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Battery Testing at Garages Simplified

by Isidor Buchmann
CEO & founder of Cadex Electronics Inc.
www.BatteryUniversity.com; answers@cadex.com
April 2011

Ever since Cadillac invented the starter motor in 1912, vehicles used lead acid batteries to crank the engine. Not much has changed in 100 years except that the starter battery must now also feed power-hungry accessories. Starting is easy, but does the battery have enough reserve for the accessories?

The purpose of battery testing is to estimate the end of battery life. Automotive technicians are most familiar with CCA, a measurement that reflects engine cranking but does not always predict the end of battery life corretly. The leading health indicator of a battery is capacity. So we ask, “At what capacity should the battery be replaced?” Capacity as a means of identifying the end of battery life is new. Car manufacturers and service garages will need to find minimum acceptable values, and in our example we take 65% as warranty replacement and 40% as the end-of-life threshold. Figure 1 illustrates the relationship between capacity and CCA on hand of a fluid-filled container. The liquid represents capacity and the taps symbolize CCA at different loads.


Figure 1: Relationship of CCA and capacity of a starter battery

Capacity represents energy content and CCA is power delivery. A battery with 40% capacity can still have a healthy crank but this indicates end-of-life.

Cranking of most starter batteries remains strong until the end of life. The capacity, on the other hand, begins to drop gradually. To study these changes, Cadex measured the capacity and CCA of 20 aging starter batteries. The results are laid out in Figure 2, sorted according to capacity levels in percentage.

Figure 2: Capacity and CCA readings of aging batteries. Batteries 1 to 9 have good CCA and high capacity; the CCA of batteries 10 to 20 remains reasonably strong but suffer from serious capacity loss. CCA tends to remain high while the capacity drops steadily as part of aging.

Batteries 1 to 9 have good CCA and capacity readings, however, batteries 10 to 20 show age related capacity fade. The motorist may be unaware of this loss until the car won’t start one morning. This is especially critical during a cold spell, which further reduces the capacity.

Car garages want simple battery testers that provide a clear and consistent assessment of the battery in seconds. This is only possible with instruments that provide CCA and capacity readings. Most battery testers give only a CCA reference based on resistance. The reason for this is clear — resistance is far easier to measure than capacity.

The Spectro CA-12 GA (Figure 3) evaluates a battery by capacity. If the reading drops to 40% or below, the battery has reached end-of-life and the message REPLACE BATTERY appears. Very seldom does a battery have high capacity and low CCA. Should CCA drop below 50%, the instrument will classify this battery as “end-of-life” also. CCA is shown in numeric value; state-of-charge in percentage, and capacity as Good/Poor. Based on these crietira, the following evaluations are possible:

To test a battery, select between flooded and AGM, enter the capacity (Ah or RC) and connect the probes. The batteries can be partially charged, but if below 40% SoC the CA-12 advises to CHARGE AND RETEST. The test begins automatically and takes about 15 seconds to complete.

Advanced instruments, such as the Spectro CA-12 GA, use matrices to estimate capacity, CCA and state-of-charge. A matrix is a multi-dimensional lookup table against which the readings are compared. Text recognition, fingerprint identification and visual imaging operate on a similar principle in that a model exists with which to equate the derived readings.

With generic matrices, the Spectro CA-12 GA provides a CCA accuracy of 90%; capacity is 80%. This sort accuracy may be seen as low, and the buyers want 90 percent and better. Such a quest is impossible with lead acid batteries. Capacity fluctuations of +/- 12 percent are common even with highly accurate discharge and charge equipment tested in a controlled laboratory environment. “Do we truly understand capacity as a measure of energy storage,” experts may ask?

Figure 3: Spectro CA-12 GA Battery Tester

The generic auto (GA) version CA-12 relies on capacity estimation to predict end-of-life, which is set to 40%. The device also provides CCA and state-of-charge measurements in numbers.

Most commercial battery testers provide a CCA accuracy of about 70 percent with no capacity reference. This leaves technicians guessing, especially if a failing battery shows a high CCA reading. The Spectro™ technology provides improved CCA accuracy to the single-frequency methods and this is evident on a questionable battery. Capacity, however, remains the leading health indicator of a battery. Figure 4 compares CCA reading derived from the Spectro CA-12 with units using single- frequecy exitation (AC Conductance).

Figure 4: Spectro CA-12 achieves higher CCA accuracies than with singe-frequency AC conductance methods

The CA-12 reads the correct CCA and is stable between 100 and 40% SoC (red). Some CCA testers may show elevated CCA numbers and are influenced by battery voltage (blue).

The Spectro CA-12 uses multi-model electrochemical impedance spectroscopy to estimate capacity and CCA. This technology had been around for a while but was reserved to research laboratories because of high cost and long test times. The development of the Spectro™ algorithm (patent) and the high-speed DSP have reduced size and price to bring this powerful tool to the garage floors. Here is how it works:

A frequency of 20 to 2,000 Hertz scans the battery as if to plot a landscape. During the 15-second test, the CA-12 compiles 40 million transactions and covnerts the data into readable information.The generic matrix of the GA model accommodates most starter batteries. If numeric capacity readout is preferred over the Good/Low classification, Cadex can offer a battery-specific matrix.

Similar to a medical test or weather forecast, rapid-testing only gives estimations. No single instrument can find all anomalies; several methods are needed to get a full assessment. There are certain battery defects that can only be found by appplying a heavy load. A micro short in a cell is such a case. A rapid-test may pass the battery as good even though the short has lowered the specific gravity due to high self-discharge and the engine won’t crank. A carbon pile or hydrometer would work best for this anomaly.

There are no ideal battery test instruments, however, scientists predict that the battery industry is moving towards electrochemical impedance spectroscopy (EIS) to estimate batter performance. The Spectro CA-12 is one of the first battery testers to use this technology outside research laboratories.

About the Author
ISIDOR BUCHMANN is the founder and CEO of Cadex Electronics Inc., a company that manufactures innovative battery test and diagnostic equipment. Active in wireless communications, Isidor has studied the behavior of rechargeable batteries in practical and everyday use. To share battery knowledge, he wrote many articles, delivered technical papers around the world, published several books, and created www.BatteryUniversity.com. For product information, please visit www.cadex.com.

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