I use the GP2100 AA in my M8.
I drilled a hole in the bottom of the transmitter to allow access to the charge plug on the battery tray.
Its the same plug that plugs inot the Novak Smart Tray.
So, inbetween rounds, I unplug my smart tray and use that same plug to charge up my transmitter.
I think the transformer puts out around .5 - .7 amps, so its a nice easy charge on them.
They go just fine, have been using them almost two years, and the pack is usually well above 10 volts. When it hits 10, I throw it on the charger for 10-15 minuts.
It has worked out really well.