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The Reason for the Car Batteries Failure (2)

Even by using highly accurate charge and discharge equipment, lead acid batteries produce disturbingly high capacity fluctuations on repetitive measurements. To demonstrate the variations, Cadex tested 91 car batteries with diverse performance levels (Figure 3). We first prepared the batteries by giving them a full charge and a 24-hour rest period. We then measured the capacity by applying a 25A discharge to 10.50V or 1.75V/cell (black diamonds).

This procedure was repeated for a second time and the resulting capacities were plotted (purple squared). This produced a whooping +/-15% variation in capacity readings across the full population. Some batteries had higher readings the second time; others were lower. Other chemistries appear to be more consistent in capacity readings than lead acid.

From the beginning, load testers have been the standard test method for car batteries. The year 1992 brought us AC conductance, a method that simplified battery testing. Now we are experimenting with multi-model electrochemical impedance spectroscopy (EIS) in a portable version at an affordable price.

Getting a fast and dependable assessment of a failing battery is difficult. Most battery testers in use only take cold cranking amps (CCA) and voltage readings. Capacity, the most important measurement of a battery, is unavailable. While taking the CCA reading alone is relatively simple, measuring the capacity is very complex and instruments offering this feature are expensive.

The Spectro CA-12 by Cadex Electronics is the first in a series of high-end battery testers capable of measuring capacity, CCA and state-of-charge (SoC) in a single, non-invasive test. The technology is based on multi-model electrochemical impedance spectroscopy (EIS). The system injects 24 excitation frequencies ranging from 20 to 2000 Hertz. The sinusoidal signals are regulated at 10mV/cell to remain within the thermal battery voltage of lead acid. This achieves stable readings for small and large batteries.

During the 30-second test, over 40 million transactions are completed. A patented algorithm analyses the data and the final results are displayed in capacity, CCA and state-of-charge.

EIS is very complex and until recently required dedicated computers and expensive laboratory equipment, not to mention chemists and engineers to interpret the readings. The hardware of a full EIS system is commonly mounted on racks and the installation runs into tens of thousands of dollars.
The tough choice
No battery tester solves all problems. Entry-level testers are low cost, simple to use and capable of servicing a broad range of batteries. However, these units only provide a rough indication of the battery condition. A lab test at Cadex demonstrates that a battery tester based on EIS is four times more accurate in detecting weak batteries than AC conductance. Conventional testers often misjudge the battery on account of low state-of-charge. Many batteries are replaced when they should have been recharged, while others are given a clean bill of health when it should have been replaced.

Acid stratification is difficult to measure, even with the EIS technology. Non-invasive testers simply take a snapshot, average the measurements and spit out the results. Stratified batteries tend to show higher state-of-charge readings because of elevated voltage. On preliminary tests, the Spectro CA-12 also shows slightly higher CCA and capacity readings than normal. After letting the battery rest, the capacity tends to normalize. This may be due to diffusion effects in the stratified as a result of resting. Little information is available on how long a stratified battery needs to rest to improve the condition, other than to note that higher temperatures will hasten the diffusion process.

Ideally, a battery tester should indicate the level of acid stratification; sulfation, surface charge and other such condition and display how to correct the problem. This feature is not yet possible. Much research is being done in finding a solution that offers a more complete battery evaluation without the need for a full discharge. The knowledge gained on lead acid batteries can then be applied to other battery systems, such as traction, military, marine, aviation and stationary batteries.

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