Comparison of Different Battery Balancing Methods for use with Second Life Batteries

Second life batteries are batteries that have previously been used in hybrid or electric vehicles, but have dropped below 80% of their original capacity. It is possible to re-purpose these batteries for second life applications, for example, small domestic units to store excess solar power. However, the process of sorting the batteries, so that similar batteries can be used together, is onerous. The main reason for having similar batteries together is that most applications rely on series strings of batteries to get a suitable voltage, and as such the string capacity is limited by the weakest battery. This paper looks at how effective different active battery balancing techniques are at balancing dissimilar battery cells in series to see if this can be used as mitigation against sorting. The paper shows that while battery balancing can be used, the time taken to balance the batteries while keeping within the current limits is long and that the full battery pack capacity is unlikely to be realised. For the majority of types of battery balancing circuit using balancing as mitigation against battery sorting is not feasible.