you can charge life packs on lipo mode,we've been doing for almost a year now with zero problems,life cells are alot more durable than lipo cells
as soon as you turn on receiver pack the nominal voltage comes down some and stays there the whole 8 minutes in 12th scale and if oval 4 minutes with good voltage to servo's thru entire run.
but you can get a small life charger pretty cheap from amain like a few mentioned