When you start to get into the 40 sized territory with electric planes they also tend to get into the expensive battery prices also.
A .40 sized motor usually require 4 to 6 cell packs as a minimum, and of that size you'll be looking at a pack with 5000 to 8000 mah as well.
A 3 cell pack will definately not be upto the job with motors of that size, 4 cell packs are a bare minimum.
For a 3D .40 size then you'll probably easily be looking at 5 cells upwards.
If your serious about going into electrics of the .40 size and maybe above, then I would buy a charger that can handle as many cells as you can afford
The more the merrier really.
But I would probably get one that can handle at least 6 cells.
But on the other hand, you are not forced to buy the larger 6 cell packs [sm=confused.gif] ................
[sm=idea.gif] You could do it a cheaper way by keeping your existing charger, and say buy two seperate 3 cell packs to run a 6 cell plane by wiring them together in series with the gold connectors when they are fitted inside your plane.
This also has the advantage of you being able to use the 3 cell packs seperatley in other planes that only need 3 cells, and also if one battery pack gets damaged by itslef or by mis-use, then you also haven't wasted a very expensive single pack.
Hope this helps you.