Rapid-testing
of batteries
When studying the characteristics relating to battery state-of-health
(SoH) and state-of-charge (SoC), some interesting and disturbing effects
can be observed - the properties are cumbersome and not linear. Worst
of all, the parameters are unique for every battery type. This inherent
complexity makes it difficult to create a formula for rapid testing that
works for all batteries.
In spite of these seemingly insurmountable odds, battery rapid testing
is possible. But the questions are asked, how accurate will the test results
be and how will the system adapt to different battery types. Instrument
cost and ease-of-use are also concerns. This paper evaluates currently
used methods, which include the load test, AC conductance test and the
six-point test developed by Cadex.
The load
test
The load test provides important battery information consisting of open
battery voltage, voltage under load and internal resistance. nickel-based
batteries should always indicate an open terminal voltage of about 1.1V/cell,
even if empty. The electro-chemical reaction of the different metals in
the cell generates this voltage potential. A depressed voltage may indicate
high self-discharge or a partial electrical short.
A lead-based battery must always have a charge and the open terminal voltage
should read 2.0V/cell and higher. If below 2 volts, a sulfation layer
builds up that makes a recharge difficult, if impossible. An open terminal
voltage of 2.10V/cell indicates that the battery is roughly 50% charged.
The voltage of a lithium-based battery can, to some extent, indicate SoC.
A fully charged cell reads about 4.0V/cell and a partially charged cell
measures between 3.0 and 4.0V/cell.
The load test applies a momentary load, during which the voltage is measured.
Voltage over current equals the resistance. More accurate results are
obtained by applying a two-stage load. Figure 1 illustrates the voltage
pattern of such a two-stage load test.
 |
Figure
1: DC load test. The DC load test measures the battery's internal resistance by reading the voltage drops of two loads of different strength. A large drop indicates high resistance. |
The AC
conductance test
An alternative method of measuring the internal battery resistance is
the AC conductance test. An alternating current of 50 to 1000 Hertz is
applied to the battery terminals. The battery's reactance causes a phase
shift between voltage and current, which reveals the condition of the
battery. AC conductance works best on single cells. Figure 2 demonstrates
the relation of voltage and current on a battery.
 |
Figure
2: AC load test. The AC method measures the phase shift between voltage and current. The battery's reactance and/or voltage deflections are used to calculate the impedance |
Some AC
resistance meters evaluate only the load factor and disregard the phase
shift information. This technique behaves similar to the pulse method
in that the AC voltage is superimposed on the battery's DC voltage and
acts as brief charge and discharge pulses. The amplitude of the ripple
is utilized to calculate the internal battery resistance.
There are some discrepancies in the resistance readings between the 'load
test' and 'AC conductance test'. The differences are more apparent on
marginal than on good batteries. So which reading is correct? In many
aspects, the AC conductance is superior to the load test, however, one
single frequency cannot provide enough data to evaluate the battery adequately.
Multi-frequency devices are being developed but their complexity rises
exponentially with the number of frequencies used.
Resistance measurement, as a whole, provides only a rough sketch of the
battery's performance because various battery conditions affect the readings.
For example, a battery that has just been charged shows a higher resistance
reading than one that has rested for a few hours. An empty or nearly empty
battery also exhibits elevated internal resistance. To obtain reliable
readings, a battery must be at least 50% charged.
Temperature further affects the internal resistance readings. A hot battery
reads a lower resistance than one at ambient temperature or one that is
cold. In addition, the chemistry, the number of cells connected in series
and the current rating (size in mAh) of a battery influence the results.
Many batteries also contain a protection circuit that further distorts
the readings.
The Cadex
QuickTest™
Cadex Electronics has developed a method to measure the state-of-health
(SoH) of a battery in 3 minutes. QuickTest™ uses a patent-pending
inference algorithm to fuse data from 6 variables, which are: capacity,
internal resistance, self-discharge, charge acceptance, discharge capabilities
and mobility of electrolyte. The data is combined with a trend-learning
algorithm to provide an accurate SoH reading in percent. Figure 3 illustrates
general structure of such a network.
 |
Figure
3: General structure of the Cadex QuickTest™ Multiple variables are fed to the micro controller, 'fuzzified' and processed through parallel logic. The information is averaged and weighted according to the battery application. |
QuickTest™
is built into the Cadex C7000-Series battery analyzers and services nickel,
lithium and lead-based batteries for two-way radios, cell phones, laptops,
scanners and medical devices. The analyzers are user-programmable and
also perform battery priming, reconditioning, fast-charging, life-testing
and boosting functions.
QuickTest™ uses battery specific matrices that are obtained with
the analyzer's trend learning process. The ability to learn allows adapting
to new batteries in the field. The matrices are stored in the battery
adapters and automatically configure the analyzer to the correct battery
setting. The adapters commonly include the matrix at time of purchase.
If missing, the matrix can be added in the field by scanning two or more
batteries on the analyzer's Learn program. The required charge level to
perform QuickTest™ is 20-90%. If outside this range, the analyzer
automatically applies a brief charge or discharge.
What is the definition of state-of-health and when should a battery be
replaced? SoH reveals the overall battery conditions based on the above
mentioned variables, which are capacity, internal resistance, self-discharge,
charge acceptance, discharge capabilities and mobility of electrolyte.
If any of these variables provide marginal readings, the end result will
be affected. A battery may have a good capacity but the internal resistance
is high. In this case, the end SoH reading will be lowered accordingly.
Similar demerit points are added if the battery has high self-discharge
or exhibits other chemical deficiencies. The battery should be replaced
if the SoH falls below 80%.
About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver
BC. Mr. Buchmann has a background in radio communications and 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.
Cadex Electronics is a manufacturer of advanced battery chargers, battery
analyzers and PC software. For product information please visit www.cadex.com.