Post by Pierpaolo Rigo

Kirchhoff’s Voltage Law (KVL) and Circulating Currents. Kirchhoff’s Voltage Law (KVL) dictates that the algebraic sum of potential differences around any closed loop is zero. While KCL governs how the total inverter current splits between parallel branches during traction, KVL rules the direct energetic interaction between those branches. This becomes critical the exact moment external load drops or shifts rapidly. When torque demand stops or decreases—during lift-off or switching transients—the thermal and electrochemical asymmetries analyzed in the previous post leave the parallel branches at mismatched States of Charge (SoC), translating into different Open Circuit Voltages (Voc). Inside the closed loop formed by two parallel branches, KVL dictates that the delta between the two internal voltages must be balanced by the potential drops across the total loop impedances: Voc1 - Voc2 = Iric * (Z1 + Z2 + Zconn) This equation isolates the generation of an endogenous circulating current (Iric). The branch with the higher energetic potential temporarily acts as a generator, dumping current directly into the parallel branch with the lower potential, which forcedly acts as a load under recharge. This parasitic current loop never reaches the inverter; it circulates exclusively within the boundaries of the battery pack. The phenomenon causes localized Joule heating (I^2 * R) even when the vehicle is stationary, stresses cell chemistry with unplanned micro-charging cycles, and drives massive asymmetries in overall degradation—deepening the unbalance drift that the BMS will later try to compensate for. #BatteryEngineering #Powertrain #ElectricVehicles #Electrochemistry #LithiumIon #BMS #Engineering