13
tion of a fixed direct current (DC) through the external cell
connections. Some precautions must be observed to get
consistent and meaningful values, however, and these are
described in Section 19.3.
Resistance measurements or microhm measurements
should be taken at the time of the installation and annually
thereafter. Initial measurements at installation become the
benchmark values and should be recorded for future moni-
toring of electrical integrity.
Specific values of connection resistance vary with cell type,
quantity of connectors, etc. It is important that the bench-
mark value for all similar connections should be no greater
than 10% or 5 microhms, whichever is greater, above the
average resistance of all such connections in the battery. If
any connection resistance exceed the average by more than
10% or 5 microhms, whichever is greater, the connection
should be remade so that an acceptable benchmark value is
established. Benchmark values for connection resistances
should also be established for terminal plates, where used,
as well as cable connections. Benchmark values should
preferably be established upon installation. However, if that
was not done, they may be established later provided the
special procedure described below is followed.
Disconnect the battery from the charger and load and disas-
semble at least three (3) of the intercell connections. Clean,
neutralize and burnish these connection components as
though they had corrosion (See Section 19.0) Reassemble
each connection per Section 7.7 and determine its resis-
tance. Measure the resistance of all similar connections in
the battery. If any connection resistance exceeds the aver-
age of the three remade connections by 10% or 5 microhms,
whichever is greater, that connection should be remade to
establish an acceptable benchmark value.
All benchmark values should be recorded. Annually, all con-
nection resistances should be remeasured. Any connection
which has a resistance value more than 20% above the
benchmark value should be corrected.
Increase in connection resistance of more than 20% above
the recorded benchmark definitely indicates a degrading
connection. Such degradation may be caused by corro-
sion (See Section 19.0) or by relaxation in hardware torque
value. If there is no sign of corrosion, the higher resistance
at the connection may be corrected by retorquing (See
Section19.2). If connection resistance is reduced to within
20% of the benchmark value, no further action will be nec-
essary. Failure to restore resistance to an acceptable value
will necessitate reworking the connection.
Maintaining electrical integrity of connections is important as
poor connection will result in reduced battery output and in
extreme cases may cause melted cell posts, circuit interrup-
tions or battery fires.
19.2 Retorquing Connections
Retorquing of connections should be performed annual-
ly (See Section 9, 14) and when connection resistances
have increased to more than 20% over the benchmark.
Retorquing should not be done if visual inspection shows
evidence of corrosion. Retorquing when corrosion is present
only restores mechanical compression but will not improve
electrical integrity.
Tests reveal that a reduction in the original torque value of
30% still provides a functional electrical connection if there
is no corrosion between contact surfaces.
Retorquing of connections should always be to the recom-
mended value (See Section 7.7).
19.3 Connection Resistance
Measurements
Connection resistances are very small, usually in microhms.
Therefore, precautions must be observed so that the mea-
sured values are meaningful and not misleading. Different
connector hook-ups require that the measurement technique
allows for these differences.
(i)
Parallel Connector Hook-Ups.
(Figure 6)
Parallel paths exist in this hook-up and measure-
ment of connection resistance include all four con-
nector post interfaces. The location of probe points
is not critical here because of the existence of par-
allel paths. An increase (decrease) in the lead mass
between post and connector interface on one side
is cancelled by an equal decrease (increase) in the
mass on the opposite side.
(ii)
Four Post, Four Connector In-Line Hook-Ups.
(Figure 7)
Cells with four post connector hook ups require two
measurements to monitor all eight post-connector
interfaces. Measurement is made in two steps—
First between points A and C and then between
points B and D. The measured values should be the
same. Values appreciably different (5 micrhoms or
more) require reworking of connections as described
in Section 19.0.
(v)
Four Post, Connector Parallel Hook-Ups.
(Figure 8)
Cells arranged end-to-end have parallel current
paths above and below the cell covers and require
that resistance measurement make allowance for
the same. The current paths above the cover are
provided by the connectors and the path under the
cover is provided by the busbars (shown by dotted
lines in Figure 10). Most resistance meters apply 10
amperes DC to the connections being monitored. If
this was done between posts A and B in Figure 10,
the current will divide through the busbars between
CAUTION!
TOO FREQUENT RETORQUING OF
CONNECTIONS IS NOT RECOMMENDED AS
THIS WILL RESULT IN DISTORTION OF CELL
POSTS, CONNECTORS, ETC., THUS
DEGRADING
RATHER THAN IMPROVING THE CONNECTIONS.
