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To a large extent, the internal resistance, also
known as impedance, determines the performance and runtime
of a battery. If measured with an AC signal, the internal
resistance of a battery is also referred to as impedance.
High internal resistance curtails the flow of energy from
the battery to the equipment.
A battery with simulated low and high internal
resistance is illustrated below. While a battery with low
internal resistance can deliver high current on demand, a
battery with high resistance collapses with heavy current.
Although the battery may hold sufficient capacity, the voltage
drops to the cut-off line and the ‘low battery’ indicator
is triggered. The equipment stops functioning and the remaining
energy is undelivered.
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Figure 6-2: Effects of impedance
on battery load.
A battery with low impedance
provides unrestricted current flow and delivers all
available energy. A battery with high impedance cannot
deliver high-energy bursts due to a restricted path,
and equipment may cut off prematurely.
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NiCd has the lowest internal resistance of all
commercial battery systems, even after delivering 1000 cycles.
In comparison, NiMH starts with a slightly higher resistance
and the readings increase rapidly after 300 to 400 cycles.
Maintaining a battery at low internal resistance
is important, especially with digital devices that require
high surge current. Lack of maintenance on nickel-based batteries
can increase the internal resistance. Readings of more than
twice the normal resistance have been observed on neglected
NiCd batteries. After applying a recondition cycle with the
Cadex 7000 Series battery analyzer, the readings
on the batteries returned to normal. Reconditioning clears
the cell plates of unwanted crystalline formations, which
restores proper current flow.
Li-ion offers internal resistance characteristics
that are between those of NiMH and NiCd. Usage does not contribute
much to the increase in resistance, but aging does. The typical
life span of a Li-ion battery is two to three years,
whether it is used or not. Cool storage and keeping the battery
in a partially charged state when not in use retard the aging
process.
The internal resistance of the Li-ion batteries
cannot be improved with cycling. The cell oxidation, which
causes high resistance, is non-reversible. The ultimate cause
of failure is high internal resistance. Energy may still be
present in the battery, but it can no longer be delivered
due to poor conductivity.
With effort and patience, lead acid batteries
can sometimes be improved by cycling or applying a topping
and/or equalizing charge. This reduces the current-inhibiting
sulfation layer but does not reverse grid corrosion.
Figure 6-3 compares the voltage signature
and corresponding runtime of a battery with low, medium and
high internal resistance when connected to a digital load.
Similar to a soft ball that easily deforms when squeezed,
the voltage of a battery with high internal resistance modulates
the supply voltage and leaves the imprint of the load. The
current pulses push the voltage towards the end-of-discharge
line, resulting in a premature cut-off.
When measuring the battery with a voltmeter after
the equipment has cut off and the load is removed, the terminal
voltage commonly recovers and the voltage reading appears
normal. This is especially true of nickel-based batteries.
Measuring the open terminal voltage is an unreliable method
to establish the state-of-charge (SoC) of the battery.
A battery with high impedance may perform well
if loaded with a low DC current such as a flashlight, portable
CD player or wall clock. With such a gentle load, virtually
all of the stored energy can be retrieved and the deficiency
of high impedance is masked.

Figure 6-3:
Discharge curve.
This chart compares the runtime
of batteries with similar capacities under low, medium and
high impedance when connected to a pulsed load.
The internal resistance of a battery can be measured
with dedicated impedance meters. Several methods are available,
of which the most common are applying DC loads and AC signals.
The AC method may be done with different frequencies. Depending
on the level of capacity loss, each technique provides slightly
different readings. On a good battery, the measurements are
reasonably close; on a weak battery, the readings between
the methods may disperse more drastically.
Modern battery analyzers offer internal resistance
measurements as a battery quick-test. Such tests can identify
batteries that would fail due to high internal resistance,
even though the capacity may still be acceptable. Internal
battery resistance measurements are available in the Cadex
7000 Series battery analyzers. (See Chapter 9: Internal
Battery Resistance.)
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