Analysis of regenerative braking and charging systems on electric vehicles from experimental can bus data

Abstract

Combining friction braking with regenerative braking in electrified vehicles is known to greatly improve efficiency. The aim of the project was to analyse the ‘cooperative control’ strategy between friction and regenerative braking systems in battery electric vehicles and understand the working principles, along with ways to improve efficiency. After performing tests on a chassis dynamometer with two vehicles to understand the working principles, a series of controlled tests on both vehicles were performed to understand the main factors impacting the efficiency of the recuperation system. The results were validated using a multiple linear regression study on over 60 trips on two routes; one dominated by inclines, the other by descents. A model with a mean error of under 4.5% was obtained for both cases, proving that the variables obtained accurately represent the regenerative braking efficiency. Further analysis was carried out to understand how speed affects the efficiency of an electric vehicle by obtaining the road load of the vehicle, and how accurately the road load impacts losses in regenerative braking. Additionally, tests were carried out to obtain the charging, drive, and battery efficiencies of the vehicle, and the internal resistance of the vehicle li-ion traction battery. From the tests, the working principles of the cooperative control braking system were identified, where it was noted that the strength and ratio of friction and regenerative braking applied by the vehicle are affected by brake pedal position and vehicle speed. It was also identified that the main aspects influencing the regenerative braking efficiency are inclines, vehicle speed and braking intensity. Additionally, it was found that one should reduce their driving speed to the lowest reasonably possible to improve range. From the analysis carried out, the overall efficiency of a Nissan LEAF AZE-0 was found to be 72.0 per cent excluding regenerative braking, consisting of a drive efficiency of 89.8 per cent, battery efficiency of 91.8 per cent, and charging efficiency of 87.4 per cent. For the route in consideration, a round-trip regenerative braking efficiency of 45.0 per cent was obtained. When plotting the trips on an efficiency map for the Nissan LEAF powertrain, the overall powertrain efficiency fell into a range of 88-90 per cent, compared to the peak powertrain efficiency of 95 per cent. It was therefore concluded that further improvements could be made to the powertrain, including modifying the reduction ratio, to account for lower-speed trips.