Research On Negative Reaction Mechanism Of VRLA Batteries For Hybrid Electric Vehicles

In recent years, more and more attention has been paid on hybrid electric vehicles (HEV). In HEV applications, the battery must operate under high-rate partial state-of-charge (HRPSoC) duty, namely within a 30-70% state-of-charge (SoC) window. In such applications, the negative plates of valve-regulated lead-acid battery (VRLA) can be progressive sulfation. Thus the lead sulfate fails to transform Pb of free state. And then the valve-regulated lead-acid battery will become invalid. It has been established that addition of carbon additives to the lead negative active material (NAM) of lead-acid batteries can retard substantially the sulfation of the negative plates during HRPSoC duty. In this paper, the influence of the conductivity of different carbon additives on the lead powder will be studied. And then the negative plates with different carbon additives on the flooded lead-acid batteries are made as to further understand the influence and mechanism of carbon additives on the performance of the battery. The following conclusions have been drawn:Carbon additives can enhance the conductivity of lead powder obviously, the ability to enhance the conductivity:expander graphite>carbon black ≈acetylene black>graphite.2 V 2.5 Ah flooded lead-acid batteries which negative plates with different proportions of different types of carbon are made. They are tested at high discharge rate. The results show that carbon additives to the negative plates can significantly improve the capacity of lead-acid batteries, where the increasing effect of expansion graphite, acetylene black, and carbon black is obvious but graphite’s is not. And when the proportion of expansion graphite, acetylene black and acetylene black is 0.5% or 1%,0.5%, and 0.5% respectively, the effect is better. In the negative plates, when the content of expansion graphite decreases, the discharge capacity increases and reaches its maximum in the ratio of 1%; and when the content of acetylene black and carbon black is between 0.1% and 0.5%, the discharge capacity increases and reaches its maximum in 0.5% content, but it decreases when the content is over 0.5%; as for graphite, when its content is from 1% to 5%, the discharge capacity increases and reaches its maximum in 5% content but it decreases when the content of graphite is between 5% and 10%.0.5% or 1% expansion graphite,0.5% acetylene black or 0.5% carbon black in negative plate can significantly improve the discharge capacity of battery at 3C,5C and 7C discharge rates. In 5C and 7C rates, the battery capacity containing 1% expansion graphite is larger than that of 0.5% only, equivalent to battery with 0.5% of acetylene black or carbon black. In 7C discharge, the capacity of carbon battery in negative plate remarkably increases. This is because carbon can raise the conductivity of the plate, promote the transmission of electron, enlarge the specific surface of negative active materials, promote the spread of sulfuric acid into the internal of plate, speed up the rate of diffusion and improve the utilization of the negative active materials.The results of SEM and BET test of carbon and negative plates show that all investigated additives contribute to increase the specific surface area of NAM and reduce the median pore radius of NAM. This shows that carbon additives can refine the negative pole of lead sulfate and simultaneously increase the specific surface area and conductivity, making electron being passed effectively. And they also inhibit the sulfation of negative pole and promote the conversion of lead sulfate to lead, increasing the capacity of battery.