Lithium-ion battery is widely used as energy storage unit in electric vehicles, mobile base stations and new energy sources. The safe operation of lithium-ion power battery is very important to ensure the normal use of the system. High internal resistance, small capacity and low state of charge (SOC) are progressive faults that often occur in the use of lithium-ion battery. A fault feature extraction method based on DBN is proposed to diagnose the three faults in this work. The learning rate of RBM composed DBN is generated by particle swarm optimization (PSO). After the feature data are extracted, a fault diagnosis model is established by extreme learning machine (ELM) to identify different faults. The experimental results show that battery fault diagnosis method proposed in the work can correctly identify each fault, and the diagnosis accuracy is 100%, which is obviously better than other feature extraction methods in fault diagnosis.