Why must we change out the  RTC NVRAM COIN Cell battery  ?    

The Coin cells fail in 5 to 7 years (makers spec) but if super lucky 10years. (seen vast fail at 8 year mark)
In the dark ages of PC's
  30+ years old, we  had this chip or a Dallas NVRAM;   Later this very tiny RAM moved deep inside the glue logic chip)
This is now gone or really moved to the Intel South bridge (ICH chip)

I Like to show a real SRAM here as seeing it with eyes, we know  that it is something real, but in fact decades ago it was very well hidden.

  • The PC Real-Time clock or it date will not keep time and the PC goes nuts. (as will Certificate check)
  • In fact BIOS can do dead, all hot keys to BIOS  dead.
  • PC only does BLACK screens. (some PC's do that)
  • The PC beeps error codes  or shows CRC text errors. or LEDs show blink codes. (any error that mentions Firmware , check sum errors or BIOS corruption replace the BATTERY NOW)
  • Some PCs act crazy  or dumb and some tell you why it fails. ( the NVRAM must work correctly or the PC will never work right again.)
  • Windows keeps showing Certificate errors. (wrong  date is this COIN battery bad)
  • If the BIOS thinks  password is set and never was used!, check this  battery first. (we use a voltmeter) 2.9 to 3.3vdc is good less is BAD.
This simple and cheap $1 battery is what is bad. Just replace it.
Do not ever install the battery backwards , as it is  in fact  POLARIZED. Plus to Plus, Negative to Negative, only. ( there are protection diodes there, but do not tempt fate)
Warning: Do not flash your BIOS thinking the vague CRC  errors are caused by BIOS FIRMWARE (bit rot), no you are wrong thinking that.
 Buy a new battery first and end your pain. (the firmware can stay good for 100+ years by design , ok? in fact microchips states 200 years on there EEPROMs)

NVRAM coin cell,  as see in most PC made today, finding it on most laptop can be a horror.

The is no need to read my full page below, on NVRAM.
Remove the battery test it with any Voltmeter, and replace it below 2.9vdc.
Some old Toshiba laptops 2001? used  soldered down to Motherboard  NiMh rechargeable battery . I will not cover this rare PC.

Do not read this page below, tech facts., just splurge and spend $1 on a new coin cell battery 2032s are common.

This is a real CMOS FET  memory cell.  This is how each bit stores a binary 1 or a 0. 
The beauty of this static RAM cell is that it uses almost no power to retain its logical state, and does not forget if the COIN cell stays good.
Adding the battery makes the static ram called  SRAM now a NVRAM (Non Volatile Random access  memory, means does not forget.)
It uses as little as 0.5uA in standby current Idd , allowing the coin cell to last a long time, PC turned off. (even in standby AC power mode,  it lasts longer)
Below is the typical MOS FET transfer curves,  The trick is not let any transistor change state, with the battery working good.
This is not our transistor ,but just common transfer curve, of the MOSFET.

Many NVRAM common lose there memory storage below 2.8vdc,
Checkout this super low VDD version SRAM from Philips Semi. (Vdd min,  1.0v !) (unlike my PC)
The actual voltage drop out point varies by PC and the diodes use to steer the battery voltage to the RAM.

CMOS RAM using  tiny chip standby power.

I have real schematics of old intel mobo here. So one can see just exactly how they are wired up.
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.
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 ,it's not rechargeable at all .
Diode keeps the COIN cell from draining dead , on the VCCSUS3_3 bus  as it surely would fast if all power to the PC was at 0vdc (line cord pulled)
The RTCRST pin resets the ICH8 if the coin is removed, but all other power needs to be removed first. 

This  below CHIP saves the BIOS settings to the NVRAM inside the ICH8 and saves RTC time and data data to the same RAM.
As you can see the battery runs the RTC clock all the time if the PC has no power  the clock goes dead (time stops just like wrest watch does) and the NVRAM loses all data it had before.
The Schottky diodes have 0.5vdc voltage drop (Vforward biased) These special diodes allow the battery to got a tiny bit more dead by 0.3 volts before the NVRAM goes nuts.
 Intel warns us never short these pins or ICH blows up;. The 1k resistor below protects ICH if the battery were to short out.. (South Bridge chip from damage)

Intel  ICH8 on this side unseen.

See power timing here) the this ICH8 above,

Desktop MOBO. The cell here is an industry standard part. child's play easy to replace.

Some laptops use same socket above, or a slot battery socket or the pig tail batter seen below.

 Below is a Laptop COIN cell , in shrink wrap, other laptops use same socket as see directly above. 

Learn that the  Pink Bunny's battery maker and ! Duracell show the  same  5-7 year life span rule.  (china no name, knock off batteries, well good luck there, marked "Sum Ting Wong")

seen only in some laptops the pig tail battery.
If savvy and you  own soldering tools one can turn the below into the above. (easy peasy lemon squeezy.)

The laptop coin cells can be pure hell to  get to , some are easy , but  others the whole laptop  needs to be disassembled, totally.
All  Desktops are cake walk easy.\
Laptop variances.  (some even have  COIN cell only hatch)
  • Under the keyboard , keyboard out.
  • Under a lid on the bottom (aka hatches)
  • 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.  and HDD removed?
  • 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?) on the bottom of the PCB MOBO. NO 2 PCs are the same unless the ARE.

Relic NVRAM battery  madness, it's really just a battery and for 36 years,  (this section is only for folks restoring many decade old PC's)

I see scalpers selling old 1993 Dallas DS1287 in amazon on ebay, and are too old and  no good,  ! do not buy NOS "new old stock"  this old. OK? If it were me, I'd go for Cute trick below !
  Some folks (even me on my TEK scope) do this, to the chip to make the coin cell modification,  and endless future replacements. (or use this cute trick)
The Dallas is also seen on 1990 Compaq 386N and other super old 1990 PCs. (best forgotten)
I see bad guys selling fake DS1287 out of China, and some with 20 year old date codes real. In fact on fleabay I see one seller selling 28 year old Dallas chip. (pure junk that)
The real ones new are made, as seen here  Maxim Integrated Products(2010) Mouse.com sells them, new.

The Dallas (Maxim now) data sheet states :
"Each Device is marked with a 4–digit date code AABB.  AA designates the year of manufacture.  BB designates the work week of manufacture."    (1 to 52 weeks)
Some more relics below, are  circa 1987 and 1994.    nobody I know uses a PC older than 17 years old, 2001 would be the oldest to even mess with at all. Nor runs virus magnet XP.

Super old Toshiba's 2001? used a  now leaking nasty phosphate, NiMh rechargeable (blue below) battery and only recharges for 7 hours, and PC must be on to do that, wow. 
Do not buy old DS chips, 93 is way too old, many "fleabay" batteries or chips are fake, or very old. Buyer beware!
Names, used, for this battery, RTC battery, NVRAM battery, CMOS battery,clock battery, BIOS configuration battery.

Slang used here : (some are mine others are industry standards , vast in scope)
Hack or hackable.  (hacks are possible sure, ,but I  mean: if you do you risk bricking the whole motherboard,  a $200+ huge mistake for most owners) (do not listen to basement goon web posters having you find and short out chips) OK?
RAM , random access memory that forgets with lost power.
BIOS , is firmware, BASIC Input Output system. now called UEFI windows 8 up. It is the lowest level of intelligence in the system.
Batteries:  A chemical power source (Cell) that is polarized DC voltage, and can  be chargeable or non chargeable, do not mix them here. Cell here does not mean CELL phone it means battery cell, chemical.
NVram , RAM that does not forget if the battery is still good) NV mean non-volatile. (means does not forget) The ram used is CMOS static RAM..
CMOS, (Is a transistor (MOS) configuration that uses almost no power, if static. NVram uses CMOS memory cells do to that 1 fact. This MOS device is used due to the amazing low power needs, in standby and is fast (2 reasons)
MOBO, = motherboard
PCB, = printed circuit board, like your MOBO is.
Coin, means small battery that is  coin like look and sized, some near quarter sized, or tad less. see photos, above.
RTC , real time clock , no lie the chip is just like the chip in your wrist watch, it is used in all PCs made to keep track of TIME and DATE and leap years. 
The OS and BIOS sees this DATA, and the OS will take this data (time /date) and then if turned on, in Windows, attempt to sync. it with NIST.com atomic clock data in Fort Collins Colorado. (national institute of standard's (NBS old name)
If BIOS can not hold TIME or DATE accurately  the NVRAM (RTC) battery is DEAD or NEAR DEAD. (same is true for CRC errors, or BIOS configurations get lost, or boot device is gone , wrong or changes in random ways.
They key word is random, here.
If Windows sees date wrong you will get Certificate errors. 

version10.  5-9-2017