Directly Interconnected High-Energy and High-Power Battery Packs

Abstract

Designing battery packs is a trade-off between power capability and capacity. Often, high power is only desired for short periods; otherwise, high capacities are preferred. To meet these requirements, hybrid packs comprising high-power and high-energy batteries can be used. However, a major drawback of these systems is the need for additional direct current to direct current converters, which increase the complexity, weight, and cost. By directly interconnecting high-power and high-energy battery strings in parallel, the current distribution is determined exclusively by Kirchhoff’s laws, which can lead to the overloading of individual batteries and thus to damage or dangerous failures. To overcome these problems, we developed a layout and control algorithm for directly interconnected packs that keeps them in a safe state by solely controlling the external power, which is governed by two additional requirements. These keep the discharge current of the high-energy element below the maximum charge current of the high-power element and the charge current of the high-energy element below the maximum discharge current of the high-power element while the pack is discharging or charging, respectively. As a proof of concept, a directly interconnected lithium-ion battery pack was successfully designed using the electrochemical simulation software Battery Simulation Studio 2021,which was tested and integrated into a Audi A3 cabriolet.