Windows 10: Why my laptops full charge capacity decreased by 15000 mWh by single day

My laptop is currently 1.5 year old ,Acer aspire 7 i5 9gen and my laptops full charge capacity starts decreasing so quickly from 42000mWh to 20000mWh by last 5 month's and by a single day it is decreased to 5000mWh. I want the reason why this happened

:)

 

Battery full charge capacity decreased?





As you can probably see from the above image, my battery's full charge capacity has been decreased. Even though I just bought this laptop like 3 months ago, the laptops full charge capacity has been decreased by a huge amount like 5000mWh. I don't know what is causing this I have three questions...

i) What causes a battery to lose its Full Charge Capacity?

ii) Can a full charge capacity be increased (reversed) by using it the right way again?

iii) How can I save my battery from losing it's full charge capacity? Will using it plugged into the charger help it not get decreased?

 

Full charge capacity of battery decreased

I thought of discharging the whole battery before charging for the whole night. The system hibernated at 10%. Then I turned on the PC to discharge the remaining 10%. But to my surprise, the PC ran for 90 min in the reserved battery level(from 7%). And the
funny thing is that, at 0% it ran for about 60 min (in the 90 min). I've reinstalled the Microsoft ACPI battery driver thinking that there might be problem with battery indicator.

Then, I've charged the pc for the whole night(not under the control of windows as told). Today, I used the laptop on battery power. The battery drained fast but with a little improvement.

Then I've generated a battery report from cmd. Full charge capacity was increased from 9302 mWh to 13954 mWh as shown.





Again the pc was working fine at the reserve battery level(while discharging the battery power today).

Then the next step as told is to charge to100% with pc turned on (present situation I was doing now).

What you want me want me to do next?

As suggested by the StefanoTamiotti, I've gone the Dell support article posted. And in ePSA
diagnostics, it was shown as- " The battery cannot provide sufficient power".

 

Laptop is charging to design capacity, not full charge capacity

I have a laptop (Dell Inspiron 3541) with Windows 10. After running powercfg /batteryreport in PowerShell, I got a battery report (at %userprofile%\battery-report.html). However, the results didn't seem right. Here's a screenshot.





As shown by the battery report, my design capacity is 41,440 mWh, but the full charge capacity is 26,758 mWh (about 65% battery health). However, it also shows that it is charging to design capacity (154%), not full charge capacity.

Some other weird parts of the battery report:




My battery has never drained 80% in 40 minutes. 80% in 140 minutes is a much better estimate.




Over 60 years turned on and on AC in a week? I was running Linux a lot that week, but 60+ years is impossible.

Is this normal (for a Windows battery report)?