battery 4 backpack w/speakers mod! need some help newbie

[dadeisgone]

hey i need a little help ive never done anything like this before but i like figuring stuff out and trying stuff, but i definitly am gonna need a lot of help!!

so i have a skull candy link pack its a back pack with speakers, and it takes 4 double A batteries what i would like to do is, replace those batteries with maybe some type of rechargeable lithium battery that will last a lot longer and i can recharge it, maybe a rc car battery or anything really, fresh batteries last about 2 hours in the back pack and rechargeable double As last even shorter

so if anyone could help me that would be amazing i will be posting pictures of what im working with very soon but i like to plan stuff before hand so any ideas or help would be greatly appreciated!!!

[exploding.monkey]

What are you trying to make?

[dadeisgone]

im trying to mod the battery setup, i have in this audio back pack