Study On Estimation Of The State Of Charge For MH-Ni Battery Based On Steady Internal Pressure For Vehicles

MH-Ni battery is one of the first choice of power on a hybrid electric vehicle(HEV)because of its advantages such as long cycle-life, excellent safety, wide working temperature range, high charge and discharge power which is higher than the lithium-ion battery especially at a low temperature. In practical application of HEV, the state of charge(SOC) must be timely and accurately informed to the system of the vehicle. After the battery production is finished, the stable pressure(Pstable) and the equilibrium hydrogen pressure(Peq, H2) of the alloy anode have a one-to-one relationship, so there is a certain relationship between the stable internal pressure and the alloy anode pressure component isotherms(p-c-t curve). The stable pressure can be used to estimate the SOC of MH-Ni battery. Therefore, it is significant to study the internal pressure of MH-Ni battery.In this paper, the D-type power batteries were fabricated(initial state), the internal pressure changes during charge and discharge processes were tested and analyzed. The results show that the battery internal pressure increases with the extension of charging time, and gradually reduced in the subsequent shelve and discharge process, and even restore to the initial pressure value. The internal pressure increases with the charge-discharge rate and the environmental temperature.The formation system, injection amount and the compaction density affect the magnitude of the internal pressure of the battery.Through the accurately extraction of the stable internal pressure of the gases inside the battery at different SOCs online, we finally set up the relationship between the Pstable and SOC of the initial state of battery under different temperature. The results show that the relation curve between the two agrees with the alloy p-c-t curve.After the different cycle times at 1 C rate and the mode, stored for different days at60℃, the battery performance were tested and the relationships between the Pstable and SOC under different temperatures have been established. The results show that which are different with the relationships of the initial state. We found that the relationship between the two gradually changed to the polynomial with the extension of storage time or use ofthe battery under the same temperature, through the fitting analysis of the relationship curves, which meet the relation of y = ax4 +bx3 + cx2 +dx +e,where y is for the Pstable, and x is an any value ranges from 0 to 100 of SOC.