@dadibrokeit is correct regarding the source of the RTC (Real Time Clock - aka cmos) battery supply in your model. It comes from the main battery.
Here are some images from the motherboard schematic showing that the RTC batt supply is from pin 7 of the main battery.
(click on images)
Perhaps you should also create a battery report just to check the status of the main battery.
Compare the Design Capacity value against the Full Charge Capacity value and work out what percentage of the Design Capacity the Full Charge capacity value is.
If it's a low percentage, the battery may be failing and would need to be replaced.
Here's the maintenance and service guide taken from this support webpage for the laptop.
Go to p.38 to view the procedure to remove/replace the battery. Search online for 761230-005 to find suppliers that suit you best.