PS2 fat vs. slim internal clock / battery differences

[azmun]

The internal battery of my slim PS2 SCPH-90000 (final revision) seems to have failed as the clock no longer works. However, was a bit surprised how its older brother, fat PS2 SCPH-35001 has a working clock. Do they use different batteries?

[Lawfer]

The internal battery of my slim PS2 SCPH-90000 (final revision) seems to have failed as the clock no longer works. However, was a bit surprised how its older brother, fat PS2 SCPH-35001 has a working clock. Do they use different batteries?

I don't think it's related to that, I have 2 SCPH-3900x, one USA NTSC and one EUR PAL, the NTSC one is still going while the PAL was dead when I first got it.