I'm not too familiar with the schp-9000x series, all I know it's that it's more work to square them off that's why they are not really used in portables. I would probably advice to get a SCPH-7900x model if you can which is quite well documented on this forum.
If you still want to go with that model we will probably need some good pictures from the front and back of your board along with some info on the battery you are trying to use.
If you still want to go with that model we will probably need some good pictures from the front and back of your board along with some info on the battery you are trying to use.
After researching here in the forums, the user Luis Mendoça shared this photo in which he shares how he made the connections on a console of your same PS2 model. Apparently, it is enough if you connect 7.4v to the motherboard's input ports. It is not necessary that you connect the four cables, with one positive and one negative is sufficient.