Why do different test methods provide dissimilar readings?
by robinner ner dfDuring 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 Sony VGP-BPS8
battery , Sony VAIO
VGN-FZ battery should be replaced. However, the battery often drops below
the critical 80% level set by IEEE before the condition can effectively be
detected. Neither does resistance alone provide a linear correlation to the
battery's capacity. Rather, the increase of cell resistance relates to aging.
When measuring the internal resistance of
brand new VRLA cells from the same batch, variations of 8% between cells are
common. Manufacturing process and materials used contribute to the
discrepancies. Rather than relying on an absolute resistance reading, service
technicians are asked to take a snapshot of the cell resistances when the
battery is installed and then measure the subtle changes as the cells age. A
25% increase in resistance over the baseline indicates a performance drop from
100% to about 80%. Battery manufacturers honor warranty replacements 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-ausu
a32-f3 battery, asus a33-f3 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.
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 hp pavilion dv7
battery, hp
pavilion dv2 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.
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 converges, 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.
Multi-frequency electro-chemical impedance
spectroscopy (EIS): Cadex Electronics has developed a rapid-test method based
on EIS toshiba
pa3399u-1brs battery, toshiba
pa3399u-1bas battery. 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..
Spectro? is the most complex of the three
methods. The 20-second test processes 40 million transactions. The instrument
is capable of reading to a very low micro-ohms level. With stored matrices as
reference, 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 hp pavilion
dv9000 battery, hp pavilion dm3
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
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. To demonstrate the relationship between resistance and capacity,
Cadex Electronics 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 closely 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 dell inspiron e1505 battery, dell vostro 1310 batteries are plotted as a percentage of their nominal value and are arranged from low-to-high on the X-axis.
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