A battery residual capacity indicator based on the battery internal resistance: an experimental study

Capacity fade of lithium-ion rechargeable battery is a slow process, driven by the passing time (calendar aging) and by the battery operating conditions (cycling aging). The operating conditions exhibit very fast dynamics (e.g., the battery current of an electric vehicle) as well as moderately slow ones (e.g., the ambient temperature variation). Because of the huge time-scale differences between the capacity fade phenomenon and the battery usage waveforms, it is very difficult to compact the waveform information and to predict the actual capacity. It is therefore necessary to extract a minimal set of indicators that are useful for the capacity estimation. For this reason, we analyse the possibility of using the battery internal resistance as a predictor for the estimation of the residual capacity. The analysis is carried out for Lithium Cobalt batteries continuously operated with random current patterns and in pulsed current tests, where the influence of the state of charge can be neglected. The results show that a linear correlation between the internal resistance and the capacity can be established, so that it is possible to use the resistance to assess the residual capacity.