Battery capacity showing sudden changes

[seglea1]

I suddenly started getting POST warnings about battery life.  I knew the battery was getting old, so I assumed it had just declined to a threshold, and bought another one; I didn't bother running a condition report because I was expecting to have to replace it some time.  However, before I'd got round to installing it, the POST warnings went away, which I thought was odd.  But I went ahead and put the new battery in,, and then ran the powercfg/battery report to check it was OK.  I then noticed the following pattern of battery capacity with the old battery (the table shows actual capacity and then design capacity, which of course does not change):

2024-05-20 - 2024-06-03 27,073 mWh 50,012 mWh
2024-06-03 - 2024-06-10 27,073 mWh 50,012 mWh
2024-06-10 - 2024-06-17 27,073 mWh 50,012 mWh
2024-06-17 - 2024-06-24 9,927 mWh 50,012 mWh
2024-06-24 - 2024-07-01 8,443 mWh 50,012 mWh
2024-07-01 - 2024-07-08 8,443 mWh 50,012 mWh
2024-07-08 - 2024-07-15 8,443 mWh 50,012 mWh
2024-07-15 - 2024-07-22 8,443 mWh 50,012 mWh
2024-07-22 8,443 mWh 50,012 mWh
2024-07-23 8,443 mWh 50,012 mWh
...
2024-07-28 8,443 mWh 50,012 mWh
2024-07-29 8,443 mWh 50,012 mWh
2024-07-30 11,898 mWh 50,012 mWh
2024-07-31 26,091 mWh 50,012 mWh
2024-08-01 26,091 mWh 50,012 mWh

So around 17th June my battery capacity shot down to a third of what it had been, and that's when the POST warnings started.  And then on 31st July it jumped back up, and the POST warnings stopped.

 It doesn't matter much, since I've already done the replacement, but I was wondering whether it was worth keeping the old one as an emergency spare.  So the practical question is whether the pattern shown above indicates a terminally sick battery.  I'd also be interested to know how on earth a battery could suddenly lose two thirds of its capacity, and then get it back; I'd certainly done nothing I can think of that would produce either effect.

The system is an Elitebook 840 G5 running 64-bit Windows 10, and the old and new batteries are both authentic HP stock.

[banhien]

@seglea1 

My HP 840 G5 machine is probably the same age or yours. My test (1 min ago)

       

I can not remember when I did the previous check, probably few years back after 3 year warranty ended. I know system caps to charger only at some % sound like 67% or only to 2/3 its capacity.

It is a good idea to keep the old one just in case.

Regards.