Double check that the battery connections are indeed making GOOD contact with the battery posts. The best way to do this is to loosen the battery post clamps, then rotate the clamps back and forth on the posts to clean the connection. RE-Tighten the clamps and try to lower the drive again.
The best way to lower the drive manually is to remove the "cross bolt" in the aft end of the lift rams. It's about 8 inches long with a nut on each end. Just remove the nut from One End of the crossbolt and withdraw the long bolt. Use a piece of wood..like a piece of 2x4 to hammer the bolt out the other side until you can withdraw it with your hands. Hold the drive up while withdrawing the bolt so the drive does not DROP when the bolt comes out. As soon as the bolt is out, the drive can be lowered by hand.

NB
EDIT: There is no such option (as suggested in post #2) to lower the drive. If you remove the large screw in the side of the drive you will just lose drive lube oil which has nothing to do with the drive lift system.