Does internal resistance reveal
battery capacity? A study on rapid-test methods for stationary
and automotive batteriesIsidor Buchmann
Cadex Electronics Inc. isidor.buchmann@cadex.com www.buchmann.ca
June 2004 During the last 20 years, three
basic battery rapid test methods have emerged: DC load, AC conductance and multi-frequency
electro-chemical impedance spectroscopy (EIS). All methods are resistance based,
a characteristic that reveals the battery's ability to deliver load current. Internal
resistance provides useful information in detecting problems and indicating when
a battery should be replaced. However, resistance alone does not provide a linear
correlation to the battery's capacity. The increase of cell resistance only relates
to aging and provides some failure indications.
When measuring the internal
resistance of brand new VRLA cells from the same batch, variations of 8% are common.
The manufacturing process and materials used are only two of many possible variables
contributing to this variance. Rather than relying on an absolute resistance reading,
service technicians take a snapshot of the cell resistances when the battery is
installed and then measure the subtle changes as the cells age. An increase in
resistance of 25% over the baseline (100%) indicates a performance drop to about
80%. Battery manufacturers honor a warranty claim if the internal resistance increases
by 50%. Before analyzing the different test methods, let's briefly brush up
on internal resistance and impedance, terms that are often used incorrectly when
addressing the conductivity of a battery.
Resistance is purely resistive
and has no reactance. There is no trailing phase shift because the voltage and
current are in unison. A heating element is such a pure resistive load. It works
equally well with direct current (DC) and alternating current (AC).
Most
electrical loads, including the battery, contain a component of reactance. The
reactive part of the load varies with frequency. For example, the capacitive reactance
of a capacitor decreases with rising frequency. A capacitor is an insulator to
DC and no current can pass through. The inductor, on the other hand, acts in the
opposite way and its reactance increases with rising frequency. DC presents an
electrical short. A battery combines ohmic resistance, as well as capacitive and
inductive reactance. The term impedance represents all three types.
The
battery may be viewed as a set of electrical elements. Figure 1 illustrates Randles'
basic lead-acid battery model in terms of resistors and a capacitor (R1, R2 and
C). The inductive reactance is commonly omitted because it plays a negligible
role in a battery at low frequency.
 | Figure
1: Randles model of a lead acid battery. The overall battery resistance
consists of pure ohmic resistance, as well as inductive and capacitive reactance.
The values of these components are different for every battery tested. |
Battery rapid test methods and how they work
Let's now look at the different battery test methods and evaluate
their strengths and limitations. It is important to know that
each method provides a different internal resistance reading
when measured on the same battery. Neither reading is right
or wrong. For example, a cell may read higher resistance readings
with the DC load method than with a 1000-hertz AC signal.
This simply implies that the battery performs better on an
AC than DC load. Manufacturers accept all variations as long
as the readings are taken with the same type of instrument.
DC load method: The pure ohmic measurement is one of
the oldest and most reliable test methods. The instrument
applies a load lasting a few seconds. The load current ranges
from 25-70 amperes, depending on battery size. The drop in
voltage divided by the current provides the resistance value.
The readings are very accurate and repeatable. Manufacturers
claim resistance readings in the 10 micro-ohm range. During
the test, the unit heats up and some cooling will be needed
between measurements on continuous use.
| The DC load
blends R1 and R2 of the Randles model into one combined
resistor and ignores the capacitor. C is a very important
component of a battery and represents 1.5 farads per 100
Ah cell capacity. |
 |
Figure
2: DC load method.
The true integrity of the Randles model cannot be seen.
R1 and R2 appear as one ohmic value. |
AC conductance method: Instead of a DC load, the instrument
injects an AC signal into the battery. A frequency of between
80-100 hertz is chosen to minimize the reactance. At this
frequency, the inductive and capacitive reactance converge,
resulting in a minimal voltage lag. Manufacturers of AC conductance
equipment claim battery resistance readings to the 50 micro-ohm
range. AC conductance gained momentum in 1992; the instruments
are small and do not heat up during use.
| The single frequency technology
sees the components of the Randles model as one complex
impedance, called the modulus of Z. The majority of the
contribution is coming from the conductance of the first
resistor. |
 |
Figure 3: AC conductance
method.
The individual components of the Randles model cannot
be distinguished and appear as a blur. |
Multi-frequency electro-chemical impedance spectroscopy
(EIS): Cadex Electronics has developed a rapid-test method
based on EIS. Called Spectro, the instrument injects
24 excitation frequencies ranging from 20-2000 Hertz. The
sinusoidal signals are regulated at 10mV/cell to remain within
the thermal battery voltage of lead acid. This allows consistent
readings for small and large batteries.
With multi-frequency impedance Spectroscopy, all three
resistance values of the Randles model can be established.
|
 |
Figure 4: Spectro
method.
R1, R2 and C can be measured separately, enabling the
estimation of battery conductivity and capacity. |
| A patented process evaluates
the fine nuances between each frequency to enable an in-depth
battery analysis. |
Spectro is the most complex of the three methods. The
30-second test processes 40 million transactions. The instrument
is capable of reading to a very low micro-ohms level. More
importantly, Spectro is capable of providing battery
capacity in Ah, conductivity (CCA) and state-of-charge.
The EIS concept is not new. In the past, EIS systems were
hooked up to dedicated computers and diverse laboratory equipment.
Trained electrochemists were required to interpret the data.
Advancements in data analysis automated this process and high-speed
signal processors shrunk the technology into a handheld device.
Capacity measurements
DC load and AC conductance have one major limitation in that
these methods cannot measure capacity. With the growing demand
of auxiliary power on cars and trucks and the need to assess
performance of stationary batteries non-invasively, testers
are needed that can estimate battery capacity. Cadex has succeeded
in doing this with car batteries. The company is working on
applying this technology to stationary batteries.
Figure 5 reveals the reserve capacity (RC) readings of 24
car batteries, arranged from low to high on the horizontal
axis. The batteries were first tested according to the SAE
J537 standard, which includes a full charge,
a rest period and a 25A discharge to 1.75V/cell during which
the reserve capacity was measured (black diamonds). The tests
were then repeated with Spectro (purple squares) using
battery-specific matrices. The derived results approach laboratory
standards, as the chart reveals.
 |
| Figure 5: Reserve
capacity of 24 batteries with a model-specific matrix.
The black diamonds show capacity readings derived
by a 25A discharge; the purple squares represent the Spectro
readings. |
Some people claim a close relationship between battery conductivity
(ohmic values) and capacity. Others say that internal ohmic
readings are of little practical use and have no relation
to capacity. The truth lies somewhere in between. An analogy
can be made with a doctor who not only takes the body temperature
to determine the health of a patient but also observes blood
pressure, glucose levels, and cholesterol readings. By taking
more than just one measurement, better health assessments
can be made.
To demonstrate the relationship between resistance and capacity,
Cadex has carried out an extensive test involving 175 automotive
batteries in which the cold cranking amps (CCA) were compared
with the RC readings. CCA represents the conductivity of the
battery and is related with the internal resistance. Figure
6 shows the test results. The CCA readings are plotted on
the vertical Y-axis and the RC on the horizontal X-axis. For
ease of reading, the batteries are plotted as a percentage
of their nominal value and are arranged from low-to-high on
the x-axis.
 |
Figure 6: CCA as a function of
reserve capacity (RC).
Internal resistance (represented by CCA) and capacity
do not follow the red line closely. Resistance values
alone do not provide accurate capacity readings.
|
____________________
Note: The CCA and RC readings were obtained according to SAE
J537 standards. CCA is defined as a discharge of a fully charged
battery at -18°C at the CCA-rated current. If the voltage
remains at or above 7.2V after 30 seconds, the battery passes.
The RC is based on a full charge, rest period and a discharge
at 25A to 1.75V/cell..
If the internal
resistance (CCA) were linear with capacity, then the blue diamonds
would be in close proximity with the red reference line. In
reality, CCA and RC wander off in both directions. For example,
the 90% CCA battery produces an RC of only 38%, whereas the
71% CCA delivers a whopping 112% capacity (green dotted line).
An important need is fulfilled
Cadex has packaged the EIS technology into an elegant hand-held
tester that is currently being beta-tested in the USA, Canada,
Europe and Japan. The Spectro CA-12 model (Figure 7) is the
first in a series of battery testers capable of reading capacity,
CCA and state-of-charge. A slightly larger unit is in the design
stages that will test stationary batteries. A patent on the
Spectro technology has been granted to Cadex Electronics.
|
|
Figure 7: Spectro CA-12
automotive battery tester.
The instrument displays CCA, reserve capacity and state-of-charge
independently. A unit for stationary batteries is in development.
|
Being able to measure battery capacity makes the Spectro
testers one of the most sought-after test systems for automotive,
marine, aviation, defense, wheeled mobility, traction and UPS
batteries. Capacity fading due to aging and other deficiencies
can be tracked and a timely replacement scheduled.
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.
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