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THE equilibrium relations of lead-acid storage batteries have been extensively studied. The double-sulfate theory proposed by Gladstone and Tribe1 has been substantiated by analytical methods,2 thermodynamic studies,3 and electromotive force measurements of galvanic cells simulating the charged state of the electrochemical system.4 It is well recognized that on discharge one equivalent each of lead dioxide and lead and two equivalents of sulfuric acid are consumed and that two equivalents of lead sulfate and two equivalents of water are formed per faraday. It is known that the reverse occurs on charge and that the lead-acid storage battery may be subjected to many cycles of charge and discharge. The battery is reversible in that chemical and electric energy may be interconverted in repeated cycles. In practice, reversibility in the thermodynamic sense is not completely realized and the voltage during discharge is somewhat lower than the reversible electromotive force. Likewise the charging voltage is correspondingly higher. The more rapid the charging and discharging the greater is the deviation from thermodynamic reversibility and the greater is the loss in energy efficiency.