Ok,
So heres a brief overview:
I have a heli.. it crashed (boohoo) and when i got it back, fixed, tried it inside a couple of times, the ni-mh battery pack died..
these battery packs cost muchos £££ and i dont want a battery pack thats gonna die again.. well, so soon.. so heres the thought, replace it with a LI-PO pack! or liion or whatever.. the issue is, the voltage of the last pack was 7.4, so 7 cells @1.2V discharged. which means when charged the heli can take 7 cells @ 1.5 = 10.5V!
So, the problem im having is, do i try it with a 2S lipo pack? or a 3S? there's an obvious weight consideration but this is a starting block and i can always try a smaller pack etc.. but obviously because cells are 3.7V for lithium, there's no real happy middle to choose!
All suggestions, help comments gratefully received!

 

It depends , what heli do you have ?
What ESC does it use (motor regulator)
And what motor does it use ?

Ni-mh and Li-ion isn't easily interchangeable , so you might have to get a new ESC

 

A freshly charged lithium is around 4.2V so 3 of them is probably a bit too much, but they spend most of their time below 4V (search for lithium ion discharge curve).
NiMH spend most of their time at around 1.2V, by the time they drop to 1.1V it's time to recharge them.
I think 2S would be safe, but might feel a little underpowered.

 

2s will be a little underpowered which for a heli means very underpowered .

 

Just in case you are not already aware, Chris, the place to go for all RC flying questions is the RC Groups website. It is by far the most heavily visited RC website in existence, and there are reams of pages there which address battery issues, especially in relation to the many cautions that apply to highly volatile LIPO batteries.

Best of luck with your flying. I just snagged a Walkera V450D03 gyro-stabilized heli, after reading the RC Groups forum topic devoted to that affordable but exceptionally well designed helicopter, which sells for about a third of the price of a 450-sized TRex, with comparable performance and durability. After many hours of crash-free sim practice, I am ready to get this bird airborne.