what rdb127 said
Also, you need to learn what your normal useage is per flight so you have an idea of how many flights with some safety factor built in. I saw where you are going to Spektrum and it's important to not let the receiver see 3.5 volts or less as that is where it will reboot so I think I'll be charging at 3.7 volts per cell when I convert all my stuff to A123. Vinny has been doing loads of research on this. Give him a call, he can certainly help you out as he is planning to use the AR9000 receiver he has and I'm planning on the AR9100 for the 2 40% planes and the AR9000 for everything else (since "everything else" already has them installed)