So far the data is less than encouraging (at least for us fans of 7.4V LiIon packs). I started out at 7.2V, the nominal voltage of a 6 cell NiMH or NiCd pack, and just under the nominal voltage of a 2 cell Lithium Ion or Lithium Polymer pack (my favorites). The motor was dead after one hour and 44 minutes. Not temporarily overheated, not slowing down, dead dead. When I took it out of the gearbox the shaft still spun freely, but it wouldn’t turn under power, even with no load, and here’s why:
In the picture above, the bottom motor was never used at all (sacrificed in the name of science!). The brushes and commutator (the brush-contact barrel) are pristine, shiny, and well-lubricated. The middle motor is one I had used for some amount of time, but only ran at or below 3V. There’s some grit in it, but the commutator is still smooth and the brushes are in fine shape. The top motor is the one I ran at 7.2V. There are grooves maybe 20 thousandths of an inch deep on the commutator, and the brushes are seriously damaged, by which I mean that one brush is just plain gone! There is also copper dust spattered all over the inside of the casing. Less than two hours earlier this was a brand new motor, just like the bottom one in the picture. Wow.
I just finished testing another motor at 6V, which did a little better, lasting for five hours and 28 minutes. It now looks the same inside as the 7.2V motor. It’s even the same brush that’s missing (the one I had connected to power, if that makes a difference), which leads me to believe that the damage is caused by arcing, and more severe in one direction. If you run your motor in reverse some of the time it may last longer, but both brushes are damaged so it probably won’t double the life. I don’t think the wear is caused by friction directly, since the 6V motor lasted for almost three times the number of actual rotations of the 7.2V motor. Anyway, here’s the performance data I have so far: