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http://nsrca.us/forum/index.php?topic=167.0
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I have well over 100 flights between two airplanes. Works great. Imagine never charging a receiver battery again (only your backup once a month) . . .
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Just don't try to run two regulators off the same motor pack, they will have a common ground and the magic smoke will escape. You want a completely separate backup system, both electrically and physically. . .
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Thanks Keith for the pointer. I suspected using only the motor batteries amounted to having a single-point-of-failure condition - but I had not thought about the "battery ejection" scenario! And the "common ground" issue did not resonate either. I really like the idea of having a small mah 2s to backup a 2-cell tap off the motor batteries. And thanks Arch for providing a 2nd opinion vote of confidence for this method!