It is a battery that keeps the CMOS stuff going. It is not a "CMOS battery".
Usually button batteries are used - say two 3V buttons.
These were often replaced by 4 AA cells on early PCs.
They are rarely rechargeable because they should last for several years reliably, 5 or more years being typical for many modern PC with (non-rechargeable) button cells. (IE - extremely low power consumption by the CMOS clock & memory).
You can probably spot the existing button cells on the MB and replace them.