Dave: Also bear in mind that if there is a given probability of failure for one battery, flying a 4x battery configuration multiplies the probability of failure for your system by four -- it does not reduce the probability of failure for the system as a whole. There is also the question of what happens when a LiPo fails? Does it drag down the voltage of the other batteries? There is one reported case of this on the forum. Perhaps the way to approach this is to use a large diode on each battery so that when forward-biased, it allows current to flow out of the battery, but when the battery fails it will not allow the failed battery to drag the other batteries down. The issue there is going to be (a) finding a diode with the current carrying capacity and (b) dealing with the 0.7 volt drop across the diode when it's forward biased. Just some thoughts to ponder... Andy.