What an amazing product, for only one dollar! Reviewers on the Dollar Tree site love this calculator, and I agree: 5 stars for value and 5 for quality.
Reviewer rastakins did some deep research into the calculator, and figured out that it uses the cpu in the Sharp EL-501X. So the EL-501X manual can be used to learn to use this calculator.
The included manual is just a 2-sided sheet of paper, in tiny print, so a complete manual will help, a lot. I made a pocket from paper and tape, taped it to the back of the calculator, and keep my reference sheet tucked in it.
Reviewer kevin1320 says the battery is a pair of LR1130. However, others say these things last for a couple years. This is a 1.5v battery, so, the device probably uses 3v power.
Maybe you could replace this with a pair of 1.5v AA batteries, soldered in, and keep using it until the keys stopped working, or the batteries started to leak.
Once you figure out how to recharge any random battery, you could just recharge these built in batteries a while, until they pooped out.
Personally, I bought this on a lark, because I was tired of opening up the Calculator app. It worked as expected, and some of the features, like the STAT mode, are keeping me from opening up my spreadsheet.
Perhaps, in the future, I will estimate some areas and volumes using circles and spheres with pi, instead of using “3” for pi. The main feature I want, that’s missing, is an equation solver.