A Study of Cell-to-Cell Interactions and Degradation in Parallel Strings: Implications for the Battery Management System

Vehicle battery systems are usually designed with a high number of cells connected in parallel to meet the stringent requirements of power and energy. The self-balancing characteristic of parallel cells allows a battery management system (BMS) to approximate the cells as one equivalent cell with a single state of health (SoH) value, estimated either as capacity fade (SoHE) or resistance increase (SoHp). A single SoH value is however not applicable if the initial SoH of each cell is different, which can occur when cell properties change due to inconsistent manufacturing processes or in-homogeneous operating environments. As such this work quantifies the convergence of SoHE and SoHp due to initial differences in cell SoH and examines the convergence factors. Four 3 Ah 18650 cells connected in parallel at 25 degrees C are aged by charging and discharging for 500 cycles. For an initial SoHE difference of 40% and SoHp difference of 45%, SoHE converge to 10% and SoHp to 30% by the end of the experiment. From this, a strong linear correlation between ASoHE and SoHp is also observed. The results therefore imply that a BMS should consider a calibration strategy to accurately estimate the SoH of parallel cells until convergence is reached. (C) 2016 The Authors. Published by Elsevier B.V.