Comparing the degradation effect of a ‘two-cell’ Supercapacitor-module with and without voltage equalization circuit(s) under experimental self-discharge and load cycling tests

Abstract

The problem of over-voltage is common in SC modules under cycling conditions. To prevent this, the voltage equalization circuit is usually used as a tool to balance SC cells in a module to increase the life expectancy of the system. There is lack of detailed experimental analysis on its performance on degradation proximity testing. This paper therefore focuses on demonstrating the importance of voltage equalization circuits specifically on their performance in degradation proximity between two-cell modules experimentally. Three types of common active balancing equalization circuits were first tested and the best performing circuit was used to compare with a commercial Maxwell voltage equalization circuit under self-discharge and load cycling tests after 1115 h of continuous cycles under high temperature. The periodic capacitance (C) and ESR parameters calculated from charge-discharge characterization test results over 1115 h shows that the active balancing circuit produced a more even/balance parameter regression between each two-cells in a module over time.

Introduction

In recent years, supercapacitors have attracted significant attention, mainly due to their high power density, long lifecycle of charge/discharge with high currents, high efficiency, a wide range of operating temperatures, environmental friendliness with low maintenance, and high safety [1], [2]. These characteristics have made supercapacitors very competitive in various applications [3], [4]. Unfortunately, even with all their advantages, supercapacitors still pose some major concerns in their cycle-life and shell-life [5], [6], [7] which causes; (1) High cell voltage as a result of electrochemical decomposition of the electrolyte under certain conditions, which drastically reduce the life expectancy [8], [9], [10], [11], (2) Parallel leakage current causing fast self-discharge, limiting specific applications such as EV/HEV [1], [12].

The most common SC degradation factors are high temperature and high voltage [8]. Beside temperature and voltage, SCs also age by constant current charge/discharge cycles, undergoing power and drive cycles on SC module [13]. Like many electrochemical devices, the chemical reactions in SCs follow the Arrhenius law which states that higher temperature causes more rapid chemical reactions—for every 10 °C increase or each 100 mV, the chemical reaction rates double and the life is halved [8], [14], [15].

To study the degradation effects of SCs, constant current cycling tests known to induce self-heating through joule heating is carried out. Moreover, researchers' have found ways to speed up the degradation process in laboratories, one of which is, by introducing thermal effect to the cycling test, consequently reducing SC efficiency [8], [15], [16], [17]. After prolonged charge-discharge cycling under thermal operations, SCs demonstrate parasitic electrochemical reactions characterized by degradation modes which include; 100% increase in resistance (ESR), 20% decrease in capacitance [18], and microscopic phenomena such as gas evolution, increase in electrode mass, and local separation of the coating layer from the metallic collector.

In applications with high voltage system such as EVs, SC cells are connected in series forming a “module” to meet system power requirement [19]. In a system with more than one SC cell, the life expectancy decreases significantly even more than a single SC, as a result of over-voltage coupled with the possibility of uneven temperature distribution across the module. SC module failure mode is characterized by a “open/short circuit” with ESR rising to infinity, as a result of electrolyte loss over time due to the absence of voltage management circuit in the module [20]. This form of failure is impetuous and can jeopardise the whole system in an instance. Hence, to prevent imbalance and over voltage between SC cells in a module the presence of a voltage equalization circuit is inevitable to increase life expectancy of the system [21]. A cell voltage equalization circuit maximizes the performance, and life of SCs installed in series [22], [23], [24].

The advantage of a balancing/equalization circuit in a SC module is demonstrated in this paper. The literature on equalization circuits in SC modules is mostly mentioned in passing [22], [23], [24], as a tool needed to balance SC cells in a module. Most research papers such as [22], [23], [24] simply acknowledges the importance of equalization/balancing circuits and incorporates it accordingly to their respective research problems, therefore insight to its application as it affects the SCs in module; either for short-term or long-term operations is lacking. This paper, however experimentally demonstrates the importance of it, especially as it relates to degradation during cycling.

Comparability studies on two balancing circuits is used to validate the importance of voltage balancing with emphases on degradation proximity between two-cells in a modules.