For a single set of 3 cells in series a 2Ah 30C cell will deliver 60A.

Consider this specimen: Monster Battery

5Ah @ 65C is 325A, and 5Ah @ 130C is 650A. The 130C max rating seems to indicate that the battery can be discharged in under half a minute.

I'm pretty sure many compact automobile engines can be started with this much current.

My thinking is I could buy a cigarette-lighter adapter, solder it to a beefy 4S battery like the one linked above, and I've got a self-contained car starter with a LOT more power than the pathetic "jump starter" products one might find in a store. With something like this, it basically could not be easier to use, and a 4S lipo won't even get overcharged. Once your car is started, leave it in there and the battery automatically gets topped off. (well 14.4V nominal voltage isn't the most ideal situation but it could be worse)

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  • \$\begingroup\$ First do note that many batteries available from hobby sources have (sometimes drastically) overblown C ratings! I'm pretty much sure that the cable on that battery can't even take the nominal 325 A discharge current. Another point is the cigarette light adapter. Assuming that the car is wired to be able to take that much current through the lighter port, I don't think that the port itself could take 325 A. Next the charging for LiPo batteries is quite complex and you can't just connect it to power and let it get topped-off. There's a good chance that it will die in such procedure. \$\endgroup\$ – AndrejaKo Mar 31 '12 at 4:41
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    \$\begingroup\$ Do note that the linked battery is a 4 cell battery and it will reach constant voltage phase of charging when its voltage reaches 16.4 V. Until that point, you need constant current charge. Once we take into account how touchy LiPos are about the voltage where they need switch from constant current to constant voltage, this can be quite a problem. And of course finally we have the balancing problem. The battery needs a balance charger to work correctly and charging it over the discharge plug will have negative effect on battery life. \$\endgroup\$ – AndrejaKo Mar 31 '12 at 4:45
  • \$\begingroup\$ Ah these are some good insights. I reckon a 12V DC-in charging circuit (that connects to each cell) will be necessary to make such a thing "safe". \$\endgroup\$ – Steven Lu Mar 31 '12 at 13:35
  • \$\begingroup\$ The only "big" problem with that seems to be that the multi-cell chargers aren't available in easy to use packages that are a bit hard to solder. The other point would be to clean up the automotive power supply, but that shouldn't be too hard. Some ICs I've seen can take up to 24 V input voltage so that would only leave voltage spikes to be fileted out. Yet another problem would be to find fast enough charger IC so that we can be sure that the battery will be charged relatively quickly. This IC can for example charge at up to 4 A. \$\endgroup\$ – AndrejaKo Mar 31 '12 at 14:42
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    \$\begingroup\$ From memory, cigar lighters are usually connected through a fuse of 15A or so. Fuses don't care which way the current is going. If you put hundreds of amps through the ligher, the fuse will blow promptly. \$\endgroup\$ – markrages Mar 31 '12 at 19:31
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Just an add on to the already excellent comments here, If you are going to use your cigarette lighter plug to "jump start" a vehicle with a low battery, Do not expect to plug your jumper pack in and turn the key. The wiring for the lighter plug is not designed to carry that many amps. The answer to this is to plug the starter into the lighter socket, then wait about 15 minutes for the battery to "charge" your car battery, Then UNPLUG your jumper battery and then attempt to start the vehicle.

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  • \$\begingroup\$ And hope that the lighter socket is designed to supply charge back to the battery. If all of the accessories (e.g. CD, AC, etc.) are powered off a common wiring bus then you basically powering everything in the car off the cigarette lighter socket rather than charging the battery which isn't particularly useful. You would have to turn the key to enable all the electronics in order to make a connection from the cigarette lighter socket to the battery. \$\endgroup\$ – Tom Carpenter Feb 2 '16 at 1:38
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The correct answer depends on multiple factors. What temperature is the engine, oil, & coolant? How big is the engine? How easily does the vehicle start & idle after it is started? Some vehicles start easily, but don't idle well when temps are cold.

Assuming you have a newer car, it starts easily (i.e. doesn't need lots of amps for more than 5 seconds), & the temperature is above freezing, a fully charged 14.8V, 5Ah, 4S1P lipo battery should provide sufficient current to get the vehicle started. If the vehicle isn't easy to start, then I'd used 2 of those batteries (i.e. 4S2P) in parallel. Twice the capacity will give you some more time to get the engine started.

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  • \$\begingroup\$ I do believe the question here is a bit outdated. These days a modern way to replace the lead-acid battery in a car might be some kind of a hybrid LiFePO4 and super-capacitor assembly. I don't know too much about that whole business, but it's sure to be both more powerful (with supercaps) and more safe than any LiCoO2 (Li-ion) or LiPo battery system. \$\endgroup\$ – Steven Lu Feb 2 '16 at 1:38
  • \$\begingroup\$ His question is quite specific about using 'affordable' lipo cells. I do not disagree with your suggestion as a solution, however, I do not think it is as affordable as using strictly lipo cells (and presumably a safe smart re/charging device). \$\endgroup\$ – zeffur Feb 2 '16 at 3:27
  • \$\begingroup\$ sure, i am him though and yeah i was originally asking the question and seeing how little engineering we could get by with the cheap higher energy density cells but armoring battery cells plus software is more complex probably than the use of some other software maybe with a capacitor bank spec'd as necessary so i'm less interested in slicing along just the low cost dimension of the situation at this juncture \$\endgroup\$ – Steven Lu Feb 2 '16 at 3:40
  • \$\begingroup\$ Lithium cells or batts kept under the hood of a vehicle without some kind of ventilation would probably be a bad idea also, since under-hood heat can be quite high--especially in hot climates. \$\endgroup\$ – zeffur Feb 3 '16 at 8:12

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