The battery online monitoring device is regarded as the research object, a scheme based on Cirrus logic's EP9315 processor and embedded Linux operating system is proposed. Short-distance wireless communication technology is adopted to transfer data. The hardware structure and software design of the system are also introduced. The battery inner resistance is measured by DC discharge, which guaranteed the reliability of online monitoring. The application of fuzzy neural network (FNN) optimized by genetic algorithm (GA) in battery capacity testing has wide prospect.