Dynamic Method for Storage Battery Diagnostic Testing 750758

Electrical techniques for field testing storage batteries have been traditionally based upon measuring static voltage. Examples include measurements of:

1. (1)
   Open circuit cell and terminal voltage;
2. (2)
   Terminal voltage during “discharge-charge” cycling -- the so-called “421 test”;
3. (3)
   Terminal voltage during high rate discharge -- the common “load test”.

Although each of these techniques has specific advantages, the load test has unquestionably been the most widely employed. Its utility is limited, however, by the large currents required. Apparatus to handle such currents is relatively heavy and cumbersome. Furthermore, a typical test consumes appreciable time and severely polarizes the battery. As a consequence of the latter, load test voltages are not repeatable.

This paper describes an alternate testing technique that correlates well with the load test but does not suffer these disadvantages. Instead of measuring static voltage, the battery's dynamic conductance is sensed electronically using a time varying signal. This quantity is proportional to the maximum power available and provides a direct measure of the battery's overall condition. Since large currents are not required, the testing apparatus can be small and lightweight. Moreover, dynamic measurements are instantaneous, uneffected by polarization, and can be routinely performed without perturbing the battery.

The dynamic measuring technique is embodied in the electronic battery tester* described herein. This device provides either an absolute determination of maximum power (in KW) available at the battery terminals, or a “pass-fail” assessment based on the battery's rating and temperature. Comparisons are made between dynamic and load test measurements at both 70°F and 0°F, and a high degree of correlation is observed. The effects of battery size, temperature, charge, and polarization are discussed. Specific uses of the dynamic measuring technique are proposed which take advantage of its inherent reproducibility, accuracy, and facility.

Electrical techniques for field testing storage batteries have been traditionally based upon measurements of static voltage. Examples include:

• Measurement of open circuit cell and terminal voltage;
• Measurement of terminal voltage during “discharge-charge” cycling -- the so-called “421 test”;
• Measurement of terminal voltage during high rate discharge -- the common “load test” and its many variations.

Each of these tests has specific advantages and disadvantages. An advantage of the “421 test”, for example, is that it can be performed on partially discharged batteries. The fact that the “421 test” requires an external power source such as the ac mains, however, is a distinct disadvantage.

For many years, the load test -- including numerous variations thereof -- has unquestionably been the most widely accepted test for assessing the serviceability of batteries in the field. Terminal voltage under heavy load is generally conceded to be a valid indication of overall battery condition. However, some disadvantages that are common to all load tests are the following:

1. (1)
   Because of the large currents required, load test apparatus is relatively heavy and cumbersome.
2. (2)
   Terminal voltage drops with time as the test proceeds. One must, therefore, observe the voltage at a precise instant after applying the load in order to obtain meaningful results.
3. (3)
   The test itself polarizes the battery and causes the resulting data to be unrepeatable. Furthermore, because of the test-induced polarization, the battery is less capable of delivering power immediately after having been load tested than immediately before.
4. (4)
   The results of a load test are affected significantly by the recent history of the battery. A battery that has just undergone charging, for example, will be temporarily polarized in the positive direction. This transitory effect is commonly referred to as “surface charge” and causes the load test voltage to be erroneously high. A similar problem occurs if the battery has recently undergone high rate discharging -- such as cold weather cranking. In this case, negative polarization (“surface discharge”) causes erroneously low load test voltages. Neither of these temporary effects is a true indication of battery capability.

The dynamic testing method described below correlates extremely well with the load test method. However, it does not suffer from any of the above disadvantages. The testing equipment, being electronic in nature, requires much less than one amp of current -- compared with hundreds of amps required by the load test. Dynamic testing equipment can, therefore, be lightweight and portable. Moreover, dynamic measurements are instantaneous, repeatable, accurate, independent of time, and do not perturb the battery being tested. In addition, the results of a dynamic test are virtually uneffected by temporary polarization effects such as “surface charge” and “surface discharge”. Consequently, one can accurately assess a battery's true capability regardless of whether or not it has recently undergone charging or discharging.

One disadvantage of both the load test and the dynamic test is that accurate battery appraisal presupposes having batteries that are nearly fully charged. In the case of the dynamic testing method, however, this caveat is not as restrictive as it may seem. First of all, automotive batteries functioning normally in service are, in fact, usually fully charged. Secondly, batteries that frequently are assumed to be discharged are actually negatively polarized. This can occur, for example, when a battery has failed during engine cranking on a cold morning*. In the case of negative polarization, the dynamic testing method will still give valid results but the traditional load test method will not.