Abstract
This paper presents the algorithms, hardware overview and testing results for controlling discharge currents from mixed battery modules placed in a parallel configuration. Battery modules with different open-circuit voltage (OCV), internal impedance or even state of charge (SOC) between modules are usually used to form a battery pack. Parallel placed mixed battery modules are typically seen in second-life, repurposed or exchangeable battery systems to increase power and energy storage capacity of a battery pack in mobile, electric vehicle (EV) and stationary energy storage application. This paper addresses battery module heterogeneity by taking advantage of buck regulators on each battery module and formulating scheduling algorithms to dispatch the buck regulators to balance the current out of each battery module. In this way, mixed battery modules can be combined and coordinated to provide a balanced power flow and guarantee safety of the total battery pack. Both open-loop and closed-loop scheduling of buck regulated battery modules are analyzed in this paper. In the open-loop algorithm, buck regulator dispatch commands are computed based on full knowledge of the OCV and impedance of each battery module, while monitoring the load impedance. In the closed-loop algorithm, dispatch commands are generated automatically by a digital proportional-integral-derivative (PID) feedback controller for which battery module current reference signals are computed recursively while monitoring the load impedance. The closed-loop scheduling method is also validated through experimental work that simulates a battery pack with several parallel placed buck regulated battery modules. The experimental results illustrate that the current from each battery module can be rated based on the SOC of each module and that the current remains balanced, despite discrepancies between OCV and internal impedance between modules. The experimental results show that the closed-loop algorithm allows scheduling of buck regulated battery modules, even in the absence of knowledge on the variations of OCV and impedance between battery modules.
Highlights
Increased investments in renewable energy technologies hope to reduce environmental pollution, improve energy security and provide economic benefits [1]
The experimental results show that the closed-loop algorithm allows scheduling of buck regulated battery modules, even in the absence of knowledge on the variations of open-circuit voltage (OCV) and impedance between battery modules
In Modular battery systems that consist of parallel placed battery modules are essential in range extension of electric vehicles and re-purposing of batteries for integration as energy storage in grid applications
Summary
Increased investments in renewable energy technologies hope to reduce environmental pollution, improve energy security and provide economic benefits [1]. Volatility and irregularity of renewable energy production necessitates distributed battery energy storage systems (BESS) to regularize power flow and make renewable energy sources economically viable solutions for grid support [2,3]. BESS with high capacity are needed in both EV applications and renewable energy grid support functions [5,6]. Most BESS use lithium-ion battery (LIBs) cells to provide high voltage with a desired energy capacity, long life span, low self-discharge rate and fast charging capabilities [7,8]. A LIB-based BESS is considered as the primary energy source for most EVs to provide a compromise between driving range and battery size [9,10]
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