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Validation of the PVToolbox Against The First Run of the Battery Capacity Cycling Test

Michael M.D. Ross
RER Renewable Energy Research

Full Text Report
Link to CETC-Varennes

Acknowledgements:

Research conducted on behalf of the Photovoltaics and Hybrid Systems Program at the CETC-Varennes (Natural Resources Canada) with partial funding from the Panel on Energy Research and Development (PERD).

Citation:

Ross, Michael M. D. Validation of the PVToolbox Against The First Run of the Battery Capacity Cycling Test. Report to CETC-Varennes (Natural Resources Canada). Montreal, Qc: RER Renewable Energy Research, 2003.

Summary:

Monitored data from the CETC—Varennes hybrid system test bench was used to perform a rudimentary validation of certain elements of PVToolbox. The test bench data provided information on the room temperature, battery temperature, outdoor temperature, battery voltage, and battery current for six cycles of discharge and recharge. A Simulink model to replicate this battery cycling was assembled from PVToolbox components as well as some purpose-built component models.

Initially the monitored data was compared to the output of a system model that used a battery model calibrated with data for the Global Yuasa battery. The behaviour of the test bench, which incorporates an Absolyte IIP battery, and the system model were significantly different, and as a consequence the existing battery model was fit to Absolyte II P data. A second comparison revealed that this did not significantly improve the performance of the battery model. As a result, the battery model was scrutinized in the light of all available data, and a number of changes were made. A third comparison showed somewhat improved performance, but nevertheless a number of important shortcomings remain.

The weaknesses of the battery model are rooted in two factors. First, we lack sufficient discharge and charge data at different temperatures and currents to adequately characterise the behaviour of the battery. As a result, we are forced to make assumptions which, while common in the literature, prove to be significantly at variance with reality. Second, we have assumed from the beginning of our model development that a single state variable can be used to characterise the state of the battery. In fact, the comparisons reported on this report suggest strongly that this is not the case: when currents vary, such as in constant voltage charging, one state variable, i.e., state-of-charge, is insufficient.

This report deals with the comparison of the simulation and the monitored data; an account of the fitting of the battery model to Absolyte data, and subsequent changes to the battery model, is given elsewhere. This report also includes recommendations concerning 1) minor changes to the hybrid test bench, 2) future tests to be run on the hybrid test bench, and 3) how the battery model’s inaccuracies should be dealt with in future investigations.