Miscellaneous/Ask Isidor/ Car Batteries 060526
What causes car batteries to fail early?
Driving habits rather than battery defect may be the cause
By Isidor Buchmann [email protected] Cadex Electronics Inc.
A German manufacturer of luxury cars reveals that of 400 SLI batteries returned under warranty, 200 are working well and have no problem. Low charge and acid stratification are the most common causes of the apparent failure. The car manufacturer says that the problem is more common on large luxury cars with power-hungry auxiliary options than on the more basic models.
In Japan, battery failure is the largest complaint among new car owners. The average car is only driven
13 km (8 miles) per day and mostly in a congested city. As a result, the batteries will never get fully charged and sulfation occurs. The batteries in Japanese cars are small and only provide enough power to crank the engine and perform some rudimentary functions. North America may be shielded from these battery problems, in part because of long distance driving.
Good battery performance is important because problems during the warranty period tarnish customer satisfaction. Any service requirement during that time is recorded and the number is published in trade magazines. This data is of great interest among prospective car buyers throughout the world.
Battery malfunction is seldom caused by a factory defect; driving habits are the more common culprits. Too much electrical power may be drawn in the distance driven. As a result, the battery does not receive the periodic fully saturated charge, a requirement that is so important for the longevity of a Lead-acid battery. According to a leading European manufacturer of car batteries, factory defects amounts to less than 7 percent.
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. For this reason, most capacity measurement instruments are still based on applying a full discharge and recharge. This procedure is typically used in a laboratory environment. Time would not permit this for the service sector.
The Spectro CA-12 by Cadex Electronics is the first in a series of high-end battery testers that is 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. The Spectro CA-12 for automotive batteries is shown in Figure 1.
Advanced battery testers are set to work within a preconfigured test range. Unlike a photocopier that duplicates any document that is placed on the glass, a high-end battery rapid-tester needs to know battery type, rated capacity, CCA and Voltage. In addition, a matrix is required against which the readings are compared. These matrices cover the common flooded SLI family, the deep cycle batteries, the Absorbent Glass Mat (AGM), the spirally wound design, and the gelled type used for wheelchairs and people movers. Matrices can also be developed to test lithium and nickel-based batteries. The Spectro CA-12 is capable of storing 5 menu-selectable matrix sets to service the most common battery groups.
When available early next year, the Spectro CA-12 will include the most common matrices. New matrices can be added by scanning batteries of the same model and various performance levels with the CA-12 running on a special program. Before scanning, the batteries must be prepared with a defined discharge/ charge program. With the help of the PC-Companion software, the collected data is sent to Cadex where the engineers assemble the code and return a working matrix to the user for download into the tester.
Bar coding car batteries
A German luxury car manufacturer has introduced a system that embeds vital battery information in a bar code label. The label also contains the battery serial number and manufacturing date. When checking batteries with a tester that supports a bar code scanner, the technician only needs to scan the bar code label and the instrument is automatically configured to the correct battery. Battery testing is thus simplified to scanning the label, attaching the leads to the battery and reading the test results. Figure 2 illustrates such a test in process.
Bar coding car batteries simplifies battery testing, eliminates configuration errors and allows less trained technicians to perform the service. The accuracy is also improved because the system selects the most suitable matrix. Batteries with no bar code can be upgraded by adding a permanent label. The battery data and serial number are entered into the PC; the Companion software translates the information into the bar code format and a label printer generates the adhesive label. Once labeled, subsequent testing on this battery will be simplified.
To eliminate unauthorized warranty claims, a process can be introduced in which both the battery serial number and the car chassis number are required. A battery replacement is only offered if the information is correct. Another system to keep honesty is generating warranty numbers that accompany the battery and paperwork of a faulty battery back to the manufacturer for replacement.
Acid stratification, a problem with luxury cars
A common cause of battery failure is acid stratification. The electrolyte on a stratified battery concentrates on the bottom, causing the upper half of the cell to be acid poor. This effect is similar to a cup of coffee in which the sugar collects on the bottom when the waitress forgets to bring the stirring spoon. Batteries tend to stratify if kept at low charge (below 80%) and never have the opportunity to receive a full charge. Short distance driving while running windshield wiper and electric heaters contributes to this. Acid stratification reduces the overall performance of the battery.
Figure 3 illustrates a normal battery in which the acid is equally distributed form top to bottom. This battery provides good performance because the correct acid concentration surrounds the plates. Figure 4 shows a stratified battery in which the acid concentration is light on top and heavy on the bottom. A light acid limits plate activation, promotes corrosion and reduces performance. High acid concentration on the bottom, on the other hand, artificially raises the open circuit voltage. The battery appears fully charged but provides a low CCA. High acid concentration also promotes sulfation and decreases the already low conductivity further. If unchecked, such a condition will eventually lead to a user-induced battery failure.
Allowing the battery to rest for a few days, applying a shaking motion or tipping the unit over tends to correct the problem. A topping charge by which the 12-Volt battery is brought up to 16 Volts for one to two hours also reverses the acid stratification. The topping charge also reduces sulfation caused by high acid concentration. Careful attention is needed to keep the battery from heating up and losing excessive electrolyte through hydrogen gassing. Always charge the battery in a well-ventilated room. Accumulation of hydrogen gas can lead to an explosion. Unlike many other gases, hydrogen is odorless and can only be detected with measuring devices.
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 battery as a result of resting. Little information is available on how long a stratified battery needs to rest to improve the condition. It is known, however, 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 offering more complete battery evaluation without the need to apply a full discharge. This knowledge can then be applied to other battery systems such as traction, military, marine, aviation and stationary batteries.
The challenge of battery testing
During the last 20 years, battery testing lagged behind other technologies. Here is one of the reasons: The battery is a very difficult animal to test, short of applying a full charge, discharge and recharge. The battery behaves similar to us humans. We still don’t know why we perform better on certain days than others.
Even by using highly accurate charge and discharge equipment, batteries produce disturbingly high capacity fluctuations. To demonstrate this phenomenon, Cadex tested over 100 car batteries with diverse performance levels (Figure 5). 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). There was a whooping +/-15% variation in capacity readings across the full population. Some batteries had higher readings the second time; others came in lower. Portable nickel and Lithium-ion batteries appear to be more consistent in capacity readings than Lead-acid. More research will be needed to find the apparent instabilities of Lead-acid batteries.
For the longest time, 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.
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 an EIS system is commonly mounted on racks and the installation runs into tens of thousands of dollars. High-speed digital signal processors and advanced data analysis made it possible to package this technology into a hand-held device.
One of the major drawbacks of older technologies is the inability to measure battery capacity. CCA alone tends to be misleading because state-of-charge and other battery conditions affect the readings. Capacity is a pivotal measurement that assesses the battery condition most accurately.
No battery tester solves all problems. Entry-level testers have the advantage of being 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. Most units have poor failure detection capabilities and simply indicate ‘good battery’, ‘recharge battery’ or ‘fail’. 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. In addition, the EIS technology can be configured to show capacity, CCA and state-of-charge readings in numbers and in percentage of the nominal value. Figure 6 shows such display formats.
While numeric test results are important to engineers in a battery research laboratory, an accurate pass/fail detection method is often sufficient for the service sector. The technician simply needs to know if a failing battery requires recharging or replacing. 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. With a quick and accurate battery test system, returned batteries can be tested, charged and kept ready for an eventual replacement. Such a program would speed up service, enhance customer satisfaction and save money.
About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC. He has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver Canada. Founded in 1980, Cadex specializes in the design and manufacturing of advanced battery charging and testing instruments. For product information please visit www.cadex.com.