My nephew has that Nyko charger. It's a big waste of money if you ask me. It takes forever to charge, about 8 hours. It uses also uses NiMH batteries, which I'm guessing is what you are currently using. I'm 90% sure they don't make rechargeable lithium Ion batteries in AA size. Like mugs said, if they do, the voltage would be double.
I would get more rechargeable batteries if I were you. Energizer now has rechargeable AA batteries with 2500 mAh per charge. They discharge at a rate of 0.5% per day when not in use. Thats pretty good, because many NiMH batteries lose about 1.5% per day.
The problem I've been having is finding a charger that can charge them to full capacity. I saw a combo pack that Walmart had, that included 4 2500 mAh batteries, and a 15 minute charger. The charger would take 19 minutes to charge the 2500 mAh batteries, but that beats the hell out of 8 hours.
It was $30 something, but I was in NC at the time, and didn't buy it, thinking I'd just get it when I got home. I haven't seen it since.
EDIT: You might also want to try Sanyo enloop rechargeable batteries. They only come with a 2000 mAh capacity for AA, but they claim to retain 90% of their charged capacity after 6 months, and 85% after a year. I think I'm going to give them a try.