RTC and BIOS reset after AC power is disconnected

When you replaced the CMOS battery did you leave a battery out of the system for at least 15 minutes before installing a brand new recently purchased battery?

@JOcean​ No, it was just like 2 or 3 minutes.

try clear cmos settings via motherboard jumper.  then change date and time to today.  test again.  sounds like your cmos battery is not able to keep the settings in NVRAM when ac power is off.

It seems that the battery is not working. Check the battery slot to see if it is abnormal or if there is a button battery that is not in good contact.

Test the new battery.  It should be 3v and size 2032.  Hopefully not a generic battery.  I prefer Energizer.  They last 5 years.  Duracell AA batteries have leaked on me, so I don't trust their 2032 battery either.

@bradthetechnut @niko_ke @redxps630 
I completely disconnected all extensions like MegaRaid card and Ethernet card, also all hard drives and keep only one nvme disk. Disconnected the power, also replaced a new battery (with at least 15 minutes disconnected everything). 

After all of this, I connected the battery, than power supply, set the date and time in bios. In the Windows 10 and in the app "Dell command update", I installed the newest version of BIOS, also synchronize the date and time in settings and till now everything is working correctly.

But I had no chance to insert back the extensions card and the rest of hard drives. So it is possible that some of those devices is creating this issue.

if the battery is able to keep bios settings, but one of the device is making it lose settings, sounds like one of the device is leeching power out of system when ac power cord is unplugged.  test each device individually until you id the culprit