Lucent Technologies Enhanced VR Series Batteries
Issue 5 December 1998 Operation 7 - 1
7 Operation
Lead-acid
Battery
Gassing Rate
This section provides general information and guidelines for
estimating the hydrogen gassing rates of lead-acid batteries on
charge. It is beyond the scope of this document to provide the
detailed methods or the engineering design required to maintain
safe levels of hydrogen in battery enclosures.
All lead-acid batteries generate hydrogen and oxygen gas at the
negative and positive plates, respectively. The generation of
these gases occurs during all conditions of use, including charge,
discharge, and while on open circuit during storage. The rate of
gas generation on discharge and open circuit is generally very
small but cannot be completely ignored. For this reason, never
place lead-acid batteries in an air-tight enclosure. Explosive
mixtures of hydrogen in air are present when the hydrogen
concentration exceeds four (4) percent by volume. The
concentration of oxygen does not significantly change the lower
explosive limit of hydrogen in air, and, therefore, only hydrogen
will be considered here.
If the concentration of hydrogen in air exceeds four (4) percent
by volume, there is a risk of explosion if the gas is ignited.
Therefore, to provide a margin of safety, lead-acid battery areas
must be ventilated to limit the accumulation of hydrogen gas
under all anticipated use conditions to a recommended
maximum of one (1) percent of the total free volume in the
enclosed battery area.
In flooded lead-acid batteries, gassing rates approach the
theoretical values calculated from the dissociation of water. In
valve-regulated lead-acid batteries (VRLA) the recombination
of oxygen gas at the negative electrode results in reduced oxygen
and hydrogen evolution from the battery. Generally, VRLA
batteries evolve from less than 1% to 20% of the hydrogen
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