Technically the CMOS backup battery is located just underneath the drive bay on the top side, the one for an optional
DVD drive. However, the battery is not directly accessible, and changing it requires dismounting of the motherboard:
Easy:
Flip the Q920 upside down and remove the plastic cover on the underside. The HDD/SDD and the RAM are accessible from this side too. There
are 4 screws visible on the underside along the perimeter: Three Phillips screws are sunk inside black plastic cylinders and the 4th is
visible directly in a corner. After unscrupulously unscrewing them and lifting out the metal frame, the motherboard can be tilted and
removed upwards (see photos). It takes a little careful tilting and fiddling to navigate the DVI and USB/audio connectors clear of of the back plate,
but there's just enough space.
The round CMOS battery and its socket are easily spotted next to the CPU cooler and its copper heat-pipes.
The CMOS battery is of the standard type CR2032, as in most PCs.
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Notes: Contrary to the images, the perimeter black metal framing and the top plastic plate can stay in place. The aforementioned 4 screws are
the only ones involved in the task.
You may want to do this on an anti-static mat to prevent damage to the electronics.
There's a Fujitsu forum (current as of May 2023), where this question
had been posted in 2019, but w/o a reply.
There is a guide for the motherboard itself available by Fujitsu, but no diagram of the mechanics
of a Q920 (current as of May2023).
Hope that helps, best wishes, have fun.