The game phone battery goes down as you play the level. What happens when it gets to zero?

A prompt to buy battery refills. Not exactly cheap for a phone game. If you enjoy the game, it might be worth it. $0.99 for one refill, $2.49 for a 24hr battery, $6.99 for 10 missions of unlimited battery, $12.99 for 20 missions. With a little patience you can avoid the gotcha and wait for the battery to naturally refill. I think that's about 5% every 10min. I enjoy the game and I consider playing for free part of the fun.