The JR DS 398/390 HV's can handle a Lipo if you want. I ran the old 388's on
a Lipo in my Supras. The LiFe is less volatile. Everyone that I have talked to that use them love them. I'm told you should "always" balance charge the 2S Hyperion LiFePO4 packs to get the longest life out of them.
The New JR servos are also much quieter than the old ones.
Originally Posted by Silent-AV8R
With the notable exception of some models of MKS servos just about any servo rated for 5-cell NiMH (6v) will easily handle an unregulated 2S LiFe pack. I run nothing but 2S Hyperion LiFePO4 packs without regulator in everything I fly. My X3.8, Aspire and AVA Pro all have Futaba S3150 servos in them, never a single issue with them (and they are rated at 4.8 volts by Futaba). I have 3 helis and two pattern planes also running the unregulated 2S LiFe packs. No issues.
I have several friends flying JR that are using 2S LiFe unregulated with the DS390HV in the fue and the DS398HV in the wings. Seems to be a solid setup.