How Lead-Acid Batteries Release Hydrogen
Lead-acid batteries produce hydrogen and oxygen gas when they are being charged. These gasses are produced by the electrolysis of water from the aqueous solution of sulfuric acid. A Vented Lead-Acid (VLA) battery cell, sometimes referred to as a “flooded” or “wet” cell, is open to the atmosphere through a flame-arresting vent, and during charging, the hydrogen and oxygen is allowed to escape through this vent. With a Valve Regulated Lead-Acid (VRLA) cell, the recombination design causes the oxygen and hydrogen to recombine within the cell. However, if the cell is overcharged, operated at an elevated temperature, or under certain fault conditions, the cell’s pressure relief valve will open, and oxygen and hydrogen will be released.
Hydrogen atoms are the smallest and lightest of all naturally occurring atoms. Accordingly, hydrogen is very easily dispersed with a minimum amount of air movement. Hydrogen also mixes with air by diffusion. Usually, unrestricted natural air movement in the vicinity of the battery is sufficient to disperse any hydrogen that might be released during normal operation. This is especially true with a VRLA battery; however, with a VLA battery, during the charging cycle, a significant amount of hydrogen may be released and forced air flow will be required. Ensuring good air movement through the battery location will prevent hydrogen concentration.
Stationary Battery Ventilation Standards
The IEEE Power and Energy Society’s Energy Storage and Stationary Battery Committee (PRE ESSB) in association with the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) has developed a document titled IEEE 1635 and ASHRAE P21 Guide for the Ventilation and Thermal management of Batteries for Stationary Applications. This document contains the following verbiage:
“7.1.1. General Gassing calculations under float conditions should generally not be used as the basis for ventilation design. Due to failure modes, human error, and code requirements, it is probably safest to use gas generation calculations from the boost/equalize/finish charge regime to determine normal ventilation requirements (it will be noted, that even assuming these conditions, gas generation is relatively low). Additional ventilation may be needed for initial charging (particularly for vented cells) based on the calculations for that mode.”
Ventilation Requirements for VRLA Batteries
There is a lot of controversy as to whether ventilation is requited for VRLA batteries. The following is an extract from IEEE Std 1187™-2015. IEEE Recommended Practice for Installation Design and Installation of Valve-Regulated Lead-Acid Storage Batteries for Stationary Applications.
“5.4.2 Ventilation for hydrogen control.
In a VRLA cell operating in a fully recombinant mode, internally there will be a slow buildup of hydrogen gas. When the cell internal pressure exceeds the valve release pressure, the hydrogen gas will be vented into the atmosphere. The following battery operating conditions have the following hydrogen generation effects:
a) Minimal gas emission: open circuit, discharge, and initial recharge (slight gas evolution can occur from cells on open circuit as a result of local action)
b) Occasional gas emission: float charge (periodic venting as a result of grid corrosion and to the extent that the recombination efficiency is less than 100 percent)
c) Potential for maximum gas emission: equalize charge and near end of recharge
d) Maximum gas emission: overcharge.
Under certain failure or extreme overcharge conditions (above the recombinant ability of the cell), VRLA batteries can evolve hydrogen at a maximum rate of 1.27 x 10-7 m3/s per ampere per cell at 25°C at standard pressure.
Adequate ventilation shall be provided in order to prevent the possible accumulation of hydrogen. Ventilation provides air circulation to help prevent hydrogen from concentrating in explosive quantities. The ventilation system shall limit hydrogen accumulation to less than 2% of the total volume of the battery area/cabinet. Either natural or forced ventilation can be used.
NOTE. Other applicable codes might be more restrictive than the above 2% requirement.”
Calculating Flammability Limits and the Flammable Range
For an explosion to occur, the fuel (hydrogen) must exist in the right concentration in air and there must be an ignition source (spark or flame). The minimum concentration of hydrogen to cause/support its combustion in air is defined as the Lower Explosive Limit (LEL) or the Lower Flammability Limit (LFL). Below this concentration the mixture is too lean to burn.
At a maximum concentration, where the mixture is too rich to burn, is the Upper Explosive Limit (UEL) or the Upper Flammability Limit (LFL). The range between the LEL and the UEL is called the flammable range. For hydrogen, the LEL is 4% by volume and the UEL is 75% by volume.
So, the aim in the design of the battery location is to ensure that the concentration is kept below the 4% mark and provide alarming in the event that this concentration is exceeded. This can be achieved, if the battery type and location warrants it, by providing forced air circulation or removal, temperature control and compensated charging, and installing a hydrogen detector.
There are several codes that specify the maximum concentration limits and almost all are below the 4% level, the most common being 1% or 2%. The IEEE recommends that the maximum average concentration in the battery area be less than 2% by volume. As indicated above, any calculation of hydrogen should be at the worst-case condition when the charge current is at the maximum, i.e. boost/equalize charge.
How to Calculate Room Volume Concentration
Contained within IEEE 1635 and ASHRAE P21 Guide for the Ventilation and Thermal Management of Batteries for Stationary Applications there are many formulae, some quite complicated, that determine the methods to calculate room volume concentration. Eagle Eye Power Solution has developed a calculator to simplify these calculations. All we need to know are the following: battery type, make, model and capacity in terms of Ah or Watts, the number of cells/strings and the room dimensions, and we can provide the ventilation requirements. When using a hydrogen detector, the installation location and calibration requirements are very important. Since hydrogen atoms are so light, the concentration will gather at the highest point in the battery location, so the detector should be installed at that point. Also, the detector needs to be calibrated, usually at six monthly intervals. So, sometimes there has to be a compromise as to where the detector is installed.