A short while ago I got a 1976 TI SR-51-II calculator from a thrift store, without the power supply that goes with it, which is important because the calculator uses its own proprietary NiCad battery pack, which has long been dead. While I partially got the calculator just for looks, it would be nice to actually be able to use it.
The first youtube video that pops up when you search this calculator on google is someone modernizing this calculator by modifying it to use phone battery. The connector for the battery in the calculator is simply 2 pins for positive and 1 for negative, similar to plugs used for fans in desktop pcs of today. I also remembered I had a breadboard kit laying around from my first semester of college, with a power supply and wires that fit in the connector. All I needed to know was the voltage needed, the comments of the youtube video mentioned that the original battery output 2.4V, while the phone batteries used in the video output 3.7V. My power supply has a 3.3V output so I was good to go, no need to figure out any in between resistors as if it was a higher voltage supply.
After connecting everything, and flipping the power switch, we have power!
Video of the procedure is in this twitch highlight, if you are wondering why I say some of the commentary I do I am talking to my stream chatters. Also you will probably want to skip around a bit in it.