Abstract
Herein, individual cell currents in parallel connected battery strings are measured using micro‐Hall‐effect sensors. Cells are routinely connected in electrical series and parallel to meet the power and energy requirements of automotive and consumer electronics applications. Cells connected in series have been extensively studied; however, cells in parallel are often assumed to be a “black box” in battery management systems. Poor pack design can result in positive feedback between current and temperature differentials along the parallel string, driving greater levels of heterogeneous behavior and uneven degradation. Herein, a noninvasive multisensor array board using Hall‐effect sensors is used to individually record the current passing through eight parallel connected cells in two different electrical configurations, showing highly heterogeneous current distribution. Characteristic “waves” of current and temperature are found to propagate along the parallel battery string and cell rebalancing is found to occur over hundreds of seconds with individual cell currents of up to 1 C rate.
Highlights
Parallel battery strings are used in most battery packs to meet the high capacity and power requirements of applications such as automotive traction.[1]
The Tesla Model S 85 kW h battery pack consists of 74 cells (18650) connected in between current and temperature differentials along the parallel string, driving parallel, and six of these in series to form greater levels of heterogeneous behavior and uneven degradation
A noninvasive multisensor array board using Hall-effect sensors is used to individually record the current passing through eight parallel connected cells in two different electrical configurations, showing highly heterogeneous current distribution
Summary
The pick-up wire, shown coming out of plane, can be seen in the centre of the ferrite core with the concentric magnetic field lines shown in perforated red This design allows the Hall-effect sensors to be integrated in close proximity to the cell electrode terminal, and even form part of the module casing. Each pick-up wire and crocodile clip assembly introduced 2.93 Æ 0.30 mΩ contact resistance between the cell and the bus bar This is less than a shunt resistor and as each pick-up is effectively in series with the cell it should not change the overall characteristics of the current distribution. The module was cycled using a Gamry Reference 3000 with 30 A booster
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