Reciever battery, Look at your servo's and figure out what there 4.8 volt tq is Verses there 6V tq, it makes a difference in there speed also. You shouldnt need a field charger if you charge you batteries the night before. I suggest a electric starter for new engines, It just saves alot of time for initial tuning and setup. The initial costs are high but after being in the hobby for a while you will realize you have extras of this and that making things way cheaper down the road. Good luck