How does my Desktop Power Supply Unit ,work? (the PSU )The PSU is sized by Watts and Ampere's ratings on each RAIL.
Power supplies , like TIME, keeps everything from happening all at once. (like the above uncontrolled lightning shows) The power supply is actually a regulator.(besides converting AC to DC)
Safety first, do not play inside the unit (dismantled) with the power cord still connected to wall A.C. or the 170volts inside will shock you very badly, the PSU is never actully off, unless unplugged from the wall.
The PSU not opened inside is safe, mostly, at 12vdc will not shock you ever but if you short the 12vdc to ground it will blow the wires to kingdom come and throw burning copper chunks in to your eyes or catch fire.
If you must open a PSU up, disconnect the line cord first, then wait 15 minutes, and if smart discharge caps with a chicken stick like seen here. (but use a 5K 5watt resistor instead (best for 170 to 300vdc or A.C)
Never trust any built in bleeder resistor in any High voltage box (device or radio RF amp. or PSU) ever. We shunt the caps with a chicken stick.
A Better DESKTOP (DT) PSU?
The #1 symptom of a dead, or near dead PSU internal FAN, same goes for up to 7 fans in servers or workstations (see Z600 spec. for huge number of fans)
If the PSU fan and all other fans are dead that means the PSU is dead or the rails of the PSU are shorted (overloaded.)
The PSU power supply you choose is for the needs of the whole PC now and say 10 years from now, even with the hope of running 150 +watt GPU cards now or later. (even gaming)
Newer more modern PSU have added features not even shown in the ATX spec. (like FULL over voltage and over current protection, note some only do under voltage on 5/12vdc, rail 2 pins ! as seen on most old cheap grade PSU's.)
A new PSU costs $15 to $50. (avoid all cheapest cost PSU's as most of them are pure junk)
The PSU is easy to test in situ or can be bench tested easy. (one good buy for PSU is the ATX12 v2.3 Thermaltake as seen here for only $35)
Question #1 how does it turn on? (if the fan is dead, the PSU is dead)
Ever clean it? (PC and PSU?)
Is the RTC 3vdc battery coin cell dead? (max life is 7 years says the pink bunny) I mention this as it fools folks, with dead PC and is not PSU caused at all.
Level one testing , calls for the hard power reset proceedure.(any desktop) We call this the long Power RESET.
Factoids on Desktop PC's
Theory of operation of the PSU and the power managment chip. ( Do not read this , as this is for understanding more complex issues, not just a dead PSU) JUMP past this now.
The PSU follows ATX rules.(and color coded wires)
The PSU is a slave, to the motherboard PMC, power managment chip, like for example the Intel ICH9 or 10. chip. (in the early days, this was ACPI, in 1996 and uses by BIOS and now UEFI new BIOS.)
This chip may be called many other things the South bridge and I/O controller Hub (or ICH) or Glue logic.(as seen on a real Intel motherboard (MOBO) here.
Later ICH became PCH, with Intels Series 5 processors. (2008 year) Nehalem CPUs up, but does all Power management now.
This chip does many things, but one small part (and complex) is the control of power. (and supports power managment logic for... see bullet list below...)
The first rule of Power managment is stand by power.
If +5vsb Stand By, rail goes dead, the MOBO will be dead as will be ICH and all wake on features of this PC. We use a voltmeter on pin 9 and see 5vdc there. good. if not , the mobo can not turn on the PSU.
The PC Case has a power switch, this does not actually pass 120vac (or 240v) to the power supply, at all, ever.
It is a 100% digital power switch (momentary closure SPST switch seen on the front of most PC cases or front side wings)
You push the button and the mobo ( motherboard ) ICH (PCH) chip saw you do that ( the pin to do that is just a port input bit) and the ICH, then sends POWER_ON (or PWR_ON) to the PSU.(pin 16)
Some PCs the BIOS can stop power on due to , a dead fan at this instant, ( the fan reads 0 RPM fans, not allowed in this model PC)
It may also have too hot sensor reading on the MOBO and fail to power on. (If say the PC just shut off overheated and is still why to hot to run safely)
The ICH (PCH) chip then and now, monitors the Power_OK pin on the PSU and the same signal from the MOBO onboard VRM (voltage regulator ) modules to see if these lines go FALSE, if they do the ICH then sets power_on line to false.
This can happen in 1 second flat , the PSU turns on then off in 1 second. (I most cases you have bad PSU or a shorted DEVICE connected to the MOBO, in rare cases the VRM is bad on the MOBO or shorted, parts there.
It can be shorted RAM. (CD/DVD/BR_D) HDD, SSD or a shorted Video card (PCIe ) or shorted NIC card, (PCI-e) or any card in ANY PCIe slot of any kind.
In most cases it is not a shorted USB device, as these have there own current limiters to avoid this horror, from ever happening. (by Design)
There is more see that here.
If the PSU never turns on , see the next section below.
If it PSU turns on and promptly turns off, then the PSU may have been told to do so by the ICH(PCH) chip, or the PSU saw overloads.
The PSU can not simply tell you way (some $300 PSU can) but it has no brain, no log files to read, sorry.
Only technician can find it or you playing as one. (go for it)
The BIOS can turn off the PSU, so if the RTC BIOS coin cell is below 2.9v, it is bad, get rid of it or BIOS can and will go nuts, the battery holds up the NVram , and if that scrambels it may set the power on schedules to off or set a false fan or overheat warning to the ICH and off she goes, turns off. or will not turn on. This logic for power works like this.
It's not windows doing this, windows is not even running yet, so it can not be Windows or if Linux , then it. (can't)
What we do first is bench test the PSU or try a spare, (on cheap PC's non gaming PCs, a spare is like $15 cost, 300w)
As seen NEXT.
End theory of op.'s:
Checks Simple. ( this is how I do the tests, briefly.
Regulation spec, (intel)
We use a scope to check ripple. (caused by BAD CAPS inside the PSU) ( some more modern PSU can do way better than this, make sure the SMPS ripple at near or above 40kHz. is not present, or too large.)
All SMPS made on earth are huge noise generators, and have most time 3 banks of huge LC filters, to snub out all noise, (most) Keep that in mind doing tests.
Using a scope, 120hz or 25k to 40kHz chopper caused, ripple max. ,mV = millivolts 1mV = 1/1000th volt
All modern 10 year or newer PC use this ATX2 spec. (as shown in the WIKI and here)
Photo Dwg. #1 PSU connections. To bench test the PSU, jump pins PS_ON to any ground pin.
Buy one, make one, or just use an unfolded paper clip to jump 2 pins.
Almost no person today has PSU with 20 pins, it's so old all are now in the land fill or fully recycled.
old relic ATX Modern ATX2 with 24 pins, the 4 pin plug below is commonly used on the GPU card, etc...
The only thing you should ever connect to a pin named Power_OK is a voltmeter, set to 10 or 20vdc range. (the ICH (PCH) chip is what monitors this pin)
To see if it goes to logic true (logical 1 state) or 5vdc. (this tells the PC that power is good, if not good, it signals fail (0v) and the PC quickly sets "PS_ON" to the off state, and the PSU goes dead and silent.
THE WIKI covers all this ATX stuff , read here what pin does what?
From WIKI, cut and pasted here. One could read the ATX spec, sure. (it's a standard, unlike silly laptops lack)
-5vdc ended 2002. V1.2 up. (very old chips that needed -5vdc are now gone forever.) Intel long ago, if a negative supply is needed, in any chip, put a charge pump on board that chip, making it a 5vdc only chip then!, and 3.3v now or less)
-12vdc is mostly only needed for relic RS232 cards, modern cards now have chip with -12v charge pump inside the chip. (I'm sure -12v will be deleted in the future )
A side show of sorts:
Some Alien non ATX PC's use SFF (small form factor )
HP Deskpro 600, PCs use this odd connector linked below... I avoid such PCs.
This SFF PC uses a 6 pin Plug, seen here (hacked to work with standard PSU)
Also a void ALL _in_ONE desktops desktops they too are non-standard, and very hard to diagnose due to the fact (hint: standard parts do not fit nor work here, only the HDD is standard inside)
Basically the ALL_In_One PC, is just Laptop missing that huge battery and mobility. ( It is just stripped down PC missing all the good parts and expandiblity and stardards we enjoy now with ATX)
EXAMPLES are GREAT:
A Random found very old ATX power supply. (old, but shows how it all works ! really nothing beats a schematic, no?) This PSU is simplistic, with only basic protections.
Drawing #2: (picked only for its simplicity and fits my page easy) -5vcd Ended back in 2002, so is over 16 years old.
AC input is the AC line cord. (if you look close you see that there is standby power all the time line power is connected.
This is a SMPS design. Q1 and Q2 are the 20khz Chopper Oscillator. (power on , activates the chopper) IC1 controls power.
This is older than 2002, PSU, 16 years old or older today. (-)5vdc supply rail is now GONE for 16 years. (it is both cheap, low parts count and simple)
Its clear as day how power on , turns on Q10 and how standby power allows that pin to pull up to 5vdc.
IC2 pin 1 tells the ICH(PCH) chip in your PC , all is good, (and can lie ,note how 3.3v is not monitors, in fact some old supples only monitor +5/+12v rails.)
What is a rail?, well one side is a ground wire and the other is the other is the output of Diode pair like D18 top seen here +12v, if you had two more diodes and one more secondary output you now have two +12v rails. But why do that?
Consider this L1 output below, if it is rated at 20 amps, total, then adding more wires to that pin (as many do) does not increase total Amp. spec, at all, no it does not, but adding 2nd RAIL does. (with new transformer first and new secondary +diodes,etc)
Even the TEGAN below has one +12v rail with 80Amp total, the 20 Amp limits are just the wire size gauge limits. (note how you fail using 20amps times 6 at 120amps, the 80amps overloads now.
Newer ATX supplies, have up to 8-10 rails inside, (up to) (newer supplies even have complex , power modulation chips that use only the power it needs, saving power and Carbon Dioxide (green) emissions. (PFC options)
The modern PSU may use SMPS control chip like this from "ON" Semi . NCP-1910 with new over current protection shut down features (OCP)
Even more modern PSU now use a DSP (super fast and smarter, digital signal processor) this is the future, as it will never catch fire nor waste power or blow up that fast $1000+ gaming motherboard.
The point here is all pins below for power must be tested by the technician to be sure the PSU is not detecting faults, even faults on supply lines not used, (-5 and -12v may or may not be monitored and are now obsolete voltages)
Photo: 3 Example of better PSU for gaming systems or any with powerful GPU card (graphic processor unit) PCI cards added.
If all this is Greek? then you learned that Electronics is not simple, find a technician to fix it. I'm retired, now and can help. (but only here with words and pictures)
Now I show REAL SCHEMATICS so all can learn this, not just with words.
As stated before above the ICH (PCH) chip (glue) Does all the power managment of the PCI (with BIOS controls too) The Chip below, at pin 33, sends the power on command to the PSU, (PS_ON*)
Keep in mind even the BIOS can self turn on the PC, using timer power on page logic. (and wake on lan, or any wake features related to power, even exit sleep mode, or hibernate)
The same chip connects to the power switch at pin marked PWRBTN# ( this is Intel parlance for power button) (the pin actual varies by the package type, and ICH0,1 and up to 10.)
My Z270 chip, pwrbtn pin is BC5 (BGA) as you can see each chip for 18 years 1999 to now, all have same pin name.
82801BA I/O Controller Hub (ICH2) below.
ICH2 uses W21 ball pin marked PWRBNT.(as one example.) I am never going to show all ICH chips made, here, just one small example.
This excerpt is from the old motherboard Intel made long ago. 2001? (acccurate detailed schematics for modern motherboards are rare)
As you can see the intel engineers even call it GLUE themselves, no surprise here. Thanks for Intel D850 actually sharing details this good ! VC820 is here
Below is and OLD ICH2 chip Ball (BGA) ICH2 82801BA, circa year 2000.
Pentium 4. +AGP motherboard by Intel , the older chips are more easy to see pins and these power features, (PS_On, and PWR_good or Power OK & PWRBTN# BGA pin W21))
Power LOGIC 101: Simple (this is all pure logic inside, and ACPI) (just enough facts to diagnose dead power or failures of power)
We test the PSU while in the system by doing the forced power reset by holding the power button in , for 10+ seconds, this force G3 state , fully off mode. G3 is 100% off. Then turn the PC on.
Here is ICH7 chip, see PWRBT_L (it has 4.8k ohm pull up to 3.3vdc) This is power button.
Rapid Desktop PSU Testing to find true cause; (in PC case tested or on a work bench)
(assumes 120vac line service power reaches the PSU (wall power) and the PSU fan is not packed solid in lint stalling it and overheating it fast.
A Dead PSU. (or suspected)
You are here because the PSU fan is dead, or the CPU fan is dead, or the Standby +5vdc LED is dead. or all 3 are dead. A silent PC? ( my Z600 has 7 fans inside, are all dead?)
Rule #1 do not hot swap (or hot plug) any DC power cables inside any PC, remove the line cord and let it sit for 5-15 minutes.
Tools: a Screwdriver to remove case side, and 1 paper clip unbent foruming "U" shape.
If all fans are dead, the PSU is bad or (self shut down protecting itself an other electronics from damage) a.k.a. a silent PC, 100% dead PC.
If one fan works and one other fails, then that dead fan is bad that is dead. (easy no?)''
The PSU can be bad or ANY of the things plugged in to the PSU (colored wires +connectors) can be shorted.
The PSU can also self shut down if finds that its own regulators went berserk (had a failure).
The PSU can be tested 2 ways, in the PC case or on a bench, easy.(both ways with the 24 pin plug removed, and all other DC power plugs in side PC .
The best way to test the PSU is with a DMM. ( a voltmeter of any kind for $20 cost)
If you have a spare PSU, (smart to have as spare? yah) try that, if you lack proper useful tools like a simple DMM.
Preliminary tests: (do the power reset test first)
A still dead PSU is present.
This section here is called the Power on bypass test:
The PSU has a fan, the fan must blow air , spin and make a fan sound. (if not the PSU IS BAD ,buy a new one now) (yes I've replaced fans on many, or upgraded them to thermal regulation and less noise)
If the fan runs, then the PSU is not totally dead, it can be a good PSU or partially good. (that is correct , there are shades of gray in bad things, all electroncs fail and a rail can fail , just 1.)
Bad ,good, weak, overheats, intermittant, are all possible in all electronics made.
Were are we, if the Fan spins now with the paper clip test, the PSU may be good, if the fan is dead the PSU IS TOAST.
The DVD eject button dead (5vdc dead) on all PSU cause said PSU to self shut down, (UVP) But DVD eject tests are smart ! testing, but some PSU have good 5 and 12v but dead 3.3v. and 3.3v not monitor and dirt cheap PSU.
If the fan spins I always use my DMM meter to see if the DC voltage meet spec.
Okay say the PSU passes your testing above.
Unplug AC cord next
Put back the 24 pin connector, and all things you removed before.
Now plug back the AC cord.
Turn on the PC if any rail is dead or the PSU shuts off, start unplugging loads, a load mean any thing that unplugs from the Octopus cables.
Keep an open mind on shorts, and failures here.
Unplug them power cord removed, 1 at time, to learn which one is shorted. (learn that shorts happen, in all electronics made on earth) luck fate or lightening damage happens.
We must then and next , do Isolation tests.
All PSU even the most cheapest and old 250watt PSU will self shut off , if the PSU goes under voltage (UVP) (P means protection)
A top brand (any) PSU has 5 protections. (cheaper or older PSU have 2 way only protections, 5 /12v only)
The wrost are the old PSU that have no OVP or UVP on 3.3v ! What I do is trash that junk PSU.
In most cases if you hear the PSU run for 1 second or less, the PSU is overloaded.
OCP, SCP, OVP, UVP, OPP protections x5)example.
The PSU works unplugged above, but not when connected to the PC fully. ?
This means we must do the fault isolation tests next.
ISOLATION TESTS ARE EASY: ( My instructor said "Remove your shorts!" and all the girls in class slapped him)
See my strip-down power isolation testing here, for both Laptops classic, or Desktops real (not fake All-in-One non ATX standardized )
GAMING/CAD 101: Fast Video cards, GPU's Graphic Processors. (or video rendering and editing worksations too) If this card overloads the PSU, bingo wrong PSU or AUX jack never connected !
The RULE of 75Watts max on the X16 PCI express slots, must be observed. Unless, you upgrade to this NEWer Pascal GPU card.( any series 10 card))
The exception to this , like my GeForce GTX 1050 graphics card by NVIDIA, launched in October 2016.
Built on the Samsung's 14 nm process, and based on the GP107 graphics processor. The Pascal GPU with 3.3 billion transistors.
The wiki does nice neat table here.
Now with TDP (total power disapation max) of 75w or less ! ( and now Laptops can do gaming 10x better too)
Not only did they shrink the GPU DIE they shrunk the card greatly 5.70" x 4.38", and now sells as , fits all PCs easy. (no need to ask they say) "Gordon Moores law" still wins.
The lowest power fast card is GTX1030 at 30 watts,
That needs no 6 pin AUX power cable
The worst card made by Nvidia (and many others is the GTX690 at 300watts, a real power hog and needs the AUX power cable direct to the PSU .
The wiki shows the true power needs in the right column TDP
I highly recommend this card, the GTX-1050 (75watts max) (it is super fast and super low current (power watts) card. Magic-like is the Moores effect, and changes forever our choices, of cards.
You get to save big money on not needing to buy a larger sized PSU. (about a $50 saving for most folks)
Most of the high performance cards sold have this power hungy Jack on top.
I bet most GPU cards like the below, sent back to the vender due top failure to read the manual , the cable needed and PSU power needs.
The Nvidia GTX650-Ti. (the power hog about 2 times what PCI slots can source) 130watts (not as bad as the 690)
<<<< See it in DM , device manager?
The RED arrows below are Heavy wires Yellow/Black (if he CARD shorts, the PSU shuts down)
If you see this jack ontop of the card, (sadly most sellers do not show the top view) then it for sure exceeds 75watts power draw.
The reason it crashes is that you overloaded the PCI-express cards slots 75watt limit on power pins of said connector slot.
Some manuals show , needs 20amps on 12vdc rails some bigger power hogs claim 30amps draw from PSU.( this means the PSU needs to exceed this total on the 12vdc rails)
The PSU was named "ATX12V v2.2" and above versions. V2.3 is best.
The 4 pin PSU Molex(tm)at the MOBO Jack runs the CPU but the 6 pins Molex runs the below card ,over 75watts (express max)
Now Imagine 2 cards in SLI mode, needs 2times more current and power.
No express slot on earth can source that power ever. (75watts is max power on PCI-e (express! is the limit)
The 1 or 2 AUX power jacks feeds are NOT OPTIONAL EVER Before Pascal chips.
The GTX-1050 uses very little power now, and is far better than the above power hog, my GTX650 is now retired to the attic. (I show this GPU as it makes a worst case for overloading any stock weak PSU)
The best PSU story: "Power Supply Unit"
First the list of bad PSU's
The best PSU have a
"Full Electrical Protection OCP, SCP, OVP, UVP, OPP" over/under voltage and over power protections are best, in all cases. $50 is not expensive. (it's not just about power)
Pay at least $50 or you will get a very poorly designed PSU. (new prices)
Newegg.com sells great PSUs look there first. (click link above to see it)
Many dirt cheap knockoffs PSU are made by reverse engineering very very old PSU designs. (and are pure junk)
Modern PSU have huge smart SMPS chip inside, (and way less or none at all tiny TO-92 sized transistors.
The very powerful 12vdc RAILS here are what matters most.
Ohms laws are simple math , Power is watts, P= V times Amps (algebra Ohm laws) Power equals Volts times Ampere draw. So 30v x 12v = 360Watt true power or dynamic power.
In most cases for sure a PC with 3 hard drives and huge GTX card, you need lots of 12vdc AMPS.
A powerful desktop can use 300watts of power an surges up to 500watts. (but at idle very low) The PSU must handle ANY LOAD, even fast gaming.
The Dynamic loads must be be handled and noise (electronic) made from such an overload.
IMO : 500watts is minimum. (if using power hog GPU card)
Better is: ( this is just my opinion, I get no cash for saying so)
A Thermaltake PSU 500w or more is also a good deal. $35 at NewEgg.com.
Make sure the amps are good, some makers of PSU lie about the watts (sold as no name PSU from China direct), read the label first.
The Below PSU , see those numbers in the +12v1/2 column, this must be correct, 384w will run the 360w GTX card. (but will not run any SLI card setup like this)
Or this nice Coolmaster.
GPU cards from H3LL? (101):
Anyone who runs expensive $60 games, knows all about this issue:
I see endless posts by folks saying bad things about a PC, Windows, and many things, all the while violating the GPU card official manual spec., they can't read? or can not understand, that AMP'S matter.
Try to know watts is combined on all RAILS, and only amps available matter per Rail. See the above photo for PSU with a really honest sticker, there. 384w just for 12v and Video cards.
Adding any OLD GFX card to a PC with 250watt PSU, is just silly, It is a wasted effort and endless grief watching said PC crash endlessly BSOD's , or even goes DEAD.
But again this card is better. GTX--1050
It is sad to see vast buyers of MOBO/GPU and PSU rejecting what they bought when in fact is the the buyers fault. (cockpit problems) . (learn to buy The PSU first, to match what your GPU and MOBO needs first)
So is it silly to try to run 360watt GPU card from only a spec. limited 75watt, express slots. (hopeless that) unless you have GPU card AUX power plugs and a matching PSU, AUX cable to drive it. ATX12 v2.3.
Learn that the GPU card is not magically smart and knows to just turn itself off if you forget the 1 or 2 AUX cables or the right sized PSU. (this is your job and doing a power budget)
We see folks doing this and then burning up there nice motherboard (the 75w rule is not guide lines, its a hard cold limit, you must keep under control)
In some cases the power traces on the Motherboard blow up and smoke lets out of the PC. (just for forgetting the AUX plug.)
What matters most is the 12vdc rail . The yellow wires are all 12vdc.
The PSU has huge power regulator inside to do 12vdc.
My PSU has run crash free for 5 year now. Not one hiccup.
if you use a proper PSU, the GPU will NOT CRASH (99% crash free, 1% are bad games,not updated) no lint packing up GPU nor CPU fans ,nor heat sinks, clean them.
Is this your PSU? and the fan stalled and packed in filth, (all PSU do this, if not cleaned every year) We just blow it out.
really? You expect this to be ok? It stalls the fan and makes all parts overheat and die. That dirt blocks air flow too)
Always clean parts OUTSIDE using Can-O-air or 30PSI shop air.
I use this wimpy supply below in my SAS hard drive cage ( no video there , no mobo there) so what you use, must match your needs, in all cases.
(bent open for easy viewing) from wiki pages. Do not bend yours like I just did. (I removed wires only for one rare application)
Learn that some PSU run a thermostat fan (speeds based on temperature only) On newer ones the main board if it see way too much heat shuts down the PSU, "self preservation" and very wise that action.
More chips ok?, for PFC controls (95%) efficient, or OCP, SCP, OVP, UVP, OPP protections are all worth having more chips or a huge one, I see now in the mighty year 2018 some makers of PSU have wised up and use a HUGE $10 or less ARM chip.
The ARM chip does it all, getting rid of other chips inside. (or DSP)
Called by some complete digital controls. (or DSP controlled or this) yah, a real fancy PSU. See how this amazing DSP techology will change all this fast. at TI.com
See this high end Thermaltake presentation.
The below is low end new product that replaces the old above photo of very old PSU.:
Many top brands say using real Japanese capacitors (a smart move) ( yah, many sold in China by SOM TING WONG are pure crap, read the wiki horror on that)
Look for more larger chips and less descrete tranistors, on all modern PSU. The huge transistor are needed, like the ones on that heat sink.
Some PSU can exceed 95% efficiency... (nice) A PSU that runs cooler lasts way longer ! A fact.
If you buy a new PSU, get one with modern ACTIVE PFC (power factor correction ) chips inside for up to 95% efficiency and way less heat. (yes it costs more to get quality) Quality is never free.
The future is DSP. for sure, only now we wait for lower end products to adopt DSP (I can't wait, to seem them), and save money they will. (more for less) (I own a DSP SDR RADIO RECEIVER and is amazing)
Digital Signal Processors (DSP) are the wave of the future, and glowing now like mad. (all a good thing ) the DSP even lowers noise. (electric and sonic noise both and less fan noise.)
Here is a hit list of the good things. (and wise match to your system)
end true and real standard desktops. (true DT, not All-in-One desktops (a new oxymoron))
THE CMOS is messed up?:
That tiny battery called RTC COIN cell runs the NVram and the RTC (real time clock chip) both go dead, if the battery fails, or drops below 2.9v, (3.3 is new typical value)
This causes the BIOS brains to go NUTS and TIME of DAY and Date, and if Windows sees time wrong, oh boy, it goes nuts too and then "Certificate errors from H3LL"
NVRAM , is a tiny RAM that must not forget things (NV = Non-volatile), if it does you will play hell. (endless pain, wild symptoms , things that make no sense at all, why let that happen ever? for $2)
If the PC is over 5 years old and you don't have a $20 DMM meter to test it, just replace it.( Walmart has them in the WATCH department, take yours with you and match it up)
On some laptop the coin cell has PigTails, and is very hard to find them or make one (as I do). Best is to search on Ebay for your pig tailed battery.
This page I made shows how the RTC and NVRAM WORK and what to do if the battery dies. (RTC COIN CELL)
version 7. 8-1-2017 (July,28,2018 last edit)