I have not done the battery box mod yet so do not know if the flat pack will fit. I do not have any, just hump packs. Batteries are never really a problem with me. I have the stock ones that come with RTR's LOL, and they do fine. I also have some for my other trucks I got and they are 1400-1600 mah and work fine. Some people like the bigger ones in the 2000's or so but when I go out I bring 3 or 4 cars with me so I never run a single one that long to drain the battery dead real quick. If this is your only truck and only battery and you are going to be running for more than an hour at a time with the same truck you might want to get one over 2000, if not the others will do.