battery and the digital load
Cadex Electronics Inc.
the move from analog to digital communications devices, new demands
are placed on the battery. Unlike analog transceivers that draw a
steady current, the digital radio loads the battery with short,
heavy current spikes. The new Tetra system (Terrestrial Trunked
Radio), which is being implemented in Europe, draws current pulses
of up to 3 Amperes when transmitting. Other systems, such as Project
25 used in North America, have similar requirements.
of the urgent requirements of a battery for two-way radios is low
internal resistance. Measured in milliohms (mΩ),
the resistance is the gatekeeper of the battery that, to a large
extent, determines the talk time. The lower the resistance, the less
restriction the battery encounters in delivering the needed power
spikes. A high mΩ
reading often triggers an early
‘low battery’ indication on a seemingly good battery because
the available energy cannot be delivered fully and remains in the
Cold and hot
temperatures also play a critical roll in battery performance.
Similar to us humans, the battery performs best at room temperature.
Depending on battery chemistry, the performance at freezing
temperatures is reduced by 20-50%.
Figure 1 we examine analog and digital radio transceivers and
compare peak power and peak current requirements, which the battery
must be able to supply during transmission.
or 3 Watts 1
1: Peak power requirements
for PMR and cell phone systems.
from analog to digital communications devices reduces the overall
energy need but increases the peak current during load pulses. The
wattage varies with signal strength.
Wattage varies with signal strength
Based on 7.2V battery
the best battery for a digital two-way radio?
Today’s battery research is
heavily focused on lithium systems, so much so that one could assume
that all future applications will be lithium based. How well do
these new battery systems perform in the rather harsh environment of
digital transceivers? In Figure 2 we examine the relationship
between energy density (capacity) and internal resistance of
Nickel-cadmium, Nickel-metal-hydride and Lithium-ion batteries. To
address longevity, we also include ‘best cycle life’. It should
be noted that periodic discharge cycles are required to achieve the
indicated cycle life of nickel-based batteries.
Energy density (Wh/Kg)
- best results
Self-discharge per month2
Best cycle life
0.5C or lower
1C or lower
300-500 3 or 2-3
2: Characteristics of NiCd, NiMH and Li-ion in terms of energy
density, internal resistance, self-discharge and cycle life.
Although most durable, NiCd requires a largest amount of maintenance
by applying monthly discharge cycles.
Internal wiring, contacts
and protection circuits are taken into account. Readings vary with
cell rating, charge state and
number of cells
connected in series.
discharge is highest in the first 24h, then tapers off.
Self-discharge increases with battery age and high temperature.
3 Cycle life
is based on depth of discharge. Shallow discharges provide more
cycles than deep discharges.
4 Cycle life
is based on maintenance procedure. Failing to apply periodic full
discharge cycles may reduce the cycle life
by a factor of
and Lithium-ion batteries have proven to perform well for cell
phones and laptop computers, the track record for
Nicked-metal-hydride on two-way radios is less encouraging. Shorter
than expected service life prompt some users to switch back to
Nickel-cadmium or experiment with the more expensive Lithium-ion
Nickel-cadmium, and to some
extend Nickel-metal-hydride, are high maintenance batteries that
must be fully discharged once per month to prevent ‘memory’. The
word ‘memory’ is a misnomer because the modern Nickel-cadmium
battery is mostly void of this phenomenon. Memory is better
explained in terms of crystalline formation that occurs on the cell
plates. If no maintenance is applied for a period of four months or
more, the capacity drops by as much as one third. Discharging the
battery to one volt per cell at this stage may restore lost
performance. However, full restoration becomes more difficult the
longer service is withheld.
is not recommended to discharge a battery before each charge because
such activity wears down the battery and shortens life. Neither is
it advisable to leave a nickel-based battery in the charger for more
than two days. When not in use, the battery should be put on a shelf
and charged before use.
is low maintenance, an advantage that most other chemistries cannot
claim. No scheduled cycling is required to prolong the battery’s
life. The pack is lighter and holds more energy than a nickel-based
pack of same size. But despite
the advertised advantages, Lithium-ion has
drawbacks. It is fragile and requires a protection circuit to
maintain safe operation. The maximum charge and discharge current is
lower than that of nickel-based batteries. Price is also an issue.
is another concern. Lithium-ion frequently fails after two or three
years, whether used or not. Keeping the battery at moderate
temperatures extends the life. Manufacturers are constantly improving Lithium-ion and the age
limitation may one day be solved.
the overall energy requirement of a digital transceiver is less than
that of the analog equivalent, batteries for digital transceivers
must be capable of delivering high current pulses, which are often
several times higher than their own rating. Let’s look at battery
rating as expressed in C-rates.
A 1C discharge of a battery
that is rated 1000 mili-ampere-hours (mAh) equals 1000mA. In
comparison, a 2C discharge of the same battery is 2000mA. A Tetra
transceiver powered by a 1000mA battery, which draws 3-ampere
pulses, loads the battery with a whopping 3C discharge pulse.
3C rate discharge is acceptable for a battery with very low internal
resistance. However, aging batteries pose a challenge because the mΩ
readings increase with usage and time.
performance can be achieved by using a larger battery, also known as
extended pack. Bulkier and heavier, the extended pack offers a
typical rating of about 2000mAh or roughly double that of the
standard pack. In terms of C-rate, the 3C discharge is reduced to
1.5C when using a 2000mAh instead of a 1000mAh battery.
do seemingly good batteries fail in digital equipment?
part of ongoing research to find the best battery system for
wireless communications devices, Cadex Electronics has examined NiCd,
NiMH and Li-ion at various discharge rates. These batteries had been
in use for a while and generated good capacity readings when tested
with a battery analyzer that draws a mild load. When discharged at a
higher rate, which is the case in a digital transceiver, the
performance dropped sharply. The reason is high internal battery
resistance on some packs. Nickel-cadmium shows a low 155 mΩ,
Nickel-metal-hydride a high 778 mΩ
and Lithium-ion a moderate 320 mΩ
resistance reading. At capacities of 113%, 107% and 94%
respectively, the batteries appear normal.
From the charts below we
observe that the talk time is in direct relationship with the
battery’s internal resistance. Nickel-cadmium performs best under
the circumstances and provides a talk time of 120 minutes at 3C
discharge. Nickel-metal-hydride performs only at 1C and fails at 3C.
The Lithium-ion allows 50 minutes talk time at 3C. Although the
batteries tested are used for GSM phones (Global System for Mobile
Communications), there is a parallel in performance with the Tetra
Discharge and resulting talk-time of a NiCd battery at 1C, 2C
and 3C under the GSM load schedule. The battery tested has a
capacity of 113%, the internal resistance is a low 155mΩ.
Discharge and resulting talk-time of a NiMH battery at 1C, 2C and 3C
under the GSM load schedule. The battery tested has a capacity of
107%, the internal resistance is a high 778mΩ.
Discharge and resulting talk-time of a Li-ion battery at 1C, 2C and
3C under the GSM load schedule. The battery tested has a capacity of
94%, the internal resistance is 320mΩ..
can battery performance be measured?
the battery performance by applying a full discharge is reliable but
the service is slow. In addition, a discharge with a steady current
does not assure battery performance under a digital load. During the
last few years, instruments have been introduced that measure
internal battery resistance. Although fast, resistance diagnosis
often provides conflicting capacity readings and the predicted
battery performance in unreliable.
Electronics has developed a method to measure the state-of-health (SoH)
of a battery through a method called QuickTest™. The
service lasts 3 minutes and tests batteries for two-way
radios, cell phones, laptops, scanners, medical equipment, video
cameras and more.
uses a unique inference algorithm to fuse data from a number of
different variables. It evaluates capacity, internal resistance,
self-discharge, charge acceptance, discharge capabilities and
mobility of electrolyte. All of these variables are combined into
one number called SoH.
QuickTest™ program is built into the Cadex C7200 and C7400
battery analyzers and services nickel, lithium and lead-based
batteries. The analyzers are user-programmable and also perform
prime, recondition, fast-charge, life-test and boost functions.
Figure 6 illustrates the four-station Cadex C7400 battery analyzer
Figure 6: The
Cadex C7400 battery analyzer features the patent-pending
QuickTest™ program. The service lasts 3 minutes and evaluates
capacity, internal resistance, self-discharge, charge
acceptance, discharge capabilities and mobility of
electrolyte. All variables are combined to display battery
use of battery specific matrices that are obtained using
the analyzer’s trend learning process. The ability to learn allows
adapting to new batteries in the field.
matrices are stored in custom-made battery adapters, which
automatically configure the analyzer to the correct battery setting.
The battery adapters commonly include the QuickTest™ matrix
at time of purchase. If missing, the user can obtain the matrix by
scanning a known good battery on the analyzer’s Learn
program. The codes can be copied to other adapters, erased and
re-entered. The required charge level to perform QuickTest™
is 20-90%. If outside this range, the analyzer automatically applies
a brief charge or discharge.
communications devices are only as reliable as the battery. To this
day, the battery remains the renegade, especially after the pack has
been in service for a while. No alternative technology is on the
horizon to replace the somewhat temperamental electro-chemical
battery in use today.
batteries have been improving, the immediate emphasis should be on
battery maintenance. This is in form of exercising batteries to
prolong life, reconditioning those that have become weak and
retiring packs that are unserviceable. Only with a properly managed
maintenance program can a battery fleet remain reliable and the
operating costs reduced. While battery maintenance for nickel-based
batteries consists of periodic discharges to eliminate ‘memory’,
maintenance-free Lithium-ion packs benefit from quick-test methods.
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.
Electronics is a manufacturer of advanced battery chargers, battery
analyzers and PC software. For product information please visit www.cadex.com.