Our team bought a spare battery last year when we were rookies. We didnt actually need it, but its paying off this year. Because of the pushing and pulling nature of the game, we used up more battery power than bottled water and mountain dew combined. Remember that if the voltage drops below 8 volts the radio is turned off and your robot disabled for the rest of the match. Yes, it happened to us our first round.
*cough*sears 12-volt 6/2 amp manual battery charger*cough*