I have (2) 4000 mah Duralites installed in mine. THe Duralite batteries already have redundancy built into the pack. You will notch the battery box further ahead of the tick marks that are on the top piece. I took another piece of light ply and built a new top piece that sits about 1/4" above the existing battery box on some standoffs, my batteries in turn slide into the notch cut in the original top and are held in place by the "new top" this allows for easy removal so I can swap out the batteries and keep on flying. BTW I get about 6 flights on the reciever and ECU batteries then change out.