I am kind of new in electricity, I bought a USB tester - KWS1902C. I can see my laptop voltage is constant 20 V (or around 20) to be more precise. Amperes are changing in sleep mode 0.002 when working depending on the load from 1 to 2 amperes. So far so good. One thing I do not understand is why mAh is constantly rising. Reading on the internet, mAh is the capacity. But why my capacity is rising all the time when battery is 80% (I set max to be 80%) on my laptop?
mAh measures the total charge which has passed through some point over all time. It is like a turnstile counter for electrons.
So mAh is a great way to keep track of capacity, just the way a turnstile counter is a good way to measure a stadium's total capacity if you reset it when the stadium is empty and then look at its value once it is full.
In other situations it is just measuring the charge which has "passed through".
It's almost the total energy, but you need to multiply it by the voltage first: electrons can pack a different punch depending on their voltage.
Note that the detail is somewhat more complex particularly with regard to the role of voltage and potential and fields, and also exotic carrier types etc, but this will do you.