I would charge them first, at 7.4 and if you have the cables for balance, do a balance charge (that makes sure each cell is the same, not 3.6 & 3.8 if you like), set to 4 amps if they are 4000Mah. Should take less than an hour and may well come off at 8.4volts, make sure you pick lipo or lipo balance from the menu before you start, you can watch the amps drop as the battery reaches full charge, I would expect a 10 minute run to be no problem, when they are back on charge you can see what you have taken out of them (Mah wise not voltage).
Do not leave in the car connected when not in use, I have seen batteries slowly discharge even if the switch is off, if they drop below about 6.5 volts your charger may/will refuse to recharge them. Remember not just a bit more power its a lot less weight in the car.
Are these hard cased lipos ?
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