“You can not measure state of charge directly with a sensor. Instead, you must measure other signals such as current, voltage and battery temperature, which can then be used to estimate the state of charge,” explained Björn Fridholm, Project Manager at Viktoria Swedish ICT.
It is important to keep track of state of charge for several reasons. The driver needs to know how much energy is left in the battery in order to judge how far it can run. It can also risk damage or shortened life of a lithium-ion battery if one drives at a high or low state of charge. A battery also loses capacity during its lifetime, and the estimation of state of charge must include the battery capacity. Björn Fridholm has invented a method to estimate the battery capacity.
“Such an estimate will contain significant uncertainties that must be dealt with, and that's really what the invention is all about. This will increase the robustness of the estimate, but it will also make it possible to identify when the battery wears abnormally quickly,” said Björn.
The method has been handed over to Volvo Cars and AB Volvo, and they will apply for a patent and commercialize the solution. Björn Fridholm expects that the solution will be found in commercial products within two to three years.
Viktoria Swedish ICT collaborated with AB Volvo, Volvo Cars, Intertek, SP and Chalmers on the State of the Function project. It was funded by the Swedish Energy Agency within the Strategic Vehicle Research and Innovation Initiative (FFI). Viktoria's part has been to develop algorithms for estimating the battery state of charge.