In some examples, a system includes a primary side with a charger and a first battery and a secondary side with a second battery. The charger on the primary side can charge both the first battery and the second battery. A hinge resistance is between the primary side and the secondary side. The primary side includes a feedback controlled active device in a current path of the first battery that compensates for the hinge resistance, for connector resistances, or for battery impedances in a current path of the second battery.