Learn about various test methods and why none is fully satisfactory.
Capacity is the leading health indicator of a battery but estimating it on the fly eludes scientists. The traditional charge/discharge/charge cycle still offers a dependable way to measure battery capacity. While portable batteries can be cycled relatively quickly, a full discharge and recharge on large lead acid batteries is not practical. Scientists continue to seek fast and non-intrusive test methods even if the readings are less accurate and applying a full discharge. This section explains what’s available in new technologies.
One would assume that capacity measurement by discharge is the most accurate method but this may not always be the case, especially with lead acid batteries. Even when using highly accurate equipment in a temperature controlled environment and following established charge and discharge standards, variations between identical tests occur. This is not fully understood except considering that batteries exhibit human-like qualities. Our IQ level also varies depending on the time of day and other conditions. Lithium- and nickel-based chemistries provide more consistent discharge results than lead acid.
To verify the capacity on repeat tests, Cadex checked 91 starter batteries with diverse performance levels and plotted the results in Figure 1. The horizontal x-axis shows the batteries from weak to strong, and the vertical y-axis reflects the capacity. The tests followed SAE J537 standards by applying a full charge and a 24-hour rest, followed by a regulated 25A discharge to 10.50V (1.75V/cell). The results in diamonds represent Test 1. The test was repeated under identical conditions and the capacities shown in squares embody Test 2. Although done within days of each other, Test 1 and 2 differ much as +/–15 percent in capacity. Other laboratories are also observing these discrepancies.
Figure 1: Capacity fluctuations on two identical charge/ discharge tests of 91 starter batteries
The capacities differ +/–15% between Test 1 and Test 2. Tests were done according to SAE J537
Courtesy of Cadex (2005)
When evaluating battery test results, the question is asked: “Against what standard are the readings compared?” If the classic charge/discharge provides inaccuracies, assessing a modern test technology is put in question by asking: “Which method is more correct?” Could electrochemical impedance spectroscopy be better than a discharge/charge cycle, we ask? This might be true in some cases but not in all.
Starter batteries have two distinct values, CCA and capacity. These two readings are close to each other like lips and teeth, but the characteristics are uniquely different; one cannot predict the other and correlation between the two is almost non-existent. (See BU-806, Tracking Battery Capacity and Resistance as part of Aging)
Most rapid-testers look at the internal resistance, which is an approximation of CCA. Reading battery resistance is relatively simple but this alone cannot predict capacity, nor can it tell when to replace a battery as end-of-life is primarily capacity related. A battery will crank the engine as long as there is enough capacity to do so and a sudden failure might occur when the capacity drops below 30 percent. There is normally no slowing down of the cranking ability beforehand to indicate the demise.
SAE specifies the capacity of a starter battery by reserve capacity (RC). RC reflects the runtime in minutes at a steady discharge of 25A. DIN and IEC assess the battery in Ah and measure the runtime at a typical discharge rate of C/20 (5h rate) for starter batteries. A 60Ah would discharge at 12A.
No accurate RC to Ah conversion exists but the most common formula is RC divided by 2 plus 16. A short method is dividing RC by 1.9. Differences in discharge current produce some inaccuracies.
A full CCA test is tedious and is seldom done. CCA cannot be “measured” but only “guessed” and the process can take a week per battery. To test CCA, apply different discharge currents to see which amperage keeps the battery above a set voltage while in frozen state. Table 2 illustrates the procedures of SAE J537, IEC and DIN. The methods are similar and only differ in the length of discharge and the cut-off voltages.
|SAE J537 CCA Test||IEC CCA Test||DIN CCA Test|
Fully charge battery according
to SAE J537 and cool to -18°C (0°F) for 24 hours. While at subfreezing temperature, apply a discharge current equal to the specified CCA. (500 CCA battery discharges at 500A.) To pass, the voltage must stay above 7.2V (1.2V/cell) for 30 seconds.
Fully charge battery according
to SAE J537 and cool to -18°C (0°F) for 24 hours. While at subfreezing temperature, apply
a discharge current equal to the specified CCA. (500 CCA battery discharges at 500A.) To pass, the voltage must stay above 8.4V for 60 seconds.
Fully charge battery according
to SAE J537 and cool to -18°C (0°F) for 24 hours. While at subfreezing temperature, apply a discharge current equal to the specified CCA. (500 CCA battery discharges at 500A.) To pass, the voltage must stay above 9V for 30s and 6V for 150s.
Table 2: CCA test methods of SAE, IEC and DIN. The methods differ in the length of discharge and the cut-off voltages.
Advancement in electrochemical impedance spectroscopy (EIS) makes estimation of battery capacity possible. The non-invasive technology known by its trademark Spectro™ combines EIS with complex modeling to attain capacity, CCA and SoC with the help of a matrix. Here is how it works:
A sinusoidal signal of multiple frequencies is injected into the battery at a few millivolts. After digital filtering, the extracted signal produces a Nyquist plot onto which Nyquist plots for various electrochemical models are superimposed. The best matching electrochemical models within allotted margins are selected; non-fitting models are rejected. Data fusion correlates the values of the key parameters to derive at capacity and CCA estimations. Figure 3 illustrates the patented process in a simplified way.
Figure 3 Spectro™ combines EIS with complex modeling to estimate battery capacity and improve CCA measurements
A sinusoidal signal produces a Nyquist plot; data fusion correlates the values of the key parameters to estimate capacity and CCA.
“How accurate are the readings,” car mechanics and battery users ask? This depends on the tester and the test methods used. Spectro™ with a generic matrix provides a correct CCA predictability of about 90 percent and capacity is about 80 percent.
As part of product evaluation, a German luxury car manufacturer compared the Spectro™ and AC conductance methods side-by-side. With a dedicated matrix, Spectro™ achieved a correct CCA prediction of 97 percent; capacity came in at 87 percent. The correct CCA prediction of AC conductance was less than 60 percent with no capacity estimation.
Such seemingly low accuracy may come as a surprise but the user must appreciate that a battery fault can only be diagnosed if measurable symptoms are present. A new battery that has not yet been fully formatted, has been in storage for a long time, or has a low charge can give false results. For unknown reasons, reversible (soft) sulfation does not display readable symptoms with Spectro™ and the battery receives a clean bill of health in spite of low capacity. Only permanent (hard) sulfation that can no longer be corrected agrees with the result. (See BU-804b: Sulfation and How to Prevent it.) CCA and capacity estimations are most accurate with “working” batteries.
Batteries are marked with capacity in Ah (or RC in minutes) and starter batteries also include CCA. These published values are assumed correct, but this may not always so. The CCA of some starter batteries can be higher or lower than marked and few consumers verify the readings. In addition, deep-cycle batteries show low capacity when new but the readings will strengthen with use as the battery is being formatted.
Last updated 2015-05-26
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