System of a battery stack and a charger device to charge the battery stack with active balancing of battery cells, which battery stack comprises: a series of battery cells with at least a first battery cell and a second battery cell connected with battery cell contacts in series between a battery stack plus contact and a battery stack minus contact and which charger device is connectable to a power source and comprises: a charger module connected with a charger plus contact to the battery stack plus contact and connected with a charger minus contact to the battery stack minus contact and built to charge the series of battery cells based on a charger program when the charger device is connected to the power source and which system comprises: a balancer module with a balancer stage for each of the battery cells of the battery stack built to transfer energy from an already fully charged battery cell to a battery cell with a lower state of charge, wherein each balancer stage of the balancer module is connected in parallel to only one sole battery cell of the battery cells of the battery stack, characterized in, that each balancer stage of the balancer module comprises an RFID antenna, wherein at least a first RFID antenna of a first balancer stage connected in parallel to the first battery cell and a second RFID antenna of a second balancer stage connected in parallel to the second battery cell are arranged in an RFID communication distance to exchange energy and information via a common first magnetic field, wherein the first balancer stage uses more or less all energy provided by the charger module at the battery cell contacts of the already fully charged battery cell to generate a magnetic field and in that way bypasses a charge current for this fully charged battery cell. a series of battery cells with at least a first battery cell and a second battery cell connected with battery cell contacts in series between a battery stack plus contact and a battery stack minus contact and which charger device is connectable to a power source and comprises: a charger module connected with a charger plus contact to the battery stack plus contact and connected with a charger minus contact to the battery stack minus contact and built to charge the series of battery cells based on a charger program when the charger device is connected to the power source and which system comprises: a balancer module with a balancer stage for each of the battery cells of the battery stack built to transfer energy from an already fully charged battery cell to a battery cell with a lower state of charge, wherein each balancer stage of the balancer module is connected in parallel to only one sole battery cell of the battery cells of the battery stack, characterized in, that each balancer stage of the balancer module comprises an RFID antenna, wherein at least a first RFID antenna of a first balancer stage connected in parallel to the first battery cell and a second RFID antenna of a second balancer stage connected in parallel to the second battery cell are arranged in an RFID communication distance to exchange energy and information via a common first magnetic field, wherein the first balancer stage uses more or less all energy provided by the charger module at the battery cell contacts of the already fully charged battery cell to generate a magnetic field and in that way bypasses a charge current for this fully charged battery cell. a balancer module with a balancer stage for each of the battery cells of the battery stack built to transfer energy from an already fully charged battery cell to a battery cell with a lower state of charge, wherein each balancer stage of the balancer module is connected in parallel to only one sole battery cell of the battery cells of the battery stack, characterized in, that each balancer stage of the balancer module comprises an RFID antenna, wherein at least a first RFID antenna of a first balancer stage connected in parallel to the first battery cell and a second RFID antenna of a second balancer stage connected in parallel to the second battery cell are arranged in an RFID communication distance to exchange energy and information via a common first magnetic field, wherein the first balancer stage uses more or less all energy provided by the charger module at the battery cell contacts of the already fully charged battery cell to generate a magnetic field and in that way bypasses a charge current for this fully charged battery cell.