I just talked to Hitec customer support about this, here is what they had to say.

Hitec rates their servo for 6 volts, they mean for use with a 6 volt pack which is usually around 7 volts when fully charged. Their reasoning is that the batteries will quickly discharge down to something less than 7 volts. OF course they have no way of knowing this for sure.

This area between 6 and 7 volts is kind of a gray area, since the voltage actually being provided to the servo under load is pretty difficult to ascertain, at least it was until I used my data recorder to track what is happening.

If you look at the graph below you will see that a 6v Nimh pack varies between 5.6 and 6.6 volts under load (Nimh is the first half of the graph, the second half is Liion).

Now, looking at it from Hitec engineer's point of view, they cannot really depend on the voltage depression, I could be running just one 5955 on 2 fully charged 2700 nimh packs (like when doing a knife edge) and the packs would certainly remain at around 6.5 volts.

So even though Hitec says 6 volts, they know we are using more and either did, or should have, designed for 7.

Their official story on using more than 6 volts is that it will work fine and cause no immediate problems but over time the more power you put into a servo the faster it will wear out. This again is kind of a non-answer. The bigger problem happens if they get into a bind or overlaoded, in that case higher voltage will rapidly shorten their lives.

I have been using Hitec servos on 6v packs for many years without any failures that could be attributed to over-voltage. Some guys even run Lipos without regulators and claim they have had no trouble. I feel that the 6.6 volts of an A123 is well within the admittedly inferred design limits of these servos.

There can be a lot of reasons for a servo failure, most of the time it comes down to being in a bind. If the motor can move and do what it's supposed to do failure is not normal even at over-voltage.

I lost two servos a few weeks ago when I was changing from the DX7 to my new 9303 in a plane that had been flying for many flights.

When I changed to the new transmitter I began with no setup, just starting from scratch. I worked on the rudder and elevators first, then the ailerons. All this took me about a half hour to read the manual and learn how it all worked. What I had forgotten was that both the throttle and choke servos were set up on the old transmitter with limits on their travel, and with the default settings of the new transmitter they were sitting there commanded at full throw and binding against the travel limitations.

By the time I got around to setting up the throttle and choke servos they were both stone dead, burned out from being in a bind. Not their fault, not over voltage, just bad planning. If I had set the travel on both to zero while working on the other settings I would have saved myself fifty bucks.

I meant no criticism of n57tc, everyone has hot buttons and things that they believe. I personally believe that regulators cause more problems than they solve and I will continue to recommend that A123 batteries be used without them.

TF