Again, the problem may be- a few of the early PS2 3V systems used AA or AAA battery holders rather than battery packs. Further, those connector systems may not have a modular plug mid harness for the newer "pack" style batteries that terminate in a connector.
So, you may find yourself either soldering or splicing connectors to take an off the shelf "BCR2" https://www.jandwelectronics.com/bcr2
designed for PS2 3V systems into the existing wiring going to that battery tray system.
A solution would be to buy the matching newer PS2 3V charging harness that would plug between the PS2 board and your BCR2 with no soldering. Find that harness using "charging" and then refine with "3V" https://www.mthpartsandsales.com/shop/search