**1. Introduction**

Batteries are DC electrochemical energy storage devices which are mainly utilized as DC source or load in applications. However, AC current injection in a battery is necessary to preheat in cold weather, to measure the internal impedance, to balance the capacity based on impedance, and to charge by sinusoidal ripple current method [1–4]. Conventionally, AC current is injected to rechargeable batteries using linear circuits which work offline (the batteries are disconnected from the applications) [5,6]. Recently, power converters are being used to inject AC current to rechargeable batteries which can be operated either offline or online (the battery is connected to the applications) [1–4,7–13]. A buck converter topology was used to inject AC current on top of DC charging current in [9]. AC current was imposed on top of discharging current in [10]. The methods available in the existing literature for AC current injection using power converters can be operated either offline or online (either while charging or discharging). A versatile AC current control technique is proposed in this manuscript which can be operated in both offline and online conditions for a broad frequency spectrum.

The AC current injection using power converters for AC system (grid tied inverter) is an well established technology where the power flows unidirectionally and has a significant voltage variation corresponding to current [14,15]. The unidirectional power flow of a single phase inverter is depicted in Figure 1a. *Vout* and *Iout* are the output voltage and current. The arrow line depicts the steady state operating condition. The tilted arrow indicates the AC signal. Power converters can operate beyond the steady state for transient conditions which is depicted as control range by shaded area. Compared to AC systems, AC

**Citation:** Islam, S.M.R.; Park, S.-Y. Versatile AC Current Control Technique for a Battery Using Power Converters. *Batteries* **2021**, *7*, 47. https://doi.org/10.3390/ batteries7030047

Academic Editors: Kai Peter Birke and Duygu Kaus

Received: 12 May 2021 Accepted: 9 July 2021 Published: 15 July 2021

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current injection to a DC system (rechargeable battery) using a power converter is a recent idea [16,17]. Power can flow bidirectionally in a rechargeable battery. The power flow of a conventional bidirectional converter is shown in Figure 1b, which can control DC current for a constant voltage. A controller for conventional bidirectional converter is not enough to inject AC current into a battery. This is because a battery has a small internal impedance and a variable DC voltage offset. A significant amount of AC current causes a small voltage ripple in a battery. The AC injector for rechargeable batteries should be able to control large current within a small voltage window as shown in Figure 1c. Therefore, AC current control for rechargeable batteries is more challenging than conventional bidirectional converters.

**Figure 1.** Power converters output: (**a**) AC current injector using an inverter for AC systems (**b**) fixed voltage bi-directional converter for DC bus, and (**c**) AC current injector required for batteries.

A versatile control technique to inject AC current into a rechargeable battery is presented in this manuscript which can inject AC current up to a magnitude of 5 A and a frequency of 2 kHz for a 40 Ah 13.8 V battery. The controller is capable of injecting AC current using two topologies: synchronous buck and H-bridge. In addition, the key contributions in this manuscript are listed as follows:


The modes of AC current injection to a battery is discussed in Section 2. Power converter topologies, small signal analysis, and controller architecture are in Sections 3–5. Finally, results are presented in Section 6.
