ORIGINAL: Mr Cox
Forget to charge a battery and you drain the healthy one. One battery shortcuts in the air, and you drain the healthy battery. Forget one switch and you have half the flight-time etc, etc.
I'm still all for one good NiMh battery, one good switch (protected from glow fuel) and a voltwatch that is visible from the outside of the plane.
Ihave actually tested the possibility of having a dead battery drain the good one. Idischarged a battery and connected it in parallel with a good one and let it sit overnight. The low battery did not get charged at all and the good battery did not get discharged. The good battery just isn't capable of charging a dead one. Even using them to power a receiver and servos showed no issues.
Shorted cells might be a different story of course.
Forgetting to turn on one switch could be a problem too but you'd likely get a good flight out of the one that was on. the next flight you would turn them both on and it would probably be no big deal.
Like I said before, I might not run a redundant system in a 40 size airplane where weight and space were a concern. I would just use quality components and keep track of the battery condition. When it comes to larger airplanes though, I like the added security. It isn't foolproof but it can make a difference.