6
Miscellaneous Tests
In addition to the tests already men-
tioned, there are also other miscella-
neous tests performed on nickel-
cadmium batteries. These tests usually
involve high rate charge and/or
discharge.
High rate discharge and charge of
nickel-cadmium batteries is possible
with today’s new and better designed
cells having advanced plate and cell
construction. The low internal resist-
ance of nickel-cadmium batteries
yields high discharge currents. If they
are discharged continuously under
short circuit conditions, however,
self-heating may do irreparable dam-
age. Continuous discharge at rates
greater than 1C should be prevented
to avoid potentially hazardous condi-
tions due to high internal gas pres-
sure build-up.
Very high currents (>2C) can be with-
drawn in low duty cycle pulses pro-
viding that internal temperatures and
pressures are maintained. Output
capacity in any type of pulse discharge
application is difficult to predict
because of the infinite number of
possible combinations of discharge
time, rest time, and EODV. Simulation
of actual events, as in the Life-Cycle
test, is the best way to quantify a
battery exposed to such conditions.
Many cells can be quick-charged at a
rate up to C/3 in as little as 3 to 5 hours
instead of the standard 12 to 15 hours
at the C/10 rate. High rate charging
should be done under controlled con-
ditions where temperature, voltage,
pressure, or some combination of
these parameters can be monitored to
assure they are within specifications.
One fast-charge method involves
charging the battery at a rate exceed-
ing the specified maximum charge rate
for a finite period of time, after which
the charge rate is reduced to currents
below C 10. This method, called “timed
fast charge,” can indeed give a quick
“boost” charge to a partially discharged
battery, but unfortunately has the
potential of permanently destroying
the battery. The destruction occurs
due to overcharging the battery be-
cause its unused capacity is unknown
prior to charging.
A safer variation of the timed fast
charge method is called “dump timed-
charge” where the battery is first
fully discharged (“dumped”) to its
EODV before recharging via the “timed
fast charge” method. The “dump
timed-charge” method has the advan-
tage of knowing just how much energy
must be pumped back into the battery
to bring it to full capacity; the risk of
overcharging is therefore eliminated.
One final test, called the “forced dis-
charge test,” determines the safety of
a battery under certain abusive con-
ditions. This test is very dangerous
because, during the test, the battery
is very likely to explode. The test
must be done under extremely well
controlled conditions in an explosion
proof safety chamber to prevent per-
sonal injury. The test involves con-
necting a current source in series
with the battery. The polarity is in
the same direction as normal or short
circuit current flow. See Figure 4. The
current source is set to a value such
that the resultant current flow is
greater than the short circuit current
flow. This test simulates what may
happen if a battery were improperly
installed in a circuit where it may not
be the only source in the application.
Ideally the battery should withstand
the stress, with some degree of margin,
when the test currents are similar to
actual conditions.
Figure 4. Forced Discharge