Most Bios Battery are a CR2032 <-- This is specification of the battery. Your motherboard is this.
It is 3 volts lithium battery. Typically lasts 5-10 years unless you turn off computer at mains power each night, in which case that time can be low as 2-4 years.
I had wondered why you were asking given they are easily visible on most motherboards, but yes yours is under the armour (yes, you have to remove it). You will find it below the cpu to the left.
Buy a name brand CR2032 a cheap one can give no end of special problems. When motherboard start doing anything funky this is first thing to change out and the last thing a lot of people think of when dealing with random strange events starting on there ageing computer.