Seven-Level Flying-Capacitor Multi-Level Converter for a Differential Power-Processing Interface in Multi-Chemistry EV Battery Packs

Lithium-ion batteries suffer from a significant increase in their internal impedances in cold environments. Electric Vehicles (EVs) require cumbersome thermal-management techniques to prevent the increased battery impedance from limiting EV driving range. This paper proposes a bottom-side differential power-processing (DPP) multi-chemistry battery (MCB) pack for improving EV battery-pack performance at sub-zero operating temperatures, while relaxing thermal-management requirements. The proposed DPP topology replaces the full-power-processing converter in traditional MCB systems with a DPP converter that has a lower power rating, resulting in a more compact system. A detailed simulation of a 60 kWh MCB pack is performed, utilizing a realistic loss model for the DPP converter, to compare its performance against a comparable single-chemistry battery (SCB) pack of high-energy cells. Simulated results show that the MCB pack improves the usable drive energy (without restricting the acceleration and deceleration performance) by up to 160 Wh/L at −10°C operating temperature, while maintaining the same pack volume and requiring no pre-heating. A 400 V-to-150V bidirectional seven-level flying-capacitor multi-level (FCML) converter utilizing 100 V GaN devices is designed to serve as the power-mix interface within the MCB pack. The functionality of the proposed MCB architecture utilizing the FCML converter is verified using an experimental prototype in closed-loop operation using 14
$\times 12\mathbf{V}$
lithium-iron-phosphate battery modules. The prototype leverages a custom liquid-cooled heatsink employing jet impingement to operate bidirectionally with up to 20 A of processed current over a wide range of voltages with a measured peak-efficiency of 98.4%.