Hi. To further amplify what Allan was saying, I use a two battery setup on all my larger planes now. Especially after what happened with my Venus II pattern plane. I was flying, oh, three flights if I recall, and decided to check my batteries. I have a pair of 1200 mah NiMH battery packs in that plane, by the way, each on a separate switch and into a separae RX channel. Anyway, I checked one battery and it was around 6.2 volts. Fine.. another one or two flighs here easilly. I checked the second one... first the voltmeter flashed a few times then went blank. I thought it was the voltmeter, so I borrowed one from soneone else. Same thing. So, I removed the battery pack and checked it 'off-line'. It was definitely bad. I had a spare so I put that in, but the battery, only a few weeks old, was dead.

Lucky I had two batteries in that plane or I would have lost it.