Due to the less awareness of the real-time operation condition and the low utilization rate of the power battery in the electric car, this paper proposes a novel design scheme of online monitoring system software for power battery. The system realizes the real-time remote monitoring battery status, fault diagnosis, data storage and estimates the state of charge (SOC) of the battery. The system using the back propagation neural network algorithm for estimating SOC, improving the accuracy of SOC estimate.