I have my pf battery box hacked with 2 lipo cells serial connected (3,7v x 2 = 7,4 volts) and i have no problem with them, for my was easy to do it, it is simple to open it and get the connection cables from batteries to the on/off switch.
BUT, Lipo batteries are risky if you overdischarge them, so you have to take care or put a lipo alarm (in rc there are a lot of cheap alarms for lipo batteries), i put a connector directly to the batteries to charge them, it is a better option, and i charge them with a rc charger