On my CineStar 6 I use a single 4S 8000 pack, good for around 10 minutes flight time hauling an AV 130 with Sony Nex 5N on it. On my Droidworx AD6 I use a pair of 4S 5000 packs in parallel, one mounted in front of its AV130 and the other behind to even out the load and get a perfect CoG, depending on the ambient temperature and what I have on the AV130 I get 12 to 14 minutes flight time from the dual battery setup and 7 or 8 on just a single 4S 5000. On my box stock MK Hexa V1 I use a pair of 2S 5000 in series and get around 15 minutes flight time hauling a GoPro HD on the Hisight II mount. on my small APV quad I use the same battery setup as on the Hexa and I only get 8 or 9 minutes due to more weight and less efficient motors.
On the smaller multis I use multiple batteries so I'm not putting a lot of weight off center on the frame, not so much for additional flight time. On the AD6 I do it for the added flight time and also for balance but with the Droidworx gear rail setup it's easy enough to slide things back and forth so dual packs for balance isn't as critical and I do often fly it with just one pack if I don't need to be in the air that long.
If I was really worried about battery failure I'd have two packs on separate power feeds to the power distribution board but I think it's easier to have a good battery maintenace routine, not overwork the packs, and retire them once they start showing signs of a cell(s) starting to decline. Once the cells won't get to full charge anymore I start watching the balancer screen during charging and when they show significant loss I retire the pack to bench test duty and order a replacement, I find it to be cheap insurance.
Ken
