Agilent Technologies AN 372-2 manual Miscellaneous Tests, Forced Discharge

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

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