Rapid-testing of batteries
(BU41)
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%.
_________________________
Created:
May 2003
