Question HP Elitebook 840 G5 won't boot unless batteries are disconnected and power button held for 1 min

Oct 19, 2021
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Hi Everyone,

I have an HP Elitebook 840 G5 that will not power up unless I disconnect the internal batteries (main and CMOS), then I have to hold down the power button for 1 min. Once the A/C is connected, it boots and operates normally.
If I power down and then reconnect the batteries, it boots and operates normally. I can then run it on just battery (it will discharge at normal rate) and if I plug it in, it will charge the battery.
Problem is if I power it off and wait a few hours, I cannot power it up again unless I repeat the entire procedure.
Once working, if I power it off and wait a bit 10-30 minutes, I can power it up and all looks good.
If I wait several hours (overnight), I cannot power it up again. Pressing the power button does nothing, no lights, no noises, ie, completely dead.

Things it is not:
  • BIOS was updated, twice to make sure it worked (maybe downgrade?)
  • I tested the A/C charger and it is good. I have an older 840G3 (same model, two years older) and swapped parts, plus I used a multimeter, I get the 19.5V DC
  • I booted to the HP diagnostics and ran the extended test on the RAM and hard drive and they passed
  • I wiped the drive and tried clean installed of Windows 11 and Windows 10 and use HP Support Assistant to make sure I have all the latest drivers
  • Re-seated the RAM, checked all internal connection and they look good
  • It is not a heat issue as once powered up, I ran it all day listening to a radio station and it worked perfectly.
Other things I tried:
  • Once working, I powered down and connected just the CMOS (thinking main battery might be bad) and then power up, all is good, unless I power off and wait a few hours and then nothing, I have to start all over
  • Once working, I powered down and connected just the main battery (thinking the CMOS might be bad) and then power up, all is good, unless I power off and wait a few hours and then nothing, I have to start all over
I'm wondering if it could be a BIOS setting?

Thanks
Richard
 
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