*Article* **Versatile AC Current Control Technique for a Battery Using Power Converters**

**S. M. Rakiul Islam \* and Sung-Yeul Park**

Electrical and Computer Engineering Department, University of Connecticut, Storrs, CT 06269, USA; sung\_yeul.park@uconn.edu

**\*** Correspondence: s.islam@uconn.edu; Tel.: +1-860-486-0915

**Abstract:** Although a battery is a DC device, AC current is often necessary for testing, preheating, impedance spectroscopy, and advanced charging. This paper presents a versatile control technique to inject AC current to a battery. Synchronous buck and H-bridge topologies are operated in bidirectional mode and controlled by uni-polar and bi-polar pulse width modulation techniques for the AC current injection. The input and output passive circuits are specially designed considering AC current and the properties of the battery. A controller is proposed considering a small internal impedance, small AC ripple voltage, and variable DC offset voltage of a battery. The controller is capable of maintaining stable operation of AC current injection in two power quadrant within a small DC voltage boundary of a battery. The controller is comprised of a feedback compensator, a feedforward term, and an estimator. The feedback gain is designed considering the internal impedance. The feedforward gain is designed based on estimated open circuit battery voltage and input voltage. The open circuit voltage estimator is designed based on filters and battery model. For validation, AC current is injected to a Valence U-12XP battery. The battery is rated for 40 Ah nominal capacity and 13.8 V nominal voltage The controller successfully injected AC current to a battery with +10 A, 0 A, and −10 A DC currents. The magnitude and frequency of the AC current was up to 5 A and 2 kHz respectively.

**Keywords:** lithium ion battery; AC current injection; bi-directional control; charger
