The CMOS battery is rechargeable and it’s getting charged when the laptop is plugged into the mains. On the picture below you see a basic coin cell CMOS battery. Basically, it’s two coin cell batteries but they are bundled together and have a cable which plugs into the system board. On the next picture you see a CMOS battery which is soldered to the system board. In order to access and replace the battery you’ll have to disassemble the whole laptop.