I've coped all this season doing one charge all day (so has been on a normal car battery). I used a 5500mAh 5S pack - went out in heats with 1 minute to go, and came in when I'd finished (1 or 2 'cool down' laps). Then I'd charge at 3C at lunch, and easily be ready for finals.
Alternatively, have two batteries!
__________________
|