IDX,
I believe it's more specifically a CR1220...for the NVRam. (real time clock and other saved settings)
Those coin cell/watch batteries are typically primary (non-rechargeable) lithium, and do
NOT get "constantly replenished by the main".
They should last about 7-ish years....plus or minus.
You can see a picture of it in this thread: https://www.findmall.com/read.php?86,2208946,page=3
Serious note:
That main board is in the under-arm compartment.
And when you open it up, all semblance of 'guaranteed water-proofing' is now out the window.
Yes, you might get it back together and have it perfectly sealed, but you should assume any remaining warrany just flew away.
If you want to replace it, I'd suggest you first:
1. Wait until your warranty is up.
2. Make certain you feel qualified to work on a computer (that's what it is).
3. Accept all the responsibility for possibly creating an expensive door-stop.
mike