Influence of H2SO4 concentration on lead-acid battery performance

With introduction of VRLA batteries the volume of electrolyte in the battery was reduced. To compensate for the reduced amount of H2SO4 in the cells, its concentration was increased from 1.28 to 1.31–1.34 relative density. This technological change was made ignoring the effect of acid concentration on the electrochemical activity of PAM, which might be the reason for the dramatic decrease in VRLAB cycle life. Investigations into the influence of acid concentration on battery performance have been carried out at LABD.

Acid concentration regions. Six concentrations of H2SO4 have been investigated – 1.15, 1.18, 1.21, 1.24, 1.27, 1.33 relative density. The investigated H2SO4 concentration range can be divided in two regions with regard to utilization of H2SO4.

  • CH2SO4 < 1.24 In this concentration region the utilization coefficient of H2SO4 is the highest (52% - 87%). Hence, H2SO4 limits cell capacity and thus battery cycle life. This H2SO4 concentration region is named H-region and batteries with electrolyte concentration within this region are called H-type batteries.

  • CH2SO4 > 1.24 Within this region the utilization of PAM is highest. The PAM structure and phase composition limit battery life. This region is called P-region and batteries with electrolyte concentration within this region are named P-type batteries. The influence of pH on PAM structure, phase composition and electrochemical activity is discussed in other section.

Battery characteristics. To investigate the influence of H2SO4 on battery capacity and cycle life performance six types of 12V/32 Ah flooded batteries have been manufactured and tested at our Department. The observed experimental data can be summarized as follow:

  • H-batteries. They have long cycle life, but their capacity is lower than the rated value. Moreover, with increase of discharge rate the cycle life increases. H-type batteries operate within the concentration region of enhanced solubility of PbSO4 and hence their charge efficiency improves. They are suitable for applications with charge voltage lower than 14 V.
  • P-batteries. These batteries have short cycle life (less than 100 cycles), but their capacity is high (above the rated value). Within this acid concentration region the positive plates undergo fast passivation, which leads to early battery end of life.


  1. D. Pavlov, V. Naidenov, S. Ruevski.  Influence of H2SO4 concentration on lead-acid performance. H-type and P-type batteries, J. Power Sources, 161 (2006) 658

Keywords: reduced amount of H2SO4 in VRLAB, acid concentration, utilization coefficient of H2SO4 in LAB, H-region of H2SO4 concentrations, H-batteries, P-region of H2SO4 concentrations, P-batteries