Thanks guys for your feedback.
Since it arrived yesterday, I have plugged it to wall charger until end of the day and took it home. Today I plugged it back to wall charger but it stays at 3 bars with no indication of activity. So in total it was about 10 hours.
At some point light went off and it went in to stand by mode.
I presume when battery is about 75% full it suppose to activate forth diode which suppose to blink, am I correct?