Ever since Cadillac invented the starter motor in 1912, car mechanics explored ways to measure cold cranking amps. CCA assures that the battery has sufficient energy to crank the engine when cold. To do this without “freezing,” testers look at internal resistance, the gatekeeper of a battery. A starter battery with low resistance assures reasonably good cranking, and a CCA reading of 400 to 500A is sufficient for most starter batteries. According to SAE J537, a CCA reading of 500A delivers 500A at -18°C (0°F) for 30 seconds without dropping below 7.2 volts. Read more about How to Measure Capacity.
Courtesy of BMW
Garages seldom do the full-fletched CCA test; this belongs to laboratories. Instead, device manufacturers offer alternatives and the carbon pile introduced in the 1980s is one of the oldest and most reliable methods. To do a pass/fail test, a fully charged starter battery is loaded with half the rated CCA for 15 seconds at a moderate temperature of 10º C (50º F) and higher. The battery will pass if the voltage stays above 9.6V. Colder temperatures cause the voltage to drop further. The DC load method has the advantage of detecting batteries with a partially shorted cell (low specific gravity) but the device cannot estimate battery capacity.
Mechanics prefer small sizes, and instead of applying the prolonged load that is typical of the carbon pile, device manufactures developed handheld testers that induce a high-current pulse. The Ohm’s law calculates the internal resistance based on the load current and voltage drop. The test conditions and results of this device are similar to the carbon pile.
Meanwhile, non-invasive test methods emerged, meaning that the battery is no longer loaded for measurements. The AC Conductance method reads CCA by injecting a single frequency of 80–90 Hertz to the battery. The units are smaller than invasive devices and the battery does not need to be fully charged. AC Conductance meters cannot read capacity and a partially shorted cell may pass as good.
Critical progress has been made in electrochemical impedance spectroscopy (EIS). Research centers have been using EIS for many years but high equipment cost, long test times and the need for trained professionals to decipher the data have kept this technology in laboratories. Fuzzy logic, advanced digital signal processors and a new algorithm to process the information have simplified this task.
Cadex took the EIS concept one step further and developed multi-model electrochemical impedance spectroscopy or Spectroäfor short. Spectro™ gives more accurate CCA estimation than what is possible with single-frequency AC Conductance, but the most important advantage is the ability to estimate capacity, the leading health indicator of a battery. Here is how it works:
A control signal ranging from 20 to 2000Hz is injected into the battery as if to capture the topography of a landscape. The scanned imprint is then compared against a matrix to derive at the reading. A matrix can be described as a multi-dimensional lookup table; and text recognition, fingerprint identification and visual imaging operate on a similar principle.
CCA works on a basic matrix that covers a broad range of starter batteries. Capacity, on the other hand, requires a complex model. To simplify testing, Cadex has developed a generic matrix that covers most starter batteries, flooded and AGM. The said generic matrix provides pass/fail information based on a capacity setting of 40 percent, which serves as the end-of-life threshold. Battery-specific matrices can be made available that offer numeric capacity values in percent. The test takes 15 seconds and works with a partially charged battery. Figure 1 shows the Spectro CA-12 with Spectro™ technology.
Figure 1: Spectro CA-12 battery tester
Multi-frequency concept Spectro™ concept displays capacity, CCA and state-of-charge; test time is 15 seconds.
“How accurate are the readings,” car mechanics ask? This depends on the tester and method used. For example, the Spectro CA-12 with a generic matrix provides a CCA accuracy of 90 percent; capacity is about 80 percent. The single-frequecy AC Conductance, on the other hand, provides a CCA accuracy of roughly 70 percent with no capacity readout. Such low accuracies may come as a surprise to many and service technicians ask for better than 90 percent. This is impossible with lead acid batteries because of inherited inaccuracies. Capacity fluctuations of +/- 12 percent are common even with highly accurate discharge and charge equipment tested in a controlled laboratory environment. Read more about How to Measure Capacity.
State-of-charge (SoC) also affects accuracy. Figure 2 compares CCA readings at different SoC, taken with the Spectro CA-12 and a device that is based on AC Conductance. While Spectro shows only a slight decrease with depleting charge, AC Conductance reflects a strong departure form the horizontal line; the readings are only similar at a 70 percent SoC. Since most batteries hover at about 70 percent when the car is brought in for service, the CCA readings of the two methods may appear similar.
Figure 2: CCA accuracy on state-of-charge
The Spectro CA-12 provides stable CCA readings between a SoC of 100–40% (red); the values on AC Conductance drop rapidly with SoC (blue).
Battery manufacturers are hesitant to endorse new test technologies. It is said that the first digital tester introduced in the early 1990s won approval by agreeing to give slightly higher CCA readings than what a lab test would provide. After all, who knows the true value! Very few service garages would go through a full SAE J537 verification that can take up to a week to complete for one battery. Showing a higher reading will indeed favor market acceptance, but this poses a problem when emerging technologies reveal correct readings that are at lower levels.
It so happened that the battery laboratory of a German luxury car manufacturer performed a comparison test as part of product qualification. The battery testers involved were the Spectro CA-12 and a device based on AC Conductance. With a dedicated matrix, the Spectro CA-12 achieved a CCA accuracy of 97%; capacity came in at 87 percent. In comparison, the AC Conductance unit produced a correct CCA prediction of only 51 percent with no capacity reference.
One can clearly see that a CCA measurement at a low accuracy provides limited information regarding battery aging and end-of-life prediction. Furthermore, the driver can guess CCA on engine cranking. Capacity is the more reliable health indicator and there is some confusion in differentiating between the two. North America focuses on CCA, and RC (reserve capacity) is usually overlooked. Europe, on the other hand, is more in tune with capacity and their batteries are clearly marked with Ah. [Formula for RC to Ah conversion: RC divided by 2 plus 16]
Figure 3 illustrates the bond between capacity and CCA on hand of a fluid-filled container. The liquid represents the capacity, and the tap symbolizes the energy delivery or CCA, best remembered as “pipe size.” While CCA stays stable through most of the battery life, the capacity decreases steadily. The illustration represents the aging process with growing “rock content” that inhibits energy storage. The capacity gradually declines until there won’t be enough “juice” one day to start the engine. Read more about Tracking Battery Capacity and Resistance as part of Aging.
Figure 3: Relationship of CCA and capacity of a starter battery
Capacity represents energy and is shown as liquid. CCA relates to internal resistance and is responsible for energy delivery, best remembered as “pipe size.” CCA tends to stay high while the capacity diminishes as part of aging.
No single instrument can evaluate all battery anomalies and rapid testing only gives a rough estimation. There are battery defects that can only be revealed by applying a heavy load, and a micro short in a cell is such a case. A rapid-test might pass the battery as good even though the short has lowered the specific gravity to almost “empty” due to high self-discharge and the engine won’t crank. A carbon pile or hydrometer is best able to find the anomaly but the test must be done after the battery has been removed from the charger for a few days. A charge will cover up the fault and everything will look normal.
There are no ideal battery test instruments; however, scientists predict that the battery industry is moving towards electrochemical impedance spectroscopy to estimate battery performance. While advancements in battery rapid-testing are noteworthy, none is foolproof.