Determination of storage loss characteristics with reasonable measurement and calculation effort

This paper presents a method how to simply determine the losses of an energy storage depending on state of charge and actual power. The proposed method only requires the measurement of electrical quantities to determine the characteristic map and therefore can be implemented without need to modify of an existing electrical storage system. Exemplary results are shown of lithium-ion and lead-acid battery packs. The losses of the used lead-acid battery pack shows a clear dependency on the state of charge, mainly a quadratic dependency on the terminal power and a difference between charge and discharge. The losses of the used lithium-ion battery packs shows in principle a similar result but with a much less significant dependency on the state of charge. The characteristic map of losses is useful to optimize the storage operation regarding minimal losses whenever a degree of freedom of the choice of storage power exists, e.g. in an energy management system of a microgrid for cost savings in the grid-coupled operation and longer back-up time in the island operation.