Highly recommended Sanyo Eneloop technology NiMh packs. The Eneloops retain their charge a long-long time, as their self-discharge rate is very low. The Hangtimes link has 'em.
A 2000 mah pack goes quite awhile between charges.
I've made homemade packs using Kodak PRECHARGED 2100 mah NiMH cells purchased at the local WalMart for about $10 a 4-pack. Two packs, some solder, and a few minutes of my time, instant long-life, low-discharge-rate TX battery with 2100 mah worth of capacity.
The Precharged cells are based on the Eneloop technology, so their self-discharge rate is low, they'll retain about 85% a full charge after sitting on the shelf for a year.
The transmitter low-voltage alarm remains calibrated, and your wall-wart charger will still fill 'em up when you need to.
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