Estimation of the state of charge of Lithium Iron Phosphate Batteries using the open circuit voltage.

Abstract

Lithium Iron Phosphate (LiFePO4) batteries are increasingly used in numerous applications from electronics to power tools due to their high energy, power densities and long cycle life. State of Charge (SOC) of a battery being an important parameter for the Battery Management System (BMS) to assess battery lifetime and efficiency. Estimating the SOC of LiFePO4 using the non-linear relationship between Open Circuit Voltage (OCV) and SOC and voltage-based techniques is difficult because of its almost flat discharge curve. However, despite the almost flat discharge curve of the LiFePO4 battery, there is a realizable change in the voltage during discharge below some voltage. The purpose of this project was to establish a relationship between SoC and OCV of the batteries by designing a simple circuit that can accurately estimate the SOC of a lithium iron phosphate battery. This circuit utilizes the OCV-based technique due to that noticeable drop in voltage during discharge of the battery. The circuit mainly consists of an ATMEGA328P microcontroller, 16-bit Analog to Digital Converter (ADC) and OLED screen to display the results.