Abstract
To recycle retired series/parallel battery packs, it is necessary to know their state-of-health (SOH) correctly. Unfortunately, voltage imbalances between the cells occur repeatedly during discharging/charging. The voltage ranges for the discharge/charge of a retired series/parallel battery pack are reduced owing to the voltage imbalances between the cells. To determine the accurate SOH of a retired series/parallel battery pack, it is necessary to calculate the total discharge capacity using fully discharging/charging tests. However, a fully discharging/charging test is impossible owing to the reduced voltage range. The SOH of a retired series/parallel battery pack with a voltage imbalance should be estimated within the reduced discharging/charging voltage range. This paper presents a regression analysis of the peak point in the incremental capacity (IC) curve from the fresh state to a 100-cycle aging state. Moreover, the SOH of the considered retired series/parallel battery pack was estimated using a regression analysis model. The error in the SOHs of the retired series/parallel battery pack and linear regression analysis model was within 1%, and hence a good accuracy is achieved.
Highlights
In recent years, with the increased use of electric vehicles (EVs), hybrid electric vehicles (HEVs), energy storage system (ESSs), and renewable energy storage system (RESSs), the development of a high-power and high-capacity lithium secondary battery have become active [1]
The incremental capacity (IC) curve was calculated in the discharge interval, and the IC curve peak value was extracted in the terminal voltage range between 3.6 V and 3.7 V
The linear regression analysis method was performed with the IC curve peak point and SOH value based on the capacity information of the fresh cell according to a 100-cycle aging test to build the capacity fading model
Summary
With the increased use of electric vehicles (EVs), hybrid electric vehicles (HEVs), energy storage system (ESSs), and renewable energy storage system (RESSs), the development of a high-power and high-capacity lithium secondary battery have become active [1]. Because the output power of one cell is limited, multiple cells are connected in series and parallel according to the size of the voltage, current, and capacity of the application. When a battery pack consists of multiple cells, a voltage imbalance occurs between the cells. When such a battery pack is charged and discharged over many cycles, the above-mentioned cell-to-cell voltage imbalance occurs owing to the temperature variation, parasitic resistance, and cell aging. A voltage imbalance between each cell degrades the overall capacity utilization efficiency of the battery pack, and accelerates the aging. The full discharge/charge of a battery pack without balancing the voltage difference can lead to an overcharge/overdischarge. Because of the high manufacturing cost and power loss, it is inefficient
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