Mike, there isn't a LiPo in the DX7 normally .
It's (just) a NiMH 4 cell battery (kinda like 4 AA batteries connected in series).

NiCd and/or NiMH are pretty easy to charge, just push current in them (don't go over 1C!!! so 1.5Amps) .
LiPo's cant be charged that easy, as their performance is totally relying on the cooperation of the cells.
And the voltage per cell CANT go below 3.45v .

Bet enough about batteries, many many discussions elsewere on this forum .