I've been meaning to do this post for sometime, I bought a rechargable battery pack for my Nintendo Wii from gamestop a few years ago, it never ever charged therefore the batteries were never used (to my knowledge anyway) when I plug it in the light on stays on only for a few seconds then goes, I bought the set brand new at the time but never had any luck and I kept it hoping I could get help with it especially since I prefer rechargable batteries over AA batteries
The storage life of batteries in these packs can vary. The batteries in the pack are essentially dead by the sounds of it, it just happens in my experience, especially if you don't use them for a long time. You can get rechargeable AA batteries, I've used these in my Wii Motes / 360 Controllers for years, they have generally lasted a lot longer than any sort of battery pack.