Determine battery charge states

Battery management systems provide an important monitoring and control component for the use of battery or accumulator packs by means of charge state detection and safety mechanisms. These packs are a parallel connection of several strings containing battery cells connected in series.

State of the art

Up to now, each string requires a separate current sensor in order to be able to determine the state of charge (SOC) of the individual battery cells on the basis of the measured partial currents. These sensors have to meet high robustness and accuracy requirements and are responsible for losses in the battery that scale with battery size. This has a negative impact on the overall cost of the battery.

Technology

Scientists at the Institute of Control Systems (IRS) at KIT have developed an efficient method to determine the charge states of all the cells in a battery for battery management systems. By using a new model and a Kalman filter, it is possible to reduce the number of current sensors required to determine the SOCs of the cells. The novel aspect here is that the string currents no longer need to be measured but are fed in as estimated values. The current strength is determined by a model that compares the measured voltages with the estimated states of the Kalman filter of each cell at any time. This is followed by an adjustment of the estimated state of charge of each cell using the currents obtained. The procedure can be applied both when the overall current is known, a single ammeter having been applied, and when a complete estimate of all currents is available.

Advantages

The new measuring method enables cost savings resulting from the saving of current sensors that are no longer required. In addition, the power loss of the current sensor is reduced by eliminating the measuring resistance.

Options for companies

KIT is looking for industrial partners for licensing and practical application of the method.