Modelling the RESS: Describing Electrical Parameters of Batteries and Electric Double-Layer Capacitors through Measurements

Being able to predict the behaviour of rechargeable energy storage systems (RESS) such as batteries and electric double-layer capacitors (EDLC) relies on the models used to represented them. Research on the development and enhancement of these models is performed on two tracks: on one hand by defining the model topology, where a trade-off is sought between the reduction of components in the model equivalent scheme and the ability of the model to describe all potential system behaviour, and on the other hand by improving the method of calculating the magnitudes of the different components in the model. The storage devices examined in this paper are divided in two groups, electrochemical devices and electrical double layer capacitors. In the first group there are lead-acid batteries, lithium batteries, Ni-Cd batteries and Ni-MH batteries. In the Second group, electric double-layer capacitors (EDLC).

For every model used to describe a battery or EDLC, different techniques are implemented to calculate the magnitude of the components. The work presented in the paper will elaborate on the calculating method used in NREL's FreedomCAR test-manual adapted to fit different battery- and EDLC models. This method consists of an iterative approach to the equations of the model using data gathered from performing tests on the battery or supercapacitor. Validation of the models obtained through this calculating method will be done by performance tests on the storage devices.

As the models of batteries and supercapacitors are a key aspect in the simulation software for hybrid vehicles more accurate models result in more accurate simulations. An example of a simulation program used to simulate the performance of hybrid drive lines is VSP. This program was developed at VUB (ETEC) by Prof. Joeri Van Mierlo.

An other well known simulation program is ADVISOR. Within this simulation program it is possible to select the battery model for the test cycles. The user of the program is asked to enter the component's magnitudes of the chosen model.

As already stated, these vehicle simulation programs strongly depend on the underlying component models, the data generated by which determines the accuracy of the program's data-output. The availability of reliable storage RESS models will thus be a key element in allowing overall vehicle modelling. To accomplish the model enhancements, the FreedomCAR calculation process is applied to the different battery-and EDLC models.

Publication year:2009

Keywords:Battery Model, EDLC (Electric Double-Layer Capacitor), Simulation