I bought a new laptop with long battery life span (12 hours) (it arrived on --04-21 in the evening) and calibrated the battery as described in this question on --04-23 to get an accurate reading on the remaining time. Regarding the unclear point (the question) there: I had the laptop turned off when charging at first but then turned it on after some time.
Then I used the laptop until --04-27 which is when it was turned on the last time until today (--05-01). I didn't discharge it anywhere as low as for the calibration. In fact, I looked for a laptop with a very long battery life time so I could avoid discharging it below 20% so it will work for a long time.
I knew that I left it at somewhere between 60% and 70% charge so I was confident to be able to use it, today (--05-01).
However, when I just turned it on, the battery was at 8%!
Note that the plot doesn't show the actual change of the battery's charge. Only the black dots are actual measurements. Unfortunately, it seems like it's not possible to make the line connecting the dots go away. There are no dots on that long slope because no measurements have been taken on it as the laptop was off during that time.
Why is this? Does it mean that the battery is already pretty much dead? What is going on?
Edit: The problem persists. I had my laptop turned off for a little over a day and the battery's charge dropped by 20 percentage points!