One more thing. I use a pair of 6 volt (5 cell) NiMH 1200 mah packs in all my larger planes.. larger meaning anything 60 or larger. I also use a single 6 volt pack on smaller planes. The reason I do this is because I am using the Spektrum DX7 radio and consider it "insurance" to have the 6 volt pack so that I don't go near that re-boot threshold that caused problems in the past.

I know.. it's been fixed. However, I still use the 6 volt packs.

As Ken pointed out, each battery has it's own switch wired as shown in his diagram. This puts the battery packs in parallel. If one should fail (personal experience with that, by the way), there is always a second one there to complete the flight. I never even realized there was a problem until I landed and checked the voltwatch. So, I also use and endorse Voltwatch. I always know the status of my battery(s) before and after every flight. Any indication of a low voltage sends the battery packs to the charger. No if-ands-or-buts about it. Simple enough, and I believe, a good policy.

CGr.