Why am I getting a Checksum error after replacing the main battery?

Hello everyone I am new to this forum.

I have an HP-Laptop 17-by1036nr. I recently opened it up to change the battery and a ram stick. I also replaced the cooling paste for the CPU. Since then, I have been having CMOS checksum errors.

CMOS errors only show up when I try to turn on the computer after leaving it off overnight. If I turn off the computer and turn it on again no CMOS Checksum error appears. After every CMOS checksum error I need te reset my BIOS settings including date and time and my action keys setting (I leave everything else in default).

I have flashed a new BIOS (upgraded from version 50 to 74), thinking that it might help, but the problem is exactly the same.

Something else that might be important: Completly shutting down the computer and leaving it overnight, as mentioned, prompts the CMOS error. If I just leave the computer off for a couple of hours no CMOS prompt comes up on startup. Also, after leaving it overnight I am not able to turn on the computer from the battery I need to plug-it in to the wall socket and then turn it on. I get no response form the computer whatsoever unless I do that. This only happens once, afterwards I can normally turn the PC on and off from battery power, and the cylce repeats after leaving the computer off overnight.

Any ideas/thoughts would be appreciated. I have not opened the computer again after replacing the battery. Plase notice that this computer does not have an independent CMOS battery.