Business Notebooks

I have an Elitebook 840 G5 that will not boot unless I disconnect both batteries (CMOS and main), then hold the power button down for 1 minute, then I connect the A/C and everything works perfectly.  I can repeat this every time to get it booted.

When I get it powered up, I can power it down and connect the batteries and then it powers up and operates normally.  HOWEVER, if I power it off and WAIT a few hours (not sure number hours but over night for sure), I cannot power it up again unless I disconnect the batteries, hold the power button, connect the A/C and start all over again.

I am a engineer with 30+ yr experience with HP products but this has me stumped.  Here are things I tried and ruled out:

- BIOS updated to latest  version with default setting (maybe try older BIOS but I can't find one)

- I checked the CMOS battery and it is good

- I tested the A/C adapter with another HP laptop and the adapter/charger is good

- I wiped the drive and tried a clean install of Windows 10 and Windows 11 and did a Windows Update, plus I used HP Support Assistant to download and install HP updates.

- I just bought a HP Elitebook 745 G5 for a client and swapped batteries thinking maybe the main battery is bad, same problem

- I found new firmware updates on HP site that Support Assistant didn't find (Elan touch screen, Thunderbolt, etc.) and installed those updates successfully, but issue is still there

- Once booted, it can run on battery and it discharges and charges normally as expected

- I ran the built in HW diagnostics as well as the extended HP diagnostics and everything passes

- I can either close the lid or enter sleep mode for a few hours and then it powers up normally.

The problem appears if I leave the unit powered off overnight with the batteries connected.  The next morning, when I push the power button nothing happens, no lights, no noise, nothing, completely dead.

When I disconnect the batteries and hold the power down (45 sec seems to work too), as soon as I plug in the A/C, the power button lights up, the keyboard lights up,  I get a beep and the white screen complaining the CMOS is disconnected.  I press Enter to continue, it reboots and operates normally.

I have run out of ideas to try.

Any suggestions would be greatly appreciated

Richard