The TI 89, like most calculators, uses AAA sized batteries. These vary from 1.5v in the fully charged disposable variants, to 1.2V NiMH rechargable batteries. I wanted to replace the batteries with a rechargable solution that could charge while the device is being used.
Conveniently, 5v fits nicely within the 4-6v input range of the calculator.
Step one is to acquire the parts. Apart from wires, tape, superglue, sidecutters and a soldering iron, you will need 3 components: The lithium polymer battery, something to charge it with, and something to boost its 3.6-4.2v to something higher. Some very low power calculators might be able to get away without boosting the voltage.
For the battery I bought a 420 mAh 3.7v LiPo battery off of ebay. Something that looks like this should do:
For the charge controller, the best option right now seems to be the TP4056 based charge + protect circuits. Make sure you get the one with both charging and protection (these will have a seperate output and battery terminal pair):
And for the boost circuit, get a tiny 5v one with a USB plug, these are very space efficient and you can remove the USB part to get a smaller size. Make sure you get one with no circuit board under the USB plug so you can save on that little space:
Dremel out the insides of your battery compartment on your calculator. For me, I removed the parts directly below and on the side of the battery, as well as the part that holds the button cell. This is what it looks on mine:
(Ignore the fact that I bypassed the battery protection, I fixed this later)
I really wanted to tap into the 5v near the top (off the USB port) but I really didn't want to risk my device after I had already destroyed one, so I played it safe and dremeled out a second usb port on the bottom.
Here is how the devices should be connected:
If you have done everyting right, turn your calculator around and it will be on and ready to go. If you can fit the stuff in less space than me, you don't need to remove the button cell. I would also add a switch to cut off battery supply if needed.