Battery management systems (BMS) are widely used for proper protection and utilization of battery systems. In this paper, a modular BMS circuit is proposed for the design of modular battery stacks that can be reconfigured depending upon utilization. The real-time evaluation is carried out using four local modules and one central module. An Arduino nano with an analog-digital converter (ADC) multiplexer and voltage divider circuit is used as central module in the BMS system. The BMS voltage measurement accuracy is evaluated with the measured value. The state of charge (SOC) measurement technique is developed to calculate the SOC of each cell. A protection circuit is designed for under-voltage, over-voltage, and short-circuit in battery cells. The circuit also provides passive cell balancing. The hardware results verify the operation of the circuit as well as the cell balancing in the battery cells during the charging and discharging cycle.