Actually, since I was using my first heli, DF#4, as spare parts (crappy machine) I had 3 stock 7-cells from that one.

I split 1 battery up and added 2 cells each to the other 2 packs making 2 9-cells packs. Confused yet?
The DF#4 has those green 7-cell battery packs. Configuration below.

O O O O
O O O

After adding 2 cells to each one they look like this:

O O O O
O O O
O O

The weight of 1 extra cell is no problem at all.