Once I found a new update firmware, I updated my Duo to v1.6 . After the update was done, the lcd screen showed sort of "output connection problem" with loud buzz; even I rebooted my charger. I thought that tells me the damage of my charger components as is described in the manual. After I updated it all over again, the problem was gone and Duo functions well now, even in the balancing mode. No sign of output circuit error. Great.