Choosing a battery that will last
Cadex Electronics Inc.
What causes a battery to wear down — is it mechanical or
chemical? The answer is “both”. A battery is a perishable
product that starts deteriorating right from the time it leaves
the factory. Similar to a spring under tension, a battery
seeks to revert back to its lowest denominator. The speed
of aging is subject to depth of discharge, environmental conditions,
charge methods and maintenance procedures, or the lack thereof.
Aging and user-conditions affect each battery chemistry differently.
As part of an ongoing research program to find the optimum
battery system for wireless applications, Cadex has performed
life cycle tests on Nickel Cadmium (NiCd), Nickel Metal Hydride
(NiMH) and Lithium Ion (Li-ion) batteries. All tests were
carried out on the Cadex C7000 battery analyzers in
the test labs of Cadex Electronics Inc., Vancouver,
The batteries received an initial full-charge, and then underwent
a regime of continued discharge/charge cycles. The internal
resistance was measured with Cadex’s proprietary OhmTest
method, and the self-discharge was obtained from time-to-time
by reading the capacity loss incurred during a 48-hour rest
period. The test program involves 53 batteries of different
models and chemistries.
When conducting battery tests in a laboratory, it should
be noted that the performance in a protected environment is
commonly superior to those in field use. Elements of stress
and inconsistency that are present in everyday use cannot
always be simulated accurately in the lab.
Battery performance as a function of cycling
In terms of life cycling, the standard NiCd is the most enduring
battery. In Figure 2 we examine the capacity, internal resistance
and self-discharge of a 7.2V, 900mA NiCd battery with standard
cells. Due to time constraints, the test was terminated after
2200 cycles. During this period, the capacity remains steady,
the internal resistance stays flat at 75 milliohms (mO) and
the self-discharge is stable. This battery receives a grade
‘A’ for almost perfect performance.
Figure 1: Characteristics of a standard
cell NiCd battery.
This battery deserves an ‘A’ for
almost perfect performance in terms of stable capacity, internal
resistance and self-discharge over many cycles. This illustration
shows results for a 7.2V, 900mA NiCd.
The readings on an ultra-high capacity NiCd are less
favorable but still better than other chemistries in terms
of endurance. Although up to 60% higher in energy density
than the standard NiCd version, Figure 3 shows a steady drop
of capacity during the 2000 cycles delivered. At the same
time, the internal resistance rises slightly. A more serious
degradation is the increase of self-discharge after 1000 cycles.
This deficiency manifests itself in shorter runtimes because
the battery consumes some energy, even if not in use.
Figure 2: Characteristics of a NiCd battery
with ultra-high capacity cells.
This battery is not as favorable
as the standard NiCd but offers higher energy densities and
performs better than other chemistries in terms of endurance.
This illustrations shows results for a 6V, 700mA NiCd.
Figure 4 examines the NiMH, a battery that offers high energy
density at low cost. We observe good performance at first
but past the 300-cycle mark, the performance starts to drift
downwards rapidly. One can observe the swift increase in internal
resistance and self-discharge after cycle count 700.
Figure 3: Characteristics of a NiMH
This battery offers good performance
at first but past the 300-cycle mark, the capacity, internal
resistance and self-discharge start to deteriorate rapidly.
This illustration shows results for a 6V, 950mA NiMH.
The Li-ion battery offers advantages that neither the NiCd
nor NiMH can meet. In Figure 5 we examine the capacity and
internal resistance of a typical Li-ion. A gentle and predictable
capacity drop is observed over 1000 cycles and the internal
resistance increases only slightly. Because of low readings,
self-discharge has been omitted on this test.
Figure 4: Characteristics of a Li‑ion
The above-average performance of
this battery may be due to the fact that the test did not
include aging. This illustration shows results for a 3.6V,
500mA Li‑ion battery.
Is the Li-ion truly superior?
Today’s battery research focuses heavily on lithium systems,
so much so that one could assume that all future applications
will be lithium based. This lithium hype is especially apparent
when attending battery conferences where battery manufacturers
from all corners of the world meet to present their latest
achievements. Just a few years ago, the argument was NiCd
against NiMH; then the speakers argued on the benefit of Li-ion
over NiMH and today the emphasis is on Li-ion Polymer. The
Nickel Cadmium (NiCd) and Nickel Metal Hydride (NiMH) are
In many aspect, the Li-ion is superior to nickel or lead
based batteries. There is one weak point with the Li-ion that,
for unknown reasons, is seldom mentioned by the battery manufacturer.
It is aging. Capacity deterioration is noticeable after one
year, whether the battery is in storage or use. Past two years,
the battery frequently fails.
Although less of a concern in the fast moving cell-phone
and notebook market, storage at operational readiness is an
important factor in most industrial applications. In Figure
6 we examine the capacity loss as a function of charge level
and storage temperature.
(recommended storage charge level)
(typical user charge level)
|| 98% after 1 year
|| 94% after 1 year
|| 96% after
|| 80% after
|| 85% after 1 year
|| 65% after 1 year
|| 75% after
|| 60% after
Figure 5: Non-recoverable capacity loss
on Li‑ion batteries after storage.
High charge levels and elevated
temperatures hasten the capacity loss. Improvements in chemistry
have increased the storage performance of some Li‑ion
It is not recommended to keep Li-ion batteries in storage
for a long time. Instead, the packs should be rotated like
perishable food. The buyer should be aware of the manufacturing
date when purchasing a replacement Li-ion battery. Unfortunately,
this information is often encoded in an encrypted serial number
and is only available to the manufacturer. The recommended
charge level is 40% and the storage temperature should be
15° C or less.
Research has brought about a variety of battery chemistries,
each offering distinct advantages but none providing a fully
satisfactory solution. However, with today’s selection of
battery systems, better choices can be made to tailor to specific
Relentless downsizing has pressured manufacturers to invent
smaller battery sizes. By packing more energy into a pack,
other qualities may be neglected, one of which is longevity.
Long service life and predictable low mΩ reading are
found in the NiCd family. However, this chemistry is being
replaced, where applicable, with systems containing higher
energy density. In addition, negative publicity about the
memory phenomenon and concerns of toxicity on careless disposal
are causing equipment manufacturers to seek alternative choices.
Once hailed as the superior battery system, the NiMH has
also failed to provide the universal battery solution for
the twenty-first century. Shorter than expected service life
remains a major complaint by users.
For a fast-moving consumer market that replaces the equipment
every two years, the Li-ion battery is the best choice. However,
for applications that need fully charged batteries that often
must be kept in warm storage conditions, the Li-ion does pose
a problem in terms of longevity.
So far, the emerging lithium polymer system has been unable
to overcome the shortcomings of the Li-ion. Other than lighter
and thinner designs, the cost-to-energy ration of Li-Polymer
is among the highest of rechargeable batteries.
With the rapid developments in technology occurring today,
battery systems may soon become viable that use neither nickel,
lead nor lithium. Fuel cells, which do not rely on electro-chemical
process but allow refueling similar to a vehicle, may solve
the portable energy needs for the future. Instead of a charger,
the user carries a bottle of liquid energy. Although still
many years from practical reality, such a battery would truly
change the way we live and work.
This article contains excerpts from the second edition book
entitled Batteries in a Portable World — A Handbook on Rechargeable
Batteries for Non-Engineers. In the book, Mr. Buchmann evaluates
the battery in everyday use and explains their strengths and
weaknesses in laymen’s terms. The 300-page book is available
from Cadex Electronics Inc. through firstname.lastname@example.org, tel. 604-231-7777
or most bookstores. For additional information on battery
technology visit www.buchmann.ca.
About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics
Inc., in Richmond (Vancouver) British Columbia, Canada. Mr.
Buchmann has a background in radio communications and has
studied the behavior of rechargeable batteries in practical,
everyday applications for two decades. The author of many
articles and books on battery maintenance technology, Mr.
Buchmann is a well-known speaker who has delivered technical
papers and presentations at seminars and conferences around
About the Company
Cadex Electronics Inc. is a world leader in the design and
manufacture of advanced battery analyzers and chargers. Their
award-winning products are used to prolong battery life in
wireless communications, emergency services, mobile computing,
avionics, biomedical, broadcasting and defense. Cadex products
are sold in over 100 countries.