Implementing a DC UPS with Battery’s State of Charge Estimation Based on Coulomb-Counting Method

Abstract

Embedded applications with requirement of portability need rechargeable battery. A novel DC Uninterruptible Power Supply (UPS) system with battery charging and battery’s State of Charge (SOC) monitoring system has been proposed in this paper. DC UPS systems are evolved in recent times due to requirement of fewer power conversion stages as compared to traditional UPS. This paper describes the DC UPS circuit which transits instantly and automatically from external power supply to battery source and vice versa when external power supply is removed and applied respectively. Modes of operation of DC UPS are explained in this paper. Proposed circuit uses rechargeable battery and thus, a battery charging circuit and battery's SOC estimation circuit are part of the proposed circuit. Battery’s SOC is estimated by Coulomb counter method. Charging circuit diagram and designing of its components are explained. SOC estimation is based on integrating current through the battery, which is done by coulomb counter - LTC4150 which is interfaced with a micro-controller. Initial SOC estimation is one of the important tasks while implementing coulomb counter method. It is estimated by measuring Open Circuit Voltage (OCV) of battery and mapping it to get corresponding initial SOC. Proposed circuit is designed in such a way that, if battery voltage reduces to 3.3V while discharging, circuit disconnects battery from system till external supply is available to charge the battery. Microcontroller is programmed to calculate and display Initial SOC, SOC, and battery voltage on serial monitor.