I have seen from testing a number of packs from different manufacturers that simply the voltage alone doesn't tell the whole store. A pack with a high internal impedance might show a lower than expected voltage under load, where a pack with a low impedance will show a higher voltage until it is nearly discharged. Doing some testing on your batteries is time well spent.

Be sure you have more battery than fuel though. I was flying a friends Cub when the throttle servo linkage failed at a fairly low throttle setting soon after takeoff. It had a large tank. We took turns flying it for about 45 minutes before it ran out of fuel. The landing was uneventful, but it would have been different if we had only had 30 minutes of battery left.

LarryC