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The
challenge of battery testing
During the last 20 years, battery testing
lagged behind other technologies. 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.
 | Figure
5: Capacity fluctuations
Capacities of 113 car batteries measured with
a conventional discharge method show a fluctuation of +/-15%. |
Summary
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.
 |  | Figure
6: Displays CCA, RC and SoC of the Spectro CA-12.
The results are displayed
in numbers and in percentage of the nominal value. |
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 an 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.
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