116.Battery voltages

LEAD ACID BATTERY

·        Generated voltage is 2  volts

·        A lead-acid cell will gas freely when fully charged but an alkaline cell gases throughout the charging period. The only indication of a fully charged alkaline cell is when its voltage remains at a steady maximum value of about 1.6-1.8 V.

·         After a 10 hour discharge a lead-acid cell voltage will have fallen to approximately 1.73 V

·        The state of charge held by a lead-acid battery is best indicated by a test on the electrolyte  specific gravity (SG) by using a hydrometer or measuring the terminal voltage when on load.

·        A fully charged lead-acid cell has an SG of about 1.27-1.285 (often written as 1270-1285) which falls to about 1.1 (or 1100) when fully discharged. the SG values quoted above for lead-acid cells are based on an ambient temperature of 15°C. (Add 0.007 to reading for each 10°C above 15 °C   & Subtract 0.007 from reading for each 10°C below 15°C.)

 NICKEL CADMIUM CELL

·         Generated voltage is  1.2 volts

·         The state of charge of an alkaline battery cell cannot be determined from its SG value. The   electrolyte density does not change during charge/discharge cycles but gradually falls during the lifetime of the battery.

·         New alkaline cells have an SG of around 1190. When this reduces to about 1145 (which may take 5-10 years depending on the duty cycle) the electrolyte must be completely renewed or the battery replaced. Discharge of alkaline cells should be discontinued when the cell voltage has fallen to about 1.1 V.