I have not found a schematic of the VB battery cartridge on the internet, so I have drawn it.
It is possible to find the ROM and SRAM datasheets on the internet, however I am not able to find the 577 (U3), which is the battery manager for the SRAM, although I know it is from the manufacturer Microchip, their old logo appears on it.
This some quick notes from top of my head. The ROM is based on common circuit 5V EPROM pin/outs from that era. I got some datasheets that reflect Nintendo OEM chips closely. Let me know and can email them.
The SRAM is very common/standard pin/outs. Can simply google data sheet for part number on OEM chips. Got a few of these p/n and datasheets at my end can email too. Modern day version is the “AS6C6264” which a drop-in replacement.
For the “Automatic Battery Backup Switch chip”. That little tricky, I find different manufactures put a new slight spin on this. You have original Nintendo OEM versions. You have modern day version. And different way to wire them up. Then you have nvSRAM and don’t need altogether. If the goal of this project opensource for any builder you can make compatible for original Nintendo OEM 4 pin, modern day versions of chip, even the nvSRAM, one design/PCB to rule them all, etc.
I’m more fan of modern day “Automatic Battery Backup Switch chips” as they are much more reliable for save data, but with chip shortage they are not as easy to get. I actually cleared out one supplier complete inventory.