Instead of a few hours interval, I just tried with 15mins.
Computer did start up 15 minutes after power down without removing the mains plug. And everything seems to be ok.
WhenI try to start 15 minutes after power down and removed the mains plug, it did not load Dell logo. Instead there was a message.
CMOS checksum error - Defaults loaded
Press F1 to continue. F2 to enter setup
I pressed F2 and in the Setup meny I found the time has been reset. When I exit without save, I got back to the above message.
Pressed F2 again, select Load Default Setting, then Save and Exit. Then computer was able to starts up.