Study Of Grid Plated Bi Or Sn And Behavior Of Positive And Negative Plates At Different Discharge Rates In VRLA Batteries

Valve-regulated lead-acid (VRLA) batteries are widely welcomed by customersdue to their advantages of maintenance-free, high performance, low cost andpollution-free environment. The positive grids of VRLA batteries generally employPb-Ca-Sn alloy at present. However, the problem of passivation layer formed on alloysurface is still one of the main reasons for the influence on the deep dischargeperformance and cycle-life of the VRLA batteries. In this paper, tin or bismuth wasplated on Pb-Ca-Sn alloy surface by electrochemical deposition method and theirinfluences on anode film properties and battery performance were investigated.Moreover, the great voltage drop of the plates in the upper to lower parts appears at ahigh discharge rate for137mm high plate, so that the current and potentialdistributions on the plate and the active mass utilization become uneven. Therefore,the current and potential distributions of12V20Ah VRLA battery at differentdischarge rates were measured so as to further understand the reasons for batterycapacity limitation.The properties of the anode films formed on Pb-Ca-Sn alloy surface with platingtin or bismuth were studied by AC voltammetry, linear sweep voltammetry andelectrochemical impedance spectroscopy (EIS). The results show that the plated layerson alloy electrodes not only inhibit the growth of the Pb(II) compounds with poorconductivity and promote the growth of PbO1+x(0<x<1) in the anode film, but alsoimprove the conductivity of the interface between grid and positive active mass (PAM)and enhance anti-corrosion. The coating film has little influence on the rate of oxygenevolution, but promotes the hydrogen evolution.The effects on battery performance of plating tin or bismuth on positive gridwere studied. The results include the following aspects: improving the dischargecapacity at a high discharge rate, reducing positive plate polarization, prolonging thedeep cycle-life of batteries, reducing the discharge internal resistance in the cyclesespecially for the positive plate, decreasing the loss of water and the corrosion rate of positive grids, changing the morphology structure of corrosion layer, increasingporosity in corrosion layer reducing the formation passivation layer. It is found thatthe PAM becomes obviously soft and the internal resistance dramatically increases atthe end of the cycling life. The high polarization of positive plate limits the capacityof battery.Current and potential distributions in the upper and lower parts of the batteryplate have been measured at a high discharge rate. The experimental results show thatthe discharge capacity decreases with the increase of discharge rate and the batterycapacity is very sensitive to temperature at a high discharge rate. From the experimentresults of simulated discharge of upper and lower parts of the battery, the upper partof the plates was discharged first at the early stage, afterwards, the discharge currentsof lower part become bigger than upper part and the currents in upper part and lowerpart tend to be equal in the end. The single cells voltages are uneven and the potentialdifference between upper and lower parts in both positive and negative plates is big ata high discharge rate. The negative plate is more sensitive to environmentaltemperature. The lower temperature the greater polarization of negative plate, thenegative plate with great polarization becomes the main factor that limits capacity.