A Thunderpower 1010C will charge lipos up to 10 cells off of a 12 volt battery but it costs about $199 US dollars. It really should be used in conjunction with the Thunderpower balancer which is designed to link with the charger and send individual cell voltage data to the charger. That adds another $90 or so to your charging system although it can charge batteries without the balancer. When charging 22.5 volt nominal, around 24 volt actual six cell batteries at 4.5 amps, the charger will be draining about 9 amps from your car battery so check to see if your car will still start after a couple of chargings. I use a 115 volt power supply to power my TP1010 (more $) just so I won't have that problem, however, 115 isn't available at all flying clubs.
Note, a car battery charger won't work as a power supply unless you also have a battery hooked up to it to smooth out the DC. Some people use a power supply from an old computer.

DON'T EVEN THINK ABOUT CHARGING LI-POS WITH A CHARGER NOT DESIGNED FOR THEM!!!!