This paper discusses new experimental work investigating the change in pH of the electrolyte of individual cells in Lead-Acid batteries during discharge with a view to predicting cell polarity reversal and thereby pre-empting potentially catastrophic failure in batteries. The discharge tests were carried out on batteries which were classified as 'new' and 'aged' as a means of indicating their state of health. A new pH phenomenon has been recorded for the first time in this paper as evidenced by experimental results including measurement of half-cell potentials. The trends observed during the discharge tests could be used as additional means of indicating impending cell reversal in series connected lead-acid cells. (C) 2016 Elsevier Ltd. All rights reserved.