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Chapter 13: Making Battery Quick-Test Feasible
When Sanyo, one of the largest battery manufacturers in the
world, was asked, “Is it feasible to quick test batteries?”
the engineer replied decisively, “No”. He based his conclusion
on the difficulty of using a universal test formula that applies
to all battery applications, — from wireless communications
to mobile computing, and from power tools to forklifts and electric
vehicles.
Several universities, research organizations
and private companies, including Cadex, are striving to find
a workable solution to battery quick testing. Many methods
have been tried, and an equal number have failed because they
were inaccurate, inconsistent and impractical.
When studying the characteristics relating to
battery state-of-health and state-of-charge (SoH and SoC,
respectively) some interesting effects can be observed. Unfortunately,
these properties are cumbersome and non-linear, and worst
of all, the parameters are unique for every battery type.
This inherent complexity makes it difficult, if not impossible,
to create a formula that works for all batteries.
In spite of these seemingly insurmountable odds,
battery quick testing is possible. But the question is asked,
“how accurate will it be, and how well will it adapt to continuously
changing battery chemistries?” The cost of a commercial quick
tester and the ease-of-use are other issues of concern.
Battery Specific Quick Testing
The secret of battery quick testing lies, to
a large extent, in understanding how the battery is being
loaded. Battery loads vary from short current bursts for a
mobile phone using the GSM protocol, to long and fluctuating
loads on laptops, and to intermittent heavy loads for power
tools.
Because of these differences in loads, a battery
for a digital mobile phone should be tested primarily for
low impedance to assure a clean delivery of the current bursts,
whereas a battery for a notebook should be examined mainly
for the bulk in energy reserve. Ultra-low impedance is of
less importance here. A battery for a power tool, on the other
hand, needs both — low impedance and good power reserve.
Some quick testers simulate the equipment load
and observe the voltage signature of the battery under these
conditions. The readings are compared with the reference settings,
which are stored in the tester. The resulting discrepancies
are calculated against the anticipated or ideal settings and
displayed as the SoH readings.
The first step in obtaining quick test readings
is measuring the battery’s internal resistance, often referred
to as impedance. Internal resistance measurements take only
a few seconds to complete and provide a reasonably accurate
indication of the battery’s condition, especially if a reference
reading from a good battery is available for comparison.
Unfortunately, the impedance measurement alone
provides only a rough sketch of the battery’s performance.
The readings are affected by various battery conditions, which
cannot always be controlled. For example, a fully charged
battery that has just been removed from the charger shows
a higher impedance reading than one that has rested for a
few hours after charge. The elevated impedance is due
to the increased interfacial resistance present after charging.
Allowing the battery to rest for an hour or two will
normalize the battery. Temperature also affects the readings.
In addition, the chemistry, the number of cells connected
in series and the rating of a battery influence the results.
Many batteries also contain a protection circuit that further
distorts the readings.
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