There are 6 volt batteries and 4.8 volt batteries. If its a 4.8 volt then at a full charge it would be about 5.5 volts. There are several things to remember. Nimh batteries will drop up to 30% of their charge soon after beingf charged. a 750MAH is not a big battery. The voltages given for batteries are the nominal voltages and MAHshould vahuely tell you how long they will last NIMH are notorious for needing topping off. I think this should explain what you think you are seeing as battery problem are really just drawbacks to the type/size you have chosen. To get better performance try topping the battery off at the feild also lookat when you are replacing it look for more MAH and possibly 6 volts/ 5 cell vice 4 cell. I use glow engines which would well equate to your setup. I was getting 3 flights from 4.8v 600MAH I upgraded them to 6.0v 2200 MAHand now Iget as many flights as I want for a weekend on one charge.