My OptiPlex 755 DT always showing System Voltage Low

FYI, currently my OptiPlex 755 DT running on Windows 7 Pro 64-Bit with Intel Core 2 Duo E6550, 2×2GB 800Mhz RAM, Phoenix ROM BIOS PLUS Version 1.10 A22

I have a Optiplex 780 MT, once while in BIOS was a warning error about low voltage, I replace and renew the CR2032 Battery  and such warning error was no longer there or disappear,  you mention you already swap the power supply and therefore to me is difficult the reason of the problem is due to a bad power supply, do you get that error every time you switch on the computer?,  

Yes sir i swapping the PSU because when im using the old one the computer is more unstable like everytime i booting up its always showing System Voltage Error message, then Boot-looping on bios screen like it goes on then off then on again and turning off again and again til my RAM destroyed, at first i tought the same as you its just CMOS battery and i swap to new battery of CR2032, and the error message still occur and still Boot-looping  until i swap the PSU its get more stable electricity, but, the Error message still occured but no Boot-looping.

BTW i swap again to new CMOS Battery when i installed the new PSU aand yep, the problem still the same but getting better, hope so..

There is a Dell forum thread about this problem, did you check already?, below there is a list of what to do, if you Clear CMOS my advise is to remove the CMOS battery and unplug also the power cord from the computer power supply this must completely clear CMOS but be aware to clear CMOS of my Optiplex  780 MT I need to swap the jumper from the normal location to a different jumper on the motherboard which is unusual to me, I have been having a variety of non DELL PC and to clear CMOS is a matter of just single jumper 

https://www.dell.com/community/Desktops-General-Read-Only/Alert-System-battery-voltage-is-low/td-p/21087

1) Clear NVRAM

2) Reset system set up options

3) Clear System Event Log in BIOS Screen

These did not resolve the problem the next step did

4) Record the System Setup Settings

5) Remove the Battery and reinstall it

The battery must have been lose because reinstalling it appears to have cleared the problem at the moment

At first, thank you for your method,

I tried it and it nothing changed and i check all forum thread and get along with some of methods and still nothing worked should i give up on this? ‍ ive got no idea what to do anymore...

I agree with redxps630 to clear CMOS you need to unplug power cord from wall And also "simultaneously" at the same time remove the CMOS  CR2032 battery And please check the Dell specifications I paste link below to more accurate step by step procedure to clear OptiPlex 755 DT CMOS

https://www.manualslib.com/manual/356816/Dell-Optiplex-755.html?page=291

Clearing CMOS Settings

I think the main problem is on the motherboard where cmos chip or the cmos port is defective due to age, as the aux led is turning on also, it may be requiring for re-soldering or changing the port or some chip on the motherboard but i have no knowledge on soldering and fixing pcb or changing part like capasitor and etc.