V1 V2 Vn - + Differential Amplifier - + Differential Amplifier - + Differential Amplifier Low Cost High Voltage Battery String Monitoring System Abstract— Battery monitoring system is used in battery operated industrial and commercial equipments to avoid any failures and increase running hours of equipments. Current technologies are examined in this paper and aims to design a system to overcome their limitations of measuring high voltage and cost. The proposed system can monitor battery banks ranging from 12 VDC to 120 VDC connected in series thus forming an effective maximum string voltage of 960 VDC which existing technologies cannot achieve at such a low cost and high reliability. Keywords—Battery String; Voltage Monitoring; Voltage to Frequency; Isolation; 1. INTRODUCTION Today SMF/VRLA Batteries are used in all kind of industries like Telecommunications, industrial UPS, generator batteries, power distribution, Electric Vehicle etc. In general the battery system is a group of 2V /4V /6V/ 12 V DC batteries connected in series to provide the system of higher DC voltage forming a battery string. Battery string performance depends on a number of factors like operating temperature, individual battery voltage, how long the battery has been used and so on. This implies, all batteries in string may not have same state of charge and they cannot be charged/discharged uniformly in a string. Voltage is one of the important factors in calculating State of charge of battery. So, battery voltage monitoring becomes an essential part of Battery Management Systems (BMS) for calculating the present health condition of string. There is plethora of dedicated BMS ICs available for battery string monitoring but all of them have a limitation of measuring maximum input voltage of 4 VDC per channel and maximum string voltage of upto 70 VDC. Measuring high voltage require isolation between string and MCU which makes the designing complex and costly. Techniques available for measuring high string voltage are either expensive or have low reliability. This paper attempts to design a low cost high voltage battery string monitoring system while maintaining a high level of accuracy and reliability. Proposed system is designed with eight input channels having per channel input voltage range of 12 VDC to 120 VDC. When there is large number of batteries in string then batteries are combined together to form battery banks of higher DC voltage. The battery bank voltage can range from 12 VDC to 120 VDC. These battery banks are in turn fed to input channels of the voltage monitoring system. 2. Current Technologies & Challenges Battery stack monitoring problem has been around for a long time. Many approaches have been tried but few are successful in accurately measuring the battery voltage in string. This problem looks deceptively simple to many engineers and many of them end up connecting a differential amplifier across each battery [4] (Fig. 1). Fig.1 Unworkable scheme for battery string voltage monitoring This approach fails miserably due to common ground of all amplifiers leading to highly inacurate voltage measurements and possibly faliure of hardware circuit . This problem can only be solved by introducing isolation in the system which electrically isolates the subject battery from the measuring MCU. In order to provide isolation in the system relays, Opto-coupler, Isolation Amplifier, isolated communication bus , transformers or capacitors can be used. 2.1 RELAY BASED BATTERY MONITORING SYSTEM Relay based monitoring [8,9] is the cheapest method for battery string voltage monitoring systems. Each battery MULTIPLEXER ADC Mandeep Singh Rajeev Khurana Prateek Jain Electrical & Instrumentation Keysight Technologies, Electrical & Instrumentation Engineering Department, Manesar, Engineering Department, Thapar University, Patiala Gurgaon Thapar University, Patiala mdsingh@thapar.edu rajeev_khurana@keysight.com pj91000@gmail.com 2015 Intl. Conference on Computing and Network Communications (CoCoNet'15), Dec. 16-19, 2015, Trivandrum, India 978-1-4673-7308-1/15/$31.00 2015 IEEE 879