What is the battery on the motherboard ?

It's the CMOS battery. It provides power to a memory chip that stores data containing the time, date and hardware settings critical to the BIOS. Without power, that chip will revert to default factory settings.

Cr2032. usually works without cmos battery but not always, dunno about newer models

Usually, The PC should continue to work if the CMOS battery is dead or dying but the BIOS won't save any of it's settings. Also, the system time will be lost.

Usually Desktop PC's use a CR2032 battery, but not always.

The battery saves the settings for the BIOS chip.

depends on the mobo. see maker's website for diagram, using your exact model number. if not know model number, run DxDiag from command prompt .. mobo model is on first page of output