I opened up my slim silver ps2 scph-90001, can't find the cmos battery anywhere. It's not under the fan, it's not under the motherboard. This is the American silver version. I have the replacement battery I just don't know where to put it.
The PS2 shown in the guide is a European model (SCPH-90004) which has a CMOS battery. The guide mentions that other region models may not have a clock battery which could be the case on 01 models (Northern America).