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The secrets of battery runtime
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The secrets of battery runtime
Isidor Buchmann, President
Cadex Electronics Inc.
isidor.buchmann@cadex.com
www.buchmann.ca
April 2001















Is the runtime of a portable device directly related to the size of the battery and the energy it can hold? In
most cases, the answer is yes. But with digital equipment, the length of time a battery can operate is not
necessarily linear to the amount of energy stored in the battery.
In this article we examine why the specified runtime of a portable device cannot always be achieved,
especially after the battery has aged. We address the four renegades that are affecting the performance of
the battery. They are: declining capacity, increasing internal resistance, elevated self-discharge, and
premature voltage cut-off on discharge.

Declining capacity
The amount of charge a battery can hold gradually decreases due to usage, aging and, with some
chemistries, lack of maintenance. Specified to deliver about 100 percent capacity when new, the battery
eventually requires replacement when the capacity drops to the 70or 60percent level. The threshold by
which a battery can be returned under warranty is typically 80percent.
The energy storage of a battery can be divided into three imaginary sections consisting of available
energy, the empty zone that can be refilled and the rock content that has become unusable. Figure 1
illustrates these three sections of a battery.
In nickel-based batteries, the rock content may be in the form of crystalline formation, also known as
memory. Deep cycling can often restore the capacity to full service. Also known as exercise, a typical
cycle consists of one or several discharges to 1V/cell with subsequent discharges. This service is best
performed with a battery analyzer.


Figure 1: Battery charge capacity.
Three imaginary sections of a battery
consisting of available energy, empty zone
and rock content.
With usage and age, the rock content grows.
Without regular maintenance, the user may
end up carrying rocks instead of batteries.
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The loss of charge acceptance of the Li-ion/polymer batteries is due to cell oxidation, which occurs
naturally during use and as part of aging. Li-ion batteries cannot be restored with cycling or any other
external means. The capacity loss is permanent because the metals used in the cells are designated to run
for a specific time only and are being consumed during their service life.
Performance degradation of the lead acid battery is often caused by sulfation, a thin layer that forms on
the negative cell plates, which inhibits current flow. In addition, there is grid corrosion that sets in on the
positive plate. With sealed lead acid batteries, the issue of water permeation, or loss of electrolyte, also
comes into play. Sulfation can be reversed to a certain point with cycling and/or topping charge but
corrosion and permeation are permanent. Adding water to a sealed lead acid battery may help to restore
operation but the long-term results are unpredictable.

Increasing internal resistance
To a large extent, the internal resistance, also known as impedance, determines the performance and
runtime of a battery. 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 in Figure 2. 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.


Figure 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.

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
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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 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 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.

Cut off
400 mOhms
Cut off
250 mOhms
Cut off
100 mOhms
100 mOhms
250 mOhms
400 mOhms
Run Time
BATTERY DISCHARGE PULSES
Cut off
voltage


Figure 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.


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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.
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.

Elevated self-discharge
All batteries exhibit a certain amount of self-discharge; the highest is visible on nickel-based batteries.
These batteries discharge 10 to 15 percent of its capacity in the first 24 hours after charge, followed by 10
to 15 percent every month thereafter.
The self-discharge on the Li-ion battery is lower compared to the nickel-based systems. The Li-ion self-
discharges about five percent in the first 24 hours and one to two percent thereafter. Adding the protection
circuit increases the self-discharge to ten percent per month.
One of the best batteries in terms of self-discharge is the lead acid system; it only self-discharges
five percent per month. It should be noted, however, that the lead acid family has also the lowest energy
density among current battery systems. This makes the system unsuitable for hand-held applications.
At higher temperatures, the self-discharge on all battery chemistries increases. Typically, the rate doubles
with every 10
°
C (18
°
F). Large energy losses occur through self-discharge if a battery is left in a hot
vehicle. On some older batteries, stored energy may get lost during the course of the day through self-
discharge rather than actual use.
The self-discharge of a battery increases with age and usage. For example, a NiMH battery is good for
300 to 400 cycles, whereas a NiCd adequately performs over 1000 cycles before high self-discharge
affects the performance of the battery. Once a battery exhibits high self-discharge, little can be done to
reverse the effect. Factors that accelerate self-discharge on nickel-based batteries are damaged separators
(induced by excess crystalline formation, allowing the packs to cook while charging), and high cycle
count,