This is a mod that I made to my Ti-84+ calculator. I took the idea from here.

The battery pack that I used was this one that I bought at a Radio Shack for 15 $ US.

I started cutting the PCB and soldering wires to it to make it as small as regular AAA batteries. Even with it removing some plastic part of my calculator was needed. Then I turned on my calculator to see if everything was OK. It started without problem. After gluing it and making holes in the case I tried it at school. It was working fine. Even if the autonomy is the half of regular AAA batteries, I can charge it really fast and using it while charging.

Edit : I added an on/off switch to avoid wasting batteries and to restart my calculator when crashing.