Rcl57mcu wants a 3.5-5V supply. So, even if I could resurrect the dead BP-7 NiCd pack/lid, it won’t work for this upgrade. I will instead recycle the tried-and-true USB charged 3.7V LiPo supply that I’ve used for HP Classic, HP-28S, and TI-58/59 supply upgrades. I will 3D print a new case lid that holds the LiPo and a small PCB with the charging circuitry. So, another PCB is needed, but it certainly doesn’t merit a 4-layer board. I created a separate KiCad project with the PSU and a “debug breakout board” on a common 2-layer PCB. The latter is to make it easy to program/debug the rcl57mcu through its 2mm pitch header pins. These pair nicely onto a single board with perforations in between. Adding this 2-layer PCB to my 4-layer PCB order only cost an additional $5 and avoided  shipping charges had it been ordered separately. 

I had it in mind that the TI-57 with rcl57mcu could be operated while charging. That’s why I put a diode between VBAT and the source terminal of the PMOS LED Anode drivers - with the MCU driving the gate at 3.3V, the source voltage should be able to be as much as 1.7V higher, or about 5V, and still keep the PMOS drivers in cutoff. The LiPo charge voltage is about 4.2V, so no problem here. Physically, however, I don’t see a nice way to attacg a USB plug, so I will just put the USB receptacle on the inside of the lid - meaning the lid has to be open to charge. I guess one could still operate the calculator with an open back and dangling wires… My experience is that these classic LED calculators running from LiPo cells don’t need to be charged very often at all. 

Maybe I will look into a wireless charging solution at some point - the lid area seems large enough to easily pull this off…