Influence of H2SO4 concentration on lead-acid battery performance: H-type and P-type batteries

With commercialization of the VRLA battery design the H2SO4 concentration of the electrolyte filled in the battery has increased to over 1.30 g cm−3. On the other hand, it has been established that the electrochemical activity of PbO2 depends on the concentration of H2SO4, the highest activity being achieved in solutions with concentrations from 1.10 to 1.28 s.g. H2SO4. At CH2SO4>1.29 g cm−3CH2SO4>1.29 g cm−3, the PbO2/PbSO4 electrode gets partially passivated. The present investigation determines the initial capacity performance and the changes in battery capacity on cycling of 12 V/32 A h batteries with six different electrolyte concentrations between 1.15 and 1.33 s.g. H2SO4. The batteries are cycled with two discharge currents, 3.2 and 8 A. The utilization of PAM is 50% against 37% NAM utilization. The utilization of H2SO4 (ηH2SO4ηH2SO4) varies between 38 and 88%, depending on the concentration of H2SO4 in the electrolyte (CH2SO4CH2SO4). At CH2SO4=1.24 g cm−3CH2SO4=1.24 g cm−3, ηH2SO4≈ηPAMηH2SO4≈ηPAM. At CH2SO4<1.24 s.g.CH2SO4<1.24 s.g., the H2SO4 concentration limits the capacity of the battery (H-region of H2SO4 concentrations), whereas at CH2SO4>1.24 s.g.CH2SO4>1.24 s.g., the capacity of the battery is limited by PAM (P-region). It has been established that in the P-region of H2SO4 concentrations, the initial capacity of the battery is higher than the rated value (C0), but the life of the battery is short (maximum 100 cycles). In the H-region of H2SO4 concentrations, the initial capacity is lower than C0, but the cycle life is considerably longer than 100 cycles and depends on the discharge current and the H2SO4 concentration. The voltage of charged cells on open circuit declines with decrease in H2SO4 concentration, which allows charging of batteries at lower voltages, as is the case with IT batteries, and reversible sulfation of the plates is avoided as well. The obtained results of the present investigation suggest that lead-acid batteries can be divided in two types depending on the concentration of H2SO4 in them: H-type batteries with CH2SO4<1.24 s.g.CH2SO4<1.24 s.g., and P-type batteries with CH2SO4>1.24 s.g.CH2SO4>1.24 s.g. Currently, VRLA batteries of the P-type are commercially produced. H-type batteries have lower initial capacity than the rated value, but long cycle life and allow to be charged at lower voltages (e.g. 2.27 V per cell). P-type batteries have initial capacity higher than or equal to the rated value, but short cycle life and require high charge voltages.