NOVELTY - The circuit (60) has substring DC-DC converters (70) connected between primary ports (72) and an isolated secondary port (74), where the DC-DC converters comprise a power switch. The DC-DC converters adjust a primary port current received at the primary ports based on a difference between a voltage (V-p1) at one of the primary ports and a reference voltage. A controller adjusts the primary port current by adjusting a control input for the power switch in the DC-DC converters. A filtering capacitor is connected across the isolated secondary port, which comprises a monitoring signal.    USE - Balancing circuit for balancing, filtering or controlling series connected cells e.g. photovoltaic cells, energy storage cells and electrochemical battery cells, in an electric power system (all claimed) for a vehicle e.g. hybrid electric vehicle, plug-in hybrid electric vehicle and electric vehicle, utilized for residential or commercial purposes.    ADVANTAGE - The circuit provides significant performance improvements in presence of mismatches so as to introduce least minimal insertion loss penalties in simple, cost effective and scalable manner with improved reliability. The circuit allows maximum power point (MPP) voltage to be occurred in a photovoltaic module with the circuit when the module is fully irradiated, so that nearly-constant voltage output of the photovoltaic module with the circuit can reduce cost and improve efficiency of follow-up power electronics in photovoltaic systems.    DETAILED DESCRIPTION - The monitoring signal comprises power indication, temperature indication, functionality indication and failure indication. The control input is a duty cycle or a switching frequency. The substring DC-DC converter is sub-module integrated converter or a resonant converter such as series-resonant converter, parallel-resonant converter, LLC resonant converter and radio-frequency resonant converter. INDEPENDENT CLAIMS are also included for the following:    (1) an electric power system    (2) a method for balancing a set of electric cell sub-strings.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a photovoltaic module and an electrical circuit including implementation of a balancing circuit for the photovoltaic module.    Voltages at primary ports (V-p1-V-p3)    Balancing circuit (60)    Substring DC-DC converter (70)    Primary port (72)    Isolated secondary port (74)