Most motherboard batteries in Laptops are NOT rechargeable...)
The battery on the motherboard, otherwise known as the CMOS battery, are rechargeable, in that they charge while on the motherboard, not in that they can be removed and recharged. This is the same for all CMOS batteries on desktops and laptops.
The reason that they are rechargeable is that they are there to provide a constant charge to the CMOS chip while the computer is turned off, as this is not provided by the main laptop battery. If they were not rechargeable then they would not be able to perform this duty for more than a few months, and because of this it is important to replace it with an appropriate battery, and not just any button cell.