Battery was discharged to 7V unnoticed when debugging using mission planner.
Recharged with B3AC charger, and 1 green light indicate fully charged. However, measured voltage only 8.66V. Further checked the voltage of each cell by measuring between 1 and 2, 2 and 3 and 3 and 4 pins of the charging cable, and read returns 4.2, 0.2 and 4.2V.
Does this mean the second cell is dead? This is only the fourth charge that I had.