Heterogeneity-induced power and capacity loss in parallel-connected cells

Battery manufacturers strive to produce cells with the same capacity and impedance but the cost and waste increase as the specifications tighten. Reused cells for second-life applications are even more heterogeneous. Pack manufacturers randomly form cell groups with cells in parallel and strings with cell groups in series. This paper studies the heterogeneity-induced capacity and power loss in parallel-connected cell groups. The pack capacity, peak branch currents, and pack power for constant current (CC) charging are analytically derived in heterogeneous parallel cell groups using Thevenin equivalent circuit models. Matching the product of the capacity times ohmic resistance (CR) for the cells within the group eliminates the current inrush associated with unmatched cells. The Current inrush occurs in lower CR cells which means that the current input to the cell group must be reduced to ensure that the current flowing through the lower CR cells is within the current limit specification, introducing power loss. Experimental results agree with the model predictions and show that power loss is proportional to CR and resistance mismatch, with 25% power loss during 1C charging for 8% CR mismatch in a 2-cell group. Matching Cr = C(R + r), where r is the polarization resistance, maximizes the cell group capacity during CC charging to be equal to the sum of the parallel cell capacities. In agreement with the model predictions, experimental results show that capacity loss is proportional to Cr mismatch and C-rate, demonstrating 3% capacity loss during 1C charging for 3% Cr mismatch in a 2-cell group.