Well, since you asked..... You might have posted this battery question in the BATTERIES AND CHARGERS forum.

But since you also asked about the value of 6V..............

If you're trying to use servos that might not be adequate in power for your application, one way to get more power out of them is to run them on higher voltage. If the servo can stand the extra available voltage they'll give you more output torque. Of course, they'll also wear out sooner and if you've got a less than good installation that stalls them, they'll be more apt to burn up.

They also move faster with higher voltage available. That is going to be harder for you to "feel" the benefit, however. You have to be rather familiar with your airplane and it will have to be one you feel is responding a bit sluggish. Very often that's because your servos aren't powerful enough, but it can also be because they're not fast enough either. Power and speed often give the same symptoms.

Not all RX nr all servos will stand the higher voltage. It's a good idea to check each component to see if it's capable of working under 6V. If you've ever read the outside of a Hitec servo box, you'll see an example of that kind of info. They list the performance of all their servos that handle 6V. They show the 4.8 specs and the 6V specs. Kinda obvious that they say their servos will take 6V.