What is  CMOS  ?       
You are here because, the PC  can't hold time of day (week,month,yr) or BIOS can't save configs,. or BIOS shows checksum errors in CMOS. (or all 3)
All you really need to know is, after about 5 years old, the cell will start to drop and go to 2.9vdc, (3.3 is new) and when it does this drop deal,  it is doomed, (that is the failure cliff)
The memory will fail, some where from 2.9 to 2.0vdc, this varies by who's chips you own. (in all cases) The key is NEVER LET IT GO Below 2.9Vdc.
 Replace the Coin Cell battery now, or it will get worse. (end story).
For the curious why, see below:

The correct words for this , is BIOS/RTC NVRAM  (or CMOS for short) 36 years in use, 1981 to now. (IBM PC)
In pure electronics we call this battery backed up NVRAM using Static RAM, non-volatile RAM, the coin cell creates that magic. (NV  magic) SRAM + COIN = NVRAM. (SRAM = Static RAM)
This device is seen inside all PCs for 36 years, 1981 to NOW.
Do not read this page, just splurge and spend $3 on a new coin cell battery 2032s are common. (below is Electronics real)

Its purpose is  2 fold, one is to store all BIOS settings and yours,  like for example which device boots first?, and 2nd to store the  time of day (&Calendar), this memory section is for the  RTC chip real time clock chip stores TIME in bank 2.
The reasons it's called CMOS is because, it is! (the transistors are CMOS static RAM) It uses almost no current to run and to hold data in standby.  (super super low current in standby)
Complimentary MOS transistors, This is also called a flip-flop or a bi-stable multi-vibrator. (or the basic binary memory CELL) CMOS uses floating gates. (see them)? If VDD drops too low, the gate thresholds are hit and it fails.
The transistor M1 to M4 repeats 2048 times  256x8 memory array.  In standby (PC off) the M5/6 are off, and the gates all hold there charges unless VDD falls to low.
This  memory cell stores is 1 BIT.  It uses as little as 0.5uA in standby current Idd , allowing the coin cell to last a long time, PC turned off.
A nice mosfet transfer curve is here.  As you can see if the gate was at 5v charge, some where below 2vdc the flip will go nuts. (toggle in a random way) The below is the typical MOS transfer characteristics.( a large one)
This is not our transistor ,ours is very tiny and has  very unique voltage response. In fact is designed to run with weak VDD. (dain) with a weak Gate voltage VGs)
The beauty of this device,  is it can run with Vdd as low as 2.0vdc( on average), (Vdd is coin cell voltage , minus 1 diode drop) the second beauty is the super low current in standby,  really nothing beats this.
When the PC is on  VDD is 5.0v
Checkout this super low VDD version SRAM from Philips Semi. (Vdd min,  1.0v !) (unlike my PC)
If you let the coin cell go bad or below 2.9v, the memory above CELL forgets every thing it ever knew, and the BIOS ROM and the RTC goes nuts, (scrambles) loss of time, data   and CRC errors abound.
 The BIOS may tell you BIOS corruption because the NVRAM is now corrupted.  (DO NOT FLASH YOUR BIOS FOR THIS SILLY COMMON FAILURE OR RISK BRICKNG YOUR PC)
This means the battery is dying fast, below 2.9vdc on the battery , means it's on the CLASSIC  cliff of total death. Put in a new coin cell, end this minor horror.
This can happen every 5 years, buying a top makers COIN is a smart idea.  (avoid no name batteries as seen on fleabay for $1 , like the brand "SOM Ting WONG")
Buy the battery with the cute bunny rabbit? (avoid NOS, new old stock on any battery) I like to get mine  at Walmart due to fast change over in stock ! (read FRESH)

CMOS RAM using  tiny chip standby power.
That lets the COIN last for 2-5 years  (some can go 10 years, but is not ever for sure)
RTC = Real time clock, it works just like your wrist watch clock chip, I runs and stores the time of day,M/D/Year in the NVRAM.
In modern times we don't use  a simple single chip NVRAM now, we use a huge glue chip, below. It's still there, just hidden deeper, like in  the huge Intel ICH10 chip or newer...

THE Intel, ICH8 BGA chip, Glue logic complex.  I/O Controller Hub 8 family, All my laptops work just like this only each year newer more powerful chips and USB 3 added.  AKA the SOUTH BRIDGE CHIP.
This chip has the RTC modules inside and the NVRAM bank inside called BIOS CMOS NVRAM, there tool and  coin cell battery, yes Intel even uses the word COIN too.
The Intel chip VCCRTC seen here has voltage spec of 2.0v to 3.465vdc.  We all know less that 2.9v is a bad battery. See my marks in red, my HP has the same setup exactly see those low Vf drop diodes.
The D2 diode will have like 0.5v Vf drop so 2.5vdc is Vbatt  min.
The 2 Diodes are current steering devices,  the lower diode prevent the PC power rails from blowing up the coin cell ,its not rechargeable at all .
Diode keeps the COIN cell from draining dead , on the VCCSUS3_3 buss as it surely would fast.
The RTCRST pin resets the ICH8 if the coin is removed, but all other power needs to be removed first. (Main batteries, and PSU /Power packs turned off.) and let stand awhile to discharge all CAP.'s.
The BIOS ROM connects directly to this same chip via direct way (relic PCs) or SPI serial way as all new PC's do. 
This CHIP saves the BIOS settings to the NVRAM inside the ICH8.

Intel data sheet, snap shot...

Intel told you , "Never short these pins, Vbat, or VCCRTC ever. "
No pins on this  page can be shorted.  (remove the coin cell, and the Laptop battery and the line cord and then push power, button, now  it is reset)  (same on desktop no huge battery there)
See power timing here) the this ICH8,

Desktop MOBO. The cell here is an industry standard part.

The laptop has this coin plugged in to any where on the MOBO, varies by all laptops made , exact spot. The connector is unique to all brands of Laptops.

The laptop coin cell can be pure hell to  get to , here are some HP examples. (all makers do this same malarkey)
  • Under the keyboard ,keyboard out.
  • Under a lid on the bottom
  • Under the lid, on the bottom for ram but ram pulled to reach it. see mine below photo.
  • Under or near the HDD bay lid off.
  • Under the blue tooth hatch.
  • Under the Wifi lid hatch.
  • Now the bad one,  remove the screen, top of case and mobo, and only now can you reach it. (sux no?)

version 2.  5-9-2017