Well the 2 cell Li-ion I bought lifts the heli about 6 inches off the ground. Its 7.2v though not 7.4v but thats not much difference is it.

I think it is because the current draw is too much for the LI-ion but the original 8.4v Nimh batteries cope ok. I checked my flight duration and I'm only getting 3-4 mins of good hovering before the heli loses power then it takes an hour to safely charge them up again so I was looking for a bit longer flight time than that.

Quaker1 seems to be getting good results from 3 cell so Im going to try it- with extra heatsinks in place of course

Just to add when measuring with a meter:

Nimh 8.4v = 8.9v when flat ( heli won't lift off ground/tail motor erratic)

Li-ion 2 cell 7.2v = 7.9v when fully charged ( heli wont lift off ground/tail motor erratic)