If its the same stick batt that the AK takes then it should be 8.4v not 7.2. The sticks are either 600, 1100 or 1400mah and should last a lot longer than 10 minutes. On an open day skirmish I would use 3-4 of them if I was being trigger happy...are you sure you are charging them for long enough in the first place.
I think the formula for working out charging time for a batt is: battery ma/charger ma=charge time. So if the battery is 600mah and charger is 60ma...600/60=10 hours charge time. Hope this helps. 
If you are referring to the Ultima Shoei 44. I have an 8.4v 1400 mah stick battery fitted (it just fits without any mods to the stock Vs the 600mah quoted in Ultima's booklet). If your compartment is a bit on the short side you could always extend the bore hole into the stock a little.