Help! Charging from 12v car battery at track?

[kaszal]

Please excuse me if this is an obvious question. I'm getting back into racing (1/10 off-road electric) and I wondered if my the battery in my Ford Focus has sufficient power to top up my rc batteries for the day e.g. practice, 4 rounds and (hopefully) a final? Will it make no difference to the 12v battery or will I be stuck there asking for jump leads at the end of the day? I'm just getting back into it so would rather put off spending £50 or more on a 12v battery and charger. Any advice appreciated.