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Emergency charging car battery with 4 li-ion cells, possible?

Hello, I'm trying to make a DIY emergency charger for my car battery. Is it possible to use 4 li-ion cells in series? That will be 4x4.2volts at full charge of the cells. I think i should limit the voltage and also the current with some sort of circuitry. Considering that I will use this when the car battery is almost fully discharged, there will be a huge power draw. Any suggestions?

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107
killerjackalope says: Nov 22, 2012. 10:49 AM
From what I understand most emergency jumpers are designed as a one or two shot booster, they're drawn from by the starter too and need to handle a fairly large current load.

Charging the car battery with other batteries wouldn't make immediate sense to me
rcs-al (author) in reply to killerjackalopeNov 22, 2012. 10:52 AM
The idea is that a car battery is way heavier then a battery pack of 4 or 8 li-ion cells (4x2). So i can have a portable power bank always available and that i can charge using li-ion chargers. It can deliver some amps to the car battery that should be enough for one start.
107
killerjackalope in reply to rcs-alNov 22, 2012. 11:23 AM
Well 16.8 volts is enough forward voltage for a quick charge but I don't know if there'd be enough amperage to do it effectively, some kind of charge controller would likely be needed to make it work...
rcs-al (author) in reply to killerjackalopeNov 23, 2012. 2:25 AM
The lead acid battery drains at least 5 amperes when I use my mains charger to charge it when it's empty. I think it would be risky for the li-ion cells to discharge at 1 or 2 times their Current/hour or more, or am i wrong ?
107
killerjackalope in reply to rcs-alNov 23, 2012. 3:48 AM
Maybe - I'm not sure what the score is with li-ons... You'd likely find the answer in some of the instructables on building electric scooters/bikes/cars...

Still I think fixing the original problem - bad battery (can't hold a charge) or faulty alternator (might be charging some of the time if it's not dying often)
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