Hardware and software development for implementation of fast and safe charging of commercial lihtium-ion batteries

Abstract

From single cells in handheld electronics to enormous packs in battery electric vehicles (BEV), batteries govern modern life. Lithium ion batteries (LIB) present the best available commercially available products for these applications; they have the highest energy densities and can output currents many times their capacity. But safely charging LIBs requires a slow and detailed process which is typically unacceptable for use in BEV and other rugged handheld devices; therefore, decreasing the required charging time would be greatly beneficial. Fast charging methods do present dangers and concerns. Unmonitored fast charging of LIBs allows for the potential of lithium plating where the lithium ions within the cell are converted to metallic lithium at the battery anode. Lithium plating can remove these ions from the charging and discharging process causing reductions in battery capacity. The metallic lithium structures formed also present the dangers of short circuit and thermal runaway. In this thesis, a charging protocol is developed using equivalent circuit models and experimentation with the goal of the elimination of lithium plating. First, equivalent models of a test cell were determined and validated. Then, this test cell was used to find the fast charging protocol both experimentally and through the use the equivalent circuit elements. Custom power electronics and software were then developed to implement the proposed charging protocol on commercial LIBs for 350 cycles. The results of this experiment show that the charging protocol did not create noticeable lithium plating while decreasing the charging time required by a typical constant current - constant voltage (CC/CV) from 50 minutes to 29 minutes. The proposed charging protocol decreased the charging time without stressing the LIB beyond its set limitation.