The battery cover disassembles via several metal clips along the edges of the battery housing. They can be easily popped up using any small flat tool.
The factory battery uses two 3.7V cells. Note that the RIGHT tab on each battery represents the POSITIVE (+) lead. This isn't indicated anywhere other than the back side of the batteries themselves.
Removal of the old cells was fairly difficult. They are glued/double-sided taped to the housing. I found that stuffing a credit card or similar item under them to pry them up was the easiest way to remove them, and still managed to tear the foil on one of them, so please be careful and remember to recycle the old cells responsibly.
After de-soldering old leads I checked to see how the two batteries were wired together. They are in a simple parallel configuration which meant I could replace them with a single 3.7v cell.
I used a Cameron Sino 4000mAh CS-KUC130SL lithium ion polymer pack, but really any 3.7v cell that will fit in the housing will work. The maximum size would be around 120mm x 58mm x 5mm, give or take.
I carefully removed the battery labelling to de-solder the battery charging circuit and wire the leads directly to the OQO charge controller. You can run the wires to either bank of terminals or split the difference as I did, whatever is easiest for your particular battery pack terminal locations.
