Improving Ultra Fast Charging of LiFePO4 batteries at low temperatures with AC heating

The speed at which electric vehicle batteries can be charged usually depends on the available charging power. At low temperatures however, another bottleneck appears when the battery is unable to accept all available charging power. The charging speed is then limited by the battery. To fix this, the battery could be equipped with a heater so that the charging can take place at a higher temperature. Using an experimental approach, the author tried to answer the following questions: \u95 How does temperature influence charging speed and battery degradation? \u95 What are the positive effects of the usage of battery heating? \u95 Is battery heating using AC an effective method from a battery point of view? \u95 Does AC heating cause battery degradation? The most important conditions that apply in this project are: \u95 Evaluated battery technology is LiFePO4. This is one of the best suitable technologies for EV purposes today due to its high energy density, high power density and safety. \u95 The experiments are focused on Ultra Fast Charging (15 minutes). Battery heating can create a significant advantage in this operating mode. Results Measurements revealed a rapid decrease of charging performance below 10ºC. Charging performance decreases to 70% at 0ºC to as low as 20% at -20ºC compared to the 25ºC level. Ageing effects also showed a strong relation with temperature. Ultra Fast Charging at 0ºC results in a five times higher ageing rate than doing the same at room temperature. The available charging power is a practical power source for heating. Converting this power into a square wave using power electronics resulted into a battery current of about 17,5A RMS per 18650 cell. Although this current level is rather high (~16C), measurements showed only little negative effects as a result of prolonged exposure to this current level. Conclusions Temperatures below 10ºC significantly limit charging speed and battery lifetime. Due to this effect, AC battery heating is able to improve performance below 10ºC. The performance improvement was determined as 25% at 0ºC and even more at lower temperatures. Due to a significant increase in ageing effects below 10ºC, also battery lifetime can be improved in an important amount. Effects on battery lifetime due to the (AC) heating current and thermal cycles were found to be minimal. From the battery point of view, AC heating thus seems a good technique to improve performance and lifetime at low temperatures.Electrical Power ProcessingElectrical Engineering, Mathematics and Computer Scienc