The batteries I am using here are Canon BP-915 or Canon BP-911 camcorder batteries as I use them in my projects for making handheld portable video game console systems. The principles and methods are however similar for other makes, brands and sizes of cells.
Li-ion batteries, like ones from camcorders, are made up in multiples of 3.7v cells, so for a 7.2v or 7.4v battery pack, you actually have two cells inside, for 11.2v you get three cells inside.
As long as you are careful, it is quite easy to segregate the cells and rewire the batteries, keeping the battery protection circuit in place, to stop over and under charging. Also in this guide, I cover how to connect the battery terminals to a charging jack.