Lead acid battery operating temperature

When temperature increases, the equilibrium voltage of a lead-acid cell, EMF or Open circuit Voltage also increases. This is 2.5 millivolts per⁰ C when electrolyte has a specific gravity range normally used in a lead-acid battery. Another factor which affects the voltage is the acid sp gr. When temperature increases, the acid expands and sp gr decreases. The expansion is about 5%. This is the reason for the drop in specific gravity. Based on the above 2 factors, the change (Correction) in OCV is 3 millivolts per degree C( mV/⁰ C )
It is a matter of concern when electrolyte temperature increases above 25-27 ⁰ C to 35⁰ C and above. The charging voltage should be set at a lower value i.e reduce charging voltage by 3 mV for every increase of 1⁰ C rise above 27 ⁰ C. Otherwise, the life of the battery will be reduced due to higher gassing and grid corrosion. At higher temperature, reduce the float voltage and at colder temperatures, increase the float voltage by 3 mV per ⁰ C