When Power Battery series of batteries are charged at constant potentials below 2.37 VPC at temperatures up to 92ºF, (33.33ºC) and below 2.25 VPC up to 107ºF, (41.67ºC) 100% of the gases produced are recombined internally so that no gas is evolved. If the batteries are charged at higher voltages or temperatures, small amounts of gas may be volved. Recombination efficiency is about 90% at 2.5 VPC @ 107ºF (41.67ºC).
Power Battery VRLA batteries internally recombine all gasses generated within the batteries if they are charged or maintained on float within the recommended charge conditions. If the batteries are charged at voltages above 2.37 VPC (14.22 volts for 12 volt batteries) small amounts of gas may be evolved, particularly if the batteries are above ordinary temperature.
A typical UPS battery installation uses battery cabinets with an internal volume of 56 cubic feet (Power CR4). Such cabinets usually contain 40 batteries occupying 23 cubic feet and electronic gear occupying 1 cubic foot leaving 32 cubic feet of free air space. The cabinet has ventilations slots with a total open area of 0.32 square feet. Hydrogen will leave the cabinet both by diffusion through the ventilation slots (and other openings) and by convection. Calculations based on the known diffusivity of hydrogen show that a maximum equilibrium concentration of 0.062% hydrogen will be reached within the CR4 cabinet when the batteries are charged at 2.5 VPC at 77ºF (25ºC) without any convection.
If the batteries are charged under the extreme condition of 2.5 VPC at 107ºF (41.67ºC) about 80ml per hour will escape from each battery so that diffusion alone will result from each battery so that diffusion alone will result in a maximum cabinet concentration of 0.23% hydrogen. Since some convection will occur, the actual hydrogen concentrations will be well below these calculated equilibrium concentrations.
The calculation of Gas Produced and Recombination Efficiency is not strictly correct since some small current will be accepted by all batteries without the production of gas.
The error is very small and, since the calculation provides a convenient way to compare recombinant batteries of different design, the method has been used by many battery researchers.
The data shown in the table has been plotted on the attached charts which show Gas Recombination Efficiency and Actual Gas Evolution as a function of Cell Voltage.
The minimum concentration of hydrogen in air that is required for combustion is 4.1%. The current IEEE standard for the maximum safe concentration of hydrogen in air is 2%. The maximum, diffusion limited, equilibrium concentration of 0.23% obtained at 2.5 VPC and 107ºF (41.67ºC) is about 10% of this. If charging is done under the more normal conditions of 2.37 VPC hydrogen concentration within the cabinet will not exceed 0.02% and will approach the natural atmostpheric hydrogen concentration of 0.01%.
If Power Battery VRLA batteries are operated within the recommended guidelines and if cabinet ventilation slots are not impeded, there is no posibility of reaching dangerous concentrations of hydrogen within the battery cabinet.