 - What is Conductance?
| Conductance is a unique electrical measurement that determines the ability of a battery to transmit current through its internal structure. As such, it gives a direct relationship to battery power. Internal conductive battery components include battery grids, active material and connectors that conduct electronic current as well as sulfuric acid electrolyte that conducts ionic current between and within the plates and plate separators.
Conductance is measured by probing the battery with a small AC current ripple that generates a small AC voltage response. Conductance is the relative ratio of the variation in current with the variation in voltage. It is a powerful tool for providing rapid and pertinent battery information without the need of discharging the battery with high currents that can be dangerous and leave the battery in a depleted condition.
As a battery ages, its internal components gradually wear out through corrosion, disintegration and shedding. When this happens, its conductance drops until it reaches a point at which the performance of the battery becomes significantly degraded. Thus, without running extensive discharge tests, conductance can be used, together with other battery information, to rapidly and safely determine a functional battery’s state of health and fitness for further service. Conductance can also be used to detect cell defects, shorts, and open circuits that can cause the battery to fail in a precipitous manner.
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- How does it compare to load testing?
| Traditionally, the battery industry has used the BCI Adjustable Load Test as a standard to determine a starting battery’s suitability for further service. This load test is a measure of the voltage drop when a load equal to one half of the battery's CCA rating is applied for 15 seconds on a charged battery. To be considered good, the voltage must not drop below 9.6 volts at 70 deg F. To use a load tester accurately, the battery cannot be in a discharged condition and therefore must be charged, often requiring extended periods of time. In addition, the battery must be at room temperature, or the operator must compensate the resultant voltage readings to determine if the battery is good or bad. All this makes it inconvenient to load test batteries until the battery has already failed leaving the motorist stranded.
In comparison, a Midtronics tester can be used to test the battery as received. This allows tests to be performed immediately without waiting from 8 to 24 hours for the battery to be charged. The Midtronics tester provides a nearly instantaneous decision so that the technician wanting to determine starting problems knows immediately if the battery is the cause or if other system diagnostics are necessary to find the real problem. And because the test is rapid and passive, it can readily be performed on batteries in service to determine battery problems before they cause a no-start condition.
Because it draws significant current, a load test can produce sparks at poor terminal connections or if weak internal components rupture under load. This can be dangerous around a previously charged battery, where flammable hydrogen and oxygen gas from the charging process may ignite and cause an explosion. The heavy load also generates appreciable heat in the test equipment that must be dissipated. In addition, the load test discharge changes the condition of the battery so that the test cannot be repeated without recharging to verify a test result.
In comparison, the Midtronics test is passive, so there is no arcing or heat generated, resulting in a test that is completely safe to perform anywhere, even in front of a customer. In addition, because there is no heat generated in the test equipment, the technician can test as many batteries as required without waiting for the tester to cool down to prevent internal damage to the tester from excessive heat. Midtronics testers last longer than adjustable load or fixed load testers with electronics; the Midtronics tester's internal electronics are not exposed to heat, which may accelerate the aging process on the equipment. Tests can also be repeated on the same battery with little change in the results.
A standard 1/2 CCA load test must be performed manually, requiring a trained operator and safety equipment. First, the user must be able to judge if the battery is in a condition to test. Second, he must be able to apply the proper load and timing to ensure an accurate test. And finally, the user must refer to complex charts of unintuitive data to interpret the results. Care must also be taken so that the battery or load is not damaged because an excessive load is applied for too long a period of time.
In comparison, a Midtronics tester takes away the guesswork because the tester does all the work. The technician inputs a minimum amount of information about the battery and presses the start button. A decision is generated in seconds, eliminating a large cause of inaccurate testing--user error. There are no complex meters to interpret or charts to consult and normally no batteries to charge. The Midtronics test method makes it simple enough so that virtually anyone can be a battery test expert.
Accuracy of Midtronics testing is very high when compared to load testing on batteries in normal use. In many instances, the test results are even higher because the necessity to charge batteries for load testing often temporarily masks bad batteries. When done correctly on normal batteries, load test results can be correlated directly with Midtronics testing. Therefore, the same testing criteria required for warranty adjustment is observed.
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- Can I measure the battery's actual CCA using conductance?
| The actual SAE J537 Cold Cranking Test is a manufacturing process control test applicable only on new, fully charged batteries. It does not produce an actual value of cold cranking amps (CCA), but is a pass/fail test based on the battery’s CCA test rating. The CCA rating is the discharge load, in amps, that a new, conditioned battery can supply for 30 seconds at 0°F/-18°C while maintaining a voltage of 1.2 volts per cell (7.2 volts per battery) or higher. Thus, the CCA test sets the minimum power requirement for a new battery as rated, which means that a battery rated at 500 CCA must measure 7.2 volts or above for 30 seconds when a load of 500 amps is applied at 0°F/ -18°C.
In automotive applications, the conductance of a new, fully charged battery under normal conditions can be directly related to its rated CCA, the industry gauge of starting power. Therefore, conductance can be translated into working units of CCA so that the power level of a battery can be compared to its cranking rating. The Midtronics CCA value derived from conductance does not, however, determine the actual Cold Cranking Amps of the battery because that test includes a lengthy discharge of 30 seconds at a very high current rate. The conductance CCA corresponds better to the starting current that the battery can normally deliver for a second or less normally needed to start a vehicle. In this regard, the conductance CCA can be considered even better than the cold cranking test. Cold cranking tests run on batteries taken from service often give poor results because they cannot sustain the long duration of discharge (30 seconds) that is not required to start a vehicle. The Midtronics CCA values are designed simply to mimic the initial starting characteristics of new batteries with a given CCA rating under the same operating conditions.
For example, if a battery that is rated at 600 CCA is measured at 500 CCA, it can be thought of as having similar starting characteristics of a new battery having a rating of 500 CCA if that battery is substituted with the test battery. It does not mean that the battery will pass a Cold Cranking Test at 500 CCA. It should also be remembered that the cranking power of all batteries increases with temperature and decreases with discharge and degradation.
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- Can I predict when a battery will fail using conductance?
| Failure means a number of things. If a fully charged battery fails to start a car, it is considered a failure. If it fails to pass a service test, it is also considered a failure. But a battery that fails a service test can often still start a vehicle under non-extreme operating conditions. Starting depends on so many factors affecting both the car and the battery.
When a battery fails a service test such as the BCI load test, it is simply deemed unreliable. That is what the test is meant to do. In other words, it is meant to separate batteries that are still adequate for service from ones that are suspect. The battery industry has set standards of performance they expect from batteries in service and those that do not pass them are judged as bad.
Because Midtronics conductance testers correlate to industry tests, they also will determine if a battery is beyond the same threshold of failure. And, as explained above, the battery can be tested more easily and frequently because of the speed and ease of use.
However, because the degradation mechanisms of battery are dependent on so many factors beside simple time, there is no easy way of predicting how long a good battery will remain good. Midtronics testers can tell how good a battery is compared to failure at any given time, but they cannot say when the battery will finally fail the test threshold. There is no tester that can do that. It is like the wear indicator on tires, we know how much of the tire has worn, but without knowledge of the amount of driving, the driving conditions, etc., there is no way of predicting when the tires need to be changed until they actually approach the failure point. To further complicate things, batteries also do not always degrade in a simple linear manner so even when the battery has degraded half way to its failure point, it does not mean it should last the same amount of time that it has already been in the vehicle.
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- Can I test batteries in the vehicle using conductance?
| Midtronics digital battery testers have several unique features to allow accurate testing while the battery remains in the vehicle, provided the engine is not running. If the battery posts are too dirty or heavily corroded, the tester will not allow a test and prompts the user to check the connection. In addition, all Midtronics digital battery testers can detect excess computer or ignition noise, which could interfere with a proper test. When detected, the tester will prompt the user to check for the cause and/or automatically retest to determine if the cause has been eliminated.
There are other factors for in-vehicle testing that have been incorporated into Midtronics testers. These allow accurate tests without having to remove the battery.
There are, however, vehicle batteries that prove difficult to test. Primarily those are batteries that are remotely located and cannot be directly connected to a Midtronics or any other tester. The extra cabling between the remote jumper posts and the actual battery will make the remote battery often look weak. Correction factors often must be vehicle specific. When in doubt, the battery must be removed and tested.
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- When should I use terminal stud adapters?
| Side terminal batteries have exposed lead bushings with embedded steel nuts that allow a connector to be bolted to the side of the battery. The face of the lead bushing is the actual conductor, not the threaded steel nut. When testing in a vehicle, care must be taken to insure that the connector is bolted tightly to the side terminal to get a good connection. Even with good connections, measurement is sometimes difficult because the only things to which the test leads can attach are the steel bolts used to hold the connector tightly to the bushing interface. Steel is not a good conductor. Midtronics has developed special algorithms to help deal with this situation, but it should be understood that these connections sometimes give misleading results when compared to direct battery testing. The best testing for side terminals (and remote terminals) is to remove the battery from the vehicle.
When a side terminal battery is tested out of a vehicle, the surfaces of side terminals should be clean and conductive and then they should be fitted with lead side terminal adapters. These are lead posts that have an embedded bolt that threads into the side terminal adapter so that the bottom of the adapter is pressed tightly by tightening with an insulated wrench or pliers against the side terminal bushing. Tester clips can then be attached to the side terminal adapters to ensure an accurate test.
Group 31 batteries have small lead bushings in the top cover with a steel threaded bolt extruding. Always install lead female stud adapters over the bolts and tighten down to the lead for best results. Testing on the steel thread will cause inconsistent and inaccurate test results.
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- Can I test Gel batteries using conductance?
| | Although Midtronics digital battery testers are designed for automotive battery applications, they can be used to test gel VRLA batteries, which are more typically used for deep cycle applications such as marine power. Gel batteries often have lower conductance for a given battery capacity because they are not required to crank. If no CCA rating is available, an appropriate reference value can be estimated by testing several good Gel batteries that are fully charged and using 85% of the average of the CCA measurement as the reference CCA rating value for testing other batteries of the same type. |
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- What about charging?
| | Midtronics battery testers use conductance to test discharged batteries to determine the condition of the battery as received. However, when a battery is shown to be good, but in a low state of charge, the battery should be charged before returning it to service. In addition, since some batteries are so discharged that their condition cannot be determined, the user is instructed to “charge and retest” before a proper decision can be rendered. If the test decision is “bad cell” or “replace”, do not charge the battery. Charging a battery with one or more bad cells could cause excessive heating and generation of explosive gasses. |
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- How do I remove “surface charge” while testing with a conductance tester?
| ”Surface charge” refers to a temporary deviation of the battery open circuit voltage from its equilibrium level. It is, however, not a static charge on the surface of the plates such as seen in a capacitor, but results from concentration and ionic imbalances caused by recent charging or discharging of the battery. In electrochemistry this voltage deviation is referred to as polarization. Because it is not a static charge, it cannot simply be removed by a dissipating discharge.
Charging a discharged battery concentrates the regenerated sulfuric acid electrolyte within the pore structures of the plates making it different from the bulk of the rest of the electrolyte. Since the concentration of the electrolyte at the electrodes governs the voltage of the battery, the higher pore concentration causes a higher voltage than would normally result from the average level of the electrolyte for the battery’s charge level. This can confuse those testing a battery into believing that the battery is at full charge when it is still decidedly discharged.
Fortunately, conductance is not affected much by this phenomenon, but the conductance level is affected by the battery charge level. Midtronics testers match measured conductance to the proper discharge state, so if this is wrong the result can reduce its accuracy.
Diffusion will normally slowly mix the high and low concentration electrolyte until an equilibrium concentration and voltage are reached. In an effort to speed this process up, batteries are often discharged for a short time to reduce the concentration gradient. It is hoped that when this occurs a stable equilibrium voltage will result more quickly. At best this is only moderately successful because the enriched concentration gradient may not even exist and can often be reversed by discharge making the voltage and the battery charge level appear lower than normal. Considerable time often must still be expended until a stable voltage is reached. This is aggravated further if the battery is very cold because diffusion is much slower.
At full charge, polarization adjustment is not needed since it is already above the level of full charge. Charge polarization, however, is a problem when the battery is in a discharged state because the high voltage can indicate the battery is already charged when it is not. This can happen after a discharged battery is jump-started and charged for a short time. This imparts an uncertainty into the reading because voltage is one of the gauges used to measure battery charge.
The BCI load test calls upon the operator to wait for hours for the voltage to stabilize, but the removal of surface charge is not normally required when using a Midtronics digital battery tester for testing batteries out of the vehicle because the voltage has normally stabilized. By measuring the conductance and voltage of a battery, Midtronics testers are able to determine the battery's true state of health and charge state.
When testing in a vehicle, however, where the battery may have received recent charge, the vehicle's headlights can be turned on for a minute, turned off, and the battery allowed to recover for several minutes until a stable voltage is reached. If the battery is truly in a discharged condition the voltage will typically remain low allowing the tester to better assess the true battery condition. If done correctly, it can prevent good, discharged batteries from being diagnosed as bad, charged ones.
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