Lucent Technologies Round Cell Batteries
7 - 24 Operation and Maintenance Issue 3 December 1999
Gassing Rate
Overview 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 airtight 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 two (2) percent of the total
free volume in the battery area.
In flooded lead-acid batteries, the gassing rate approaches the
theoretical value calculated from the dissociation of water. From
electrochemical theory, if all the charging current is used to generate
gas, each cell will generate 0.016 cubic feet of hydrogen per hour,
per ampere of charging current at 77°F (25°C) and one atmosphere
pressure.
For example, a 48V string of lead-acid batteries requiring 0.1 Amp of
float current at 77°F can produce as much as 0.04 cubic feet of hydrogen
per hour (0.016ft
3
/hr x 0.1 Amp x 24 cells).
The quantity of fresh air required to maintain an explosion safe
environment in the battery area will vary greatly depending on many
factors including, but not limited to, the age and condition of the battery,
the number of cells in the battery area, the battery temperature and the
current flowing through the batteries. Therefore, the design of a
ventilation system for batteries in a specific application requires careful
consideration of factors other than the gassing rate of new batteries in
typical float operation. Typical gassing rates may be useful as a “best
case” condition, but cannot adequately address “worst case” or any other
operating condition that may occur during the service life of the battery.
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