Lithium battery pack cell matching

Battery packs with well-matched cells perform better than those in which the cell or group of cells differ in serial connection. Quality Li-ion cells have uniform capacity and low self-discharge when new. Adding cell balancing is beneficial especially as the pack ages and the performance of each cell decreases at its own pace.

We demonstrate the importance of resistance matching in battery packs. At 4.5C charge and discharge, 20% resistance mismatch reduces lifetime by 40%. We quantitatively explain experimental results using a model of SEI formation. Resistance mismatch causes uneven current sharing.

Cell matching according to capacity is important, especially for industrial batteries, and no perfect match is possible. If slightly off, nickel-based cells adapt to each other after a few charge/discharge cycles similar to the players on a winning sports team.

Assuming the battery pack will be balanced the first time it is charged and in use. Also, assuming the cells are assembled in series. If the cells are very different in State of Charge (SoC) when assembled the Battery Management System (BMS) will have to gross balance the cells on the first charge.

When cycled, all batteries show large capacity losses over 18 cycles, but the greatest decrease occurs with the pack exhibiting 12 percent capacity mismatch. Battery packs with well-matched cells perform better than those in which the cell or group of cells differ in serial connection.

Six battery packs (each containing two cells connected in parallel, as depicted in Fig. 5) were tested using the method described below. For further reference within this paper, two parallel-connected cells are called a “cell group”. The current to each cell and the temperature of each cell were recorded.