31. October 2011The report from WP 1.5 is about the battery modeling and degradation tests
EXECUTIVE SUMMARY
Range has been the Achilles heel of EVs compared to cars with internal combustion engines for the last century. The properties of the electrochemical energy storage medium also known as the battery decides how much energy can be stored and subsequently transformed into propulsion. Development of the lithium ion and lithium polymer battery families has shown significant improvement in gravimetric and volumetric energy capacity during the last 20 years. Based on the current market situation and estimated utilisation areas for different chemistries the EDISON project chose to purchase Li-ion manganese spinel and lithium iron phosphate batteries for laboratory studies. Later in this chapter more information about battery module tests can be found. Information on tests on EV sized battery packs can be found in the chapter from WP6A. During the project period comprehensive research programs on battery technology have been established e.g. the ARPA-E managed by department of energy in the USA. Some of the envisioned products promise an increase in gravimetric energy capacity with factor of 2-4. . Tejs Vegge, DTU is currently heading a research project funded by DSF about the very promising Li-air technology.
In WP 1.5 a simulation-model was developed in order to evaluate the effects that use-pattern has on EV battery degradation and lifetime. From a user perspective, the important parameters relating to an EV battery is energy capacity, efficiency and remaining lifetime. These parameters are strongly dependent on a number of variables such as temperature, depth-of-discharge (DOD), charge and discharge rates and cycle number. This means that battery use affects battery performance. The following aging effects are taken into consideration in the model: Depth of discharge (DOD), number of cycles, state of charge (SOC), C-rate, temperature, temperature cycling and calendar life.
A battery module test setup was developed at Risø DTU, ABF in order to test a 75 Ah 8-cell NMC Kokam battery module and a 50 Ah 10-cell LFP Byd battery module. Results from the battery module tests were used as input for the battery model. The setup was used to measure the module impedance as well as the impedance of the individual cells in the module. It was observed that when a cell in a module collapses and the cell voltage drops to 0V due to harsh or prolonged operation of the module, the impedance of the collapsed cell increases dramatically. This means that even though such a cell collapse will have limited impact on the electrical performance of the EV battery pack, local heating of the collapsed cell during pack operation may lead to overheating and subsequent violent collapses of the adjacent cells. This potential safety hazard must be
addressed and properly handled by the battery pack BMS system.
Further in WP 1.5, an equivalent circuit (EC) model describing the dynamic behaviour of the batteries during operation was developed. The EC model can be represented as a set of ordinary differential equations and offers a good compromise between accuracy and simulation speed. In relation to the development of the EC model, a technique to find the open circuit voltage (OCV) as a function of state of charge (SOC) was developed and applied to the batteries. The EC model combines the OCV vs. SOC data with SOH data and measurements of the battery impedance to provide real-time information about the battery voltage and SOC as a function of the battery use. A literature study was also conducted in the Edison WP 1.5 to ensure the relevant public knowledge was present in order to accomplish the tasks executed in the WP.
A battery module test setup was developed at Risø DTU, ABF in order to test a 75 Ah 8-cell NMC Kokam battery module and a 50 Ah 10-cell LFP Byd battery module. Results from the battery module tests were used as input for the battery model. The setup was used to measure the module impedance as well as the impedance of the individual cells in the module. It was observed that when a cell in a module collapses and the cell voltage drops to 0V due to harsh or prolonged operation of the module, the impedance of the collapsed cell increases dramatically. This means that even though such a cell collapse will have limited impact on the electrical performance of the EV battery pack, local heating of the collapsed cell during pack operation may lead to overheating and subsequent violent collapses of the adjacent cells. This potential safety hazard must be
