Personally I use LIFE, you can use any flavor, A123 or the LIFE flat packs like the Hyperion or the batteries Tower sell. I use them with no regulator, any 6v servo will handle the 6.6v and the batteries drop to this voltage as soon as a load is applied, perfectly safe and reliable. These batteries are easy to charge, do not have any memory effect like NiCd or Nimh and suffer extremely little self discharge, they will not burn your house down either. That is unless you abuse them, but that can be said for ANY battery.