Part 21 Report - 1997-322
ACCESSION NO #: 9705120062
Bell Labs Lucent Technologies
Innovations for Lucent Bell Labs Innovations
Technologies
Fred C. Laman Lucent Technologies Inc.
Micro Electronics Group Room 1Q-105
Power Systems 3000 Skyline Drive
Mesquite, Texas 75149-1802
Telephone 972-284-2871 EMAIL flaman@lucent.com
Facsimile 972-284-3579
March 3, 1997
REPORT OF DISCOVERY
HARNESS/POSITIVE PLATE LUG DISCONNECT
IN ROUND CELL BATTERY
FRED LAMAN
03/03/1997
REPORT OF DISCOVERY
Observation
On February 12, 1997 a four cell string of high gravity Round Cells
failed a capacity discharge test. The test was carried out at the C&D
Technical Center in Conshohocken PA., as part of the Nuclear Round Cell
User's Council test program. The discharge test failed because one cell
went into reversal immediately following the start of the discharge.
This cell was manufactured in 1991 (serial # R9105-97461) and used in a
spare battery at McGuire's Nuclear Generating Station. The remaining
three cells in the string were new cells recently manufactured. The
discharge was done at a constant current of 508 amps, corresponding to a
2 hour discharge rate, to an end voltage of 1.75/cell. The reversed cell
reached a potential of -2.63 volt within 1 second and -3.32 volt after 47
seconds, at which time the discharge was terminated. During this time
the reversed cell was gassing heavily and produced a smell of hydrogen
sulfide. Subsequent autopsy of the failed cell revealed that the harness
of the positive electrode was completely disconnected from the four lugs
connected to the top positive plate.
Cell history
The failed cell from the Mcguire spare battery had been subjected to the
same discharge testing as the batteries in service at McGuire. During a
total number of five tests consisting of a combination of capacity
discharge testing and service load profile testing, no additional voltage
loss associated with poor harness/lug connections had been observed.
After the cell was transferred to Conshohocken the first capacity
discharge test using a two hour rate, showed an additional voltage loss
of approximately 100 mV and a delivered capacity of approximately 80% of
rated capacity. In the subsequent discharge done under the same
conditions the cell was driven into reversal.
Cause of failure
Further examination of the disconnected harness and positive plate
towards incomplete welds as the most likely contributor to the
disconnection. The weld burns were narrow and shallow. The fact that no
discharge problems were experienced during testing done at the McGuire
Nuclear Generating Station indicates that extensive handling with
associated mechanical and thermal stresses possibly contributed to the
final failure.
Consequences of such failure
Consequences of such a cell reversal, if it occurs in a battery servicing
a load, are loss in capacity and excessive gassing. The major factor in
excessive gassing is the generation of hydrogen gas which can form
explosive mixtures in air. Smaller amounts of hydrogen gas also be
formed during cell reversal, especially when the cell temperature is
raised. Although this gas is highly toxic its smell can be detected at
very low concentrations. The extent of capacity loss and gas generation
depends on the specific application, such as battery configuration,
battery cut-off voltage and most importantly initial discharge current in
the load profile.
Capacity loss
The Coup de Fouet, which is a rapid voltage loss occurring at the very
discharge of a lead-acid battery, is critical when the initial discharge
current is very high. The service load profile for the Nuclear
Generating Plant is characterized by a high initial current, which
generally lasts approximately on minute. If the initial discharge
current is enough to lower the Coup de Fouet discharge voltage below the
battery cut-off voltage, then the battery will shut down almost
immediately and the deliverable capacity is very small. In this case the
amount of hydrogen gas generated during reversal is also limited.
Hydrogen gas generation
Assuming that water decomposition is the main electrochemical reaction
following cell reversal, then the total amount of hydrogen gas generated
in the reversed cell can estimated based on the discharge current and
battery string capacity delivered before the cut-off voltage is reached,
according to:
Volume of hydrogen in liters = 0.42 x delivered capacity in
Ampere-hours.
Summary
Based on the available data, values for delivered capacity and the volume
of hydrogen gas, generated is estimated for two different initial
discharge currents. The lower current represents the case where the Coup
de Fouet voltage is above the battery voltage, while the higher current
represents the case where the Coup de Fouet voltage is below the battery
cut-off voltage. This example is based on one reversed cell in a 60 cell
string battery with 100% capacity and a cut off voltage of 105 volt.
initial available amount of
current battery hydrogen gas
capacity generated
500 A 60% 250 liters
1000 A < 1% < 4 liters
Conclusion
Test records from the manufacturer show that the cell serial #
97461, did pass the bond resistance test as per Round Cell specification
KS-20724 drawing L-905503. This test is required to evaluate the harness
to positive plate lug weld. The cell also passed a subsequent discharge
test specified by McGuire Nuclear Generating Station. Most importantly
in the entire Round Cell manufacturing history of 800,000 cells and 25
years this is the first time that a complete disconnection of harness and
positive plate lugs have been reported as the cause of failure. From
this Round Cell manufacturing history, combined with the weld test
results as the manufacturer, it is concluded that the cell failure
described in this ROD is an anomaly, occurring extremely rarely.
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