What is the perfect battery?
Isidor Buchmann, President
Cadex Electronics Inc.
We often get puzzled by announcements of new batteries that
are said to offer very high energy densities, deliver 1000
charge/discharge cycle and are paper-thin. Are they real?
Perhaps — but not in one and the same battery. While one battery
type may be designed for small size and long runtime, this
pack will not last and wear out prematurely. Another battery
may be built for long life, but the size is big and bulky.
A third battery may provide all the desirable attributes,
but the price would be too high for commercial use.
Battery manufacturers are well aware of customer needs and
have responded by offering packs that best suit the specific
applications. The mobile phone industry is an example of clever
adaptation. Emphasis is placed on small size, high energy
density and low price. Longevity comes in second.
The inscription of NiMH on a battery pack does not
automatically guarantee high energy density. A prismatic Nickel-Metal
Hydride battery for a mobile phone, for example, is made for
slim geometry. Such a pack provides an energy density of about
60Wh/kg and the cycle count is around 300. In comparison,
a cylindrical NiMH offers energy densities of 80Wh/kg and
higher. Still, the cycle count of this battery is moderate
to low. High durability NiMH batteries, which endure 1000
discharges, are commonly packaged in bulky cylindrical cells.
The energy density of these cells is a modest 70Wh/kg.
Compromises also exist on lithium-based batteries. Li‑ion
packs are being produced for defense applications that far
exceed the energy density of the commercial equivalent. Unfortunately,
these super-high capacity Li‑ion batteries are deemed
unsafe in the hands of the public and the high price puts
them out of reach of the commercial market.
In this article we look at the advantages and limitations
of the commercial battery. The so-called miracle battery that
merely live in controlled environments is excluded. We scrutinize
the batteries not only in terms of energy density but also
longevity, load characteristics, maintenance requirements,
self-discharge and operational costs. Since NiCd remains a
standard against which other batteries are compared, we evaluate
alternative chemistries against this classic battery type.
Nickel Cadmium (NiCd) — mature and well understood
but relatively low in energy density. The NiCd is used where
long life, high discharge rate and economical price are important.
Main applications are two-way radios, biomedical equipment,
professional video cameras and power tools. The NiCd contains
toxic metals and is environmentally unfriendly.
Nickel-Metal Hydride (NiMH) — has a higher energy
density compared to the NiCd at the expense of reduced cycle
life. NiMH contains no toxic metals. Applications include
mobile phones and laptop computers.
Lead Acid — most economical for larger power applications
where weight is of little concern. The lead acid battery is
the preferred choice for hospital equipment, wheelchairs,
emergency lighting and UPS systems.
Lithium Ion (Li‑ion) — fastest growing battery
system. Li‑ion is used where high-energy density and
lightweight is of prime importance. The technology is fragile
and a protection circuit is required to assure safety. Applications
include notebook computers and cellular phones.
Lithium Ion Polymer (Li‑ion polymer) — offers
the attributes of the Li-ion in ultra-slim geometry and simplified
packaging. Main applications are mobile phones.
Figure 1 compares the characteristics of the six most commonly
used rechargeable battery systems in terms of energy density,
cycle life, exercise requirements and cost. The figures are
based on average ratings of commercially available batteries
at the time of publication.
Energy Density (Wh/kg)
|| 80 (initial)
(includes peripheral circuits) in mW
| 100 to 2001
| 200 to 3001
| 150 to 2501
| 200 to 3001
| 200 to 20001
Life (to 80% of initial capacity)
|| 300 to 5002,3
|| 200 to
| 500 to 10003
|| 300 to
|| 1h typical
|| very low
/ Month (room temperature)
- best result
0.5C or lower
1C or lower
1C or lower
0.2C or lower
Temperature (discharge only)
|| -40 to
| -20 to
| -20 to
| -20 to
| 0 to
| 0 to
|| 30 to 60 days
|| 60 to 90 days
|| 3 to 6 months9
|| not req.
|| not req.
|| not req.
(US$, reference only)
per Cycle (US$)11
Figure 1: Characteristics of commonly used
- Internal resistance of a battery pack varies with cell
rating, type of protection circuit and number of cells.
Protection circuit of Li‑ion and Li-polymer adds about
- Cycle life is based on battery receiving regular maintenance.
Failing to apply periodic full discharge cycles may reduce
the cycle life by a factor of three.
- Cycle life is based on the depth of discharge. Shallow
discharges provide more cycles than deep discharges.
- The discharge is highest immediately after charge, then
tapers off. The NiCd capacity decreases 10% in the first
24h, then declines to about 10% every 30 days thereafter.
Self-discharge increases with higher temperature.
- Internal protection circuits typically consume 3% of the
stored energy per month.
- 1.25V is the open cell voltage. 1.2V is the commonly
used value. There is no difference between the cells; it
is simply a method of rating.
- Capable of high current pulses.
- Applies to discharge only; charge temperature range is
- Maintenance may be in the form of ‘equalizing’ or ‘topping’
- Cost of battery for commercially available portable devices.
- Derived from the battery price divided by cycle life.
Does not include the cost of electricity and chargers.
Observation: It is interesting to note that NiCd has
the shortest charge time, delivers the highest load current
and offers the lowest overall cost-per-cycle, but has the
most demanding maintenance requirements.
The Nickel Cadmium (NiCd) battery
The NiCd prefers fast charge to slow charge and pulse charge
to DC charge. All other chemistries prefer a shallow discharge
and moderate load currents. The NiCd is a strong and silent
worker; hard labor poses no problem. In fact, the NiCd is
the only battery type that performs well under rigorous working
conditions. It does not like to be pampered by sitting in
chargers for days and being used only occasionally for brief
periods. A periodic full discharge is so important that, if
omitted, large crystals will form on the cell plates (also
referred to as memory) and the NiCd will gradually
lose its performance.
Among rechargeable batteries, NiCd remains a popular choice
for applications such as two-way radios, emergency medical
equipment and power tools. Batteries with higher energy densities
and less toxic metals are causing a diversion from NiCd to
Advantages and Limitations of NiCd
Fast and simple charge — even after prolonged storage.
High number of charge/discharge cycles — if properly
maintained, the NiCd provides over 1000 charge/discharge
Good load performance — the NiCd allows recharging
at low temperatures.
Long shelf life – in any state-of-charge.
Simple storage and transportation — most airfreight
companies accept the NiCd without special conditions.
Good low temperature performance.
Forgiving if abused — the NiCd is one of the most rugged
Economically priced — the NiCd is the lowest cost battery
in terms of cost per cycle.
Available in a wide range of sizes and performance
options — most NiCd cells are cylindrical.
Relatively low energy density — compared with newer
Memory effect — the NiCd must periodically be exercised
to prevent memory.
Environmentally unfriendly — the NiCd contains toxic
metals. Some countries are limiting the use of the NiCd
Has relatively high self-discharge — needs recharging
Figure 2: Advantages and limitations of
The Nickel-Metal Hydride (NiMH) battery
Research of the NiMH system started in the 1970s as a means
of discovering how to store hydrogen for the nickel hydrogen
battery. Today, nickel hydrogen batteries are mainly used
for satellite applications. They are bulky, contain high-pressure
steel canisters and cost thousands of dollars per cell.
In the early experimental days of the NiMH battery, the metal
hydride alloys were unstable in the cell environment and the
desired performance characteristics could not be achieved.
As a result, the development of the NiMH slowed down. New
hydride alloys were developed in the 1980s that were stable
enough for use in a cell. Since the late 1980s, NiMH has steadily
The success of the NiMH has been driven by its high energy
density and the use of environmentally friendly metals. The
modern NiMH offers up to 40 percent higher energy density
compared to NiCd. There is potential for yet higher capacities,
but not without some negative side effects.
The NiMH is less durable than the NiCd. Cycling under heavy
load and storage at high temperature reduces the service life.
The NiMH suffers from high self-discharge, which is considerably
greater than that of the NiCd.
The NiMH has been replacing the NiCd in markets such as wireless
communications and mobile computing. In many parts of the
world, the buyer is encouraged to use NiMH rather than NiCd
batteries. This is due to environmental concerns about careless
disposal of the spent battery.
Experts agree that the NiMH has greatly improved over the
years, but limitations remain. Most of the shortcomings are
native to the nickel-based technology and are shared with
the NiCd battery. It is widely accepted that NiMH is an interim
step to lithium battery technology.
Advantages and Limitations of NiMH
30 – 40 percent higher capacity over a standard
NiCd. The NiMH has potential for yet higher energy densities.
Less prone to memory than the NiCd. Periodic exercise
cycles are required less often.
Simple storage and transportation — transportation
conditions are not subject to regulatory control.
Environmentally friendly — contains only mild toxins;
profitable for recycling.
Limited service life — if repeatedly deep cycled, especially
at high load currents, the performance starts to deteriorate
after 200 to 300 cycles. Shallow rather than deep discharge
cycles are preferred.
Limited discharge current — although a NiMH battery
is capable of delivering high discharge currents, repeated
discharges with high load currents reduces the battery’s
cycle life. Best results are achieved with load currents
of 0.2C to 0.5C (one-fifth to one-half of the rated
More complex charge algorithm needed — the NiMH generates
more heat during charge and requires a longer charge
time than the NiCd. The trickle charge is critical and
must be controlled carefully.
High self-discharge — the NiMH has about 50 percent
higher self-discharge compared to the NiCd. New chemical
additives improve the self-discharge but at the expense
of lower energy density.
Performance degrades if stored at elevated temperatures
— the NiMH should be stored in a cool place and at a
state-of-charge of about 40 percent.
High maintenance — battery requires regular full discharge
to prevent crystalline formation.
About 20 percent more expensive than NiCd — NiMH
batteries designed for high current draw are more expensive
than the regular version.
Figure 3: Advantages and limitations of NiMH
The Lead Acid battery
Invented by the French physician Gaston Planté in 1859, lead
acid was the first rechargeable battery for commercial use.
Today, the flooded lead acid battery is used in automobiles,
forklifts and large uninterruptible power supply (UPS) systems.
During the mid 1970s, researchers developed a maintenance-free
lead acid battery that could operate in any position. The
liquid electrolyte was transformed into moistened separators
and the enclosure was sealed. Safety valves were added to
allow venting of gas during charge and discharge.
Driven by different applications, two battery designations
emerged. They are the small sealed lead acid (SLA), also known
under the brand name of Gelcell, and the large valve regulated
lead acid (VRLA). Technically, both batteries are the same.
(Engineers may argue that the word ‘sealed lead acid’ is a
misnomer because no lead acid battery can be totally sealed.)
Because of our emphasis on portable batteries, we focus on
Unlike the flooded lead acid battery, both the SLA and VRLA
are designed with a low over-voltage potential to prohibit
the battery from reaching its gas-generating potential during
charge. Excess charging would cause gassing and water depletion.
Consequently, these batteries can never be charged to their
The lead acid is not subject to memory. Leaving the battery
on float charge for a prolonged time does not cause damage.
The battery’s charge retention is best among rechargeable
batteries. Whereas the NiCd self-discharges approximately
40 percent of its stored energy in three months, the SLA self-discharges
the same amount in one year. The SLA is relatively inexpensive
to purchase but the operational costs can be more expensive
than the NiCd if full cycles are required on a repetitive
The SLA does not lend itself to fast charging — typical charge
times are 8 to 16 hours. The SLA must always be stored in
a charged state. Leaving the battery in a discharged condition
causes sulfation, a condition that makes the battery difficult,
if not impossible, to recharge.
Unlike the NiCd, the SLA does not like deep cycling. A full
discharge causes extra strain and each cycle robs the battery
of a small amount of capacity. This wear-down characteristic
also applies to other battery chemistries in varying degrees.
To prevent the battery from being stressed through repetitive
deep discharge, a larger SLA battery is recommended.
Depending on the depth of discharge and operating temperature,
the SLA provides 200 to 300 discharge/ charge cycles. The
primary reason for its relatively short cycle life is grid
corrosion of the positive electrode, depletion of the active
material and expansion of the positive plates. These changes
are most prevalent at higher operating temperatures. Cycling
does not prevent or reverse the trend.
The optimum operating temperature for the SLA and VRLA battery
is 25°C (77°F). As a rule of thumb, every 8°C (15°F) rise
in temperature will cut the battery life in half. VRLA that
would last for 10 years at 25°C will only be good for 5 years
if operated at 33°C (95°F). The same battery would endure
a little more than one year at a temperature of 42°C (107°F).
Among modern rechargeable batteries, the lead acid battery
family has the lowest energy density, making it unsuitable
for handheld devices that demand compact size. In addition,
performance at low temperatures is poor.
The SLA is rated at a 5-hour discharge or 0.2C. Some batteries
are even rated at a slow 20-hour discharge. Longer discharge
times produce higher capacity readings. The SLA performs well
on high pulse currents. During these pulses, discharge rates
well in excess of 1C can be drawn.
In terms of disposal, the SLA is less harmful than the NiCd
battery but the high lead content makes the SLA environmentally
Advantages and Limitations of Lead
Inexpensive and simple to manufacture in terms
of cost per watt hours, the SLA is the least expensive.
Mature, reliable and well-understood technology
when used correctly, the SLA is durable and provides
Low self-discharge the self-discharge rate is
among the lowest in rechargeable batterysystems.
Low maintenance requirements no memory; no electrolyte
Capable of high discharge rates.
Cannot be stored in a discharged condition.
Low energy density poor weight-to-energy density
limits use to stationary and wheeled applications.
Allows only a limited number of full discharge cycles
well suited for standby applications that require
only occasional deep discharges.
Environmentally unfriendly the electrolyte and
the lead content can cause environmental damage.
Transportation restrictions on flooded lead acid
there are environmental concerns regarding spillage
in case of an accident.
Thermal runaway can occur with improper charging.
Figure 4: Advantages and limitations of
lead acid batteries.
The Lithium Ion battery
Pioneer work with the lithium battery began in 1912 under
G.N. Lewis but it was not until the early 1970s that the first
non-rechargeable lithium batteries became commercially available.
Lithium is the lightest of all metals, has the greatest electrochemical
potential and provides the largest energy density per weight.
Attempts to develop rechargeable lithium batteries followed
in the 1980s, but failed due to safety problems. Because of
the inherent instability of lithium metal, especially during
charging, research shifted to a non-metallic lithium battery
using lithium ions. Although slightly lower in energy density
than lithium metal, the Li‑ion is safe, provided certain
precautions are met when charging and discharging. In 1991,
the Sony Corporation commercialized the first Li‑ion
battery. Other manufacturers followed suit. Today, the Li‑ion
is the fastest growing and most promising battery chemistry.
The energy density of the Li‑ion is typically twice
that of the standard NiCd. Improvements in electrode active
materials have the potential of increasing the energy density
close to three times that of the NiCd. In addition to high
capacity, the load characteristics are reasonably good and
behave similarly to the NiCd in terms of discharge characteristics
(similar shape of discharge profile, but different voltage).
The flat discharge curve offers effective utilization of the
stored power in a desirable voltage spectrum.
The high cell voltage allows battery packs with only one
cell. Most of today’s mobile phones run on a single cell,
an advantage that simplifies battery design. To maintain the
same power, higher currents are drawn. Low cell resistance
is important to allow unrestricted current flow during load
The Li‑ion is a low maintenance battery, an advantage
that most other chemistries cannot claim. There is no memory
and no scheduled cycling is required to prolong the battery’s
life. In addition, the self-discharge is less than half compared
to NiCd, making the Li‑ion well suited for modern fuel
gauge applications. Li‑ion cells cause little harm when
Despite its overall advantages, Li‑ion also has its
drawbacks. It is fragile and requires a protection circuit
to maintain safe operation. Built into each pack, the protection
circuit limits the peak voltage of each cell during charge
and prevents the cell voltage from dropping too low on discharge.
In addition, the cell temperature is monitored to prevent
temperature extremes. The maximum charge and discharge current
is limited to between 1C and 2C. With these precautions in
place, the possibility of metallic lithium plating occurring
due to overcharge is virtually eliminated.
Aging is a concern with most Li‑ion batteries and many
manufacturers remain silent about this issue. Some capacity
deterioration is noticeable after one year, whether the battery
is in use or not. Over two or perhaps three years, the battery
frequently fails. It should be noted that other chemistries
also have age-related degenerative effects. This is especially
true for the NiMH if exposed to high ambient temperatures.
Storing the battery in a cool place slows down the aging
process of the Li‑ion (and other chemistries). Manufacturers
recommend storage temperatures of 15°C (59°F). In addition,
the battery should be partially charged during storage.
Manufacturers are constantly improving the chemistry of the
Li‑ion battery. New and enhanced chemical combinations
are introduced every six months or so. With such rapid progress,
it is difficult to assess how well the revised battery will
The most economical Li-ion battery in terms of cost-to-energy
ratio is the cylindrical 18650 cell. This cell is used
for mobile computing and other applications that do not demand
ultra-thin geometry. If a slimmer pack is required (thinner
than 18 mm), the prismatic Li‑ion cell is the best choice.
There are no gains in energy density over the 18650, however,
the cost of obtaining the same energy may double.
For ultra-slim geometry (less than 4 mm), the only choice
is Li‑ion polymer. This is the most expensive system
in terms of cost-to-energy ratio. There are no gains in energy
density and the durability is inferior to the rugged 18560
Advantages and Limitations of Li-ion
High energy density — potential for yet higher capacities.
Relatively low self-discharge — self-discharge is less
than half that of NiCd and NiMH.
Low Maintenance — no periodic discharge is needed;
Requires protection circuit — protection circuit limits
voltage and current. Battery is safe if not provoked.
Subject to aging, even if not in use — storing the
battery in a cool place and at 40 percent state-of-charge
reduces the aging effect.
Moderate discharge current.
Subject to transportation regulations — shipment of
larger quantities of Li-ion batteries may be subject
to regulatory control. This restriction does not apply
to personal carry-on batteries.
Expensive to manufacture — about 40 percent higher
in cost than NiCd. Better manufacturing techniques and
replacement of rare metals with lower cost alternatives
will likely reduce the price.
Not fully mature — changes in metal and chemical combinations
affect battery test results, especially with some quick
Figure 5: Advantages and limitations of Li?ion
The Lithium Polymer battery
The Li-polymer differentiates itself from other battery systems
in the type of electrolyte used. The original design, dating
back to the 1970s, uses a dry solid polymer electrolyte. This
electrolyte resembles a plastic-like film that does not conduct
electricity but allows an exchange of ions (electrically charged
atoms or groups of atoms). The polymer electrolyte replaces
the traditional porous separator, which is soaked with electrolyte.
The dry polymer design offers simplifications with respect
to fabrication, ruggedness, safety and thin-profile geometry.
There is no danger of flammability because no liquid or gelled
electrolyte is used. With a cell thickness measuring as little
as one millimeter (0.039 inches), equipment designers are
left to their own imagination in terms of form, shape and
Unfortunately, the dry Li-polymer suffers from poor conductivity.
Internal resistance is too high and cannot deliver the current
bursts needed for modern communication devices and spinning
up the hard drives of mobile computing equipment. Heating
the cell to 60°C (140°F) and higher increases the conductivity
but this requirement is unsuitable for portable applications.
To make a small Li-polymer battery conductive, some gelled
electrolyte has been added. Most of the commercial Li-polymer
batteries used today for mobile phones are a hybrid and contain
gelled electrolyte. The correct term for this system is Lithium
Ion Polymer. For promotional reasons, most battery manufacturers
mark the battery simply as Li-polymer. Since the hybrid
lithium polymer is the only functioning polymer battery for
portable use today, we will focus on this chemistry.
With gelled electrolyte added, what then is the difference
between classic Li‑ion and Li‑ion polymer? Although
the characteristics and performance of the two systems are
very similar, the Li‑ion polymer is unique in that solid
electrolyte replaces the porous separator. The gelled electrolyte
is simply added to enhance ion conductivity.
Technical difficulties and delays in volume manufacturing
have deferred the introduction of the Li‑ion polymer
battery. In addition, the promised superiority of the Li‑ion
polymer has not yet been realized. No improvements in capacity
gains are achieved — in fact, the capacity is slightly less
than that of the standard Li‑ion battery. For the present,
there is no cost advantage. The major reason for switching
to the Li-ion polymer is form factor. It allows wafer-thin
geometries, a style that is demanded by the highly competitive
mobile phone industry.
Advantages and Limitations of Li-ion
Very low profile — batteries that resemble the profile
of a credit card are feasible.
Flexible form factor — manufacturers are not bound
by standard cell formats. With high volume, any reasonable
size can be produced economically.
Light weight – gelled rather than liquid electrolytes
enable simplified packaging, in some cases eliminating
the metal shell.
Improved safety — more resistant to overcharge; less
chance for electrolyte leakage.
Lower energy density and decreased cycle count compared
to Li-ion — potential for improvements exist.
Expensive to manufacture — once mass-produced, the
Li-ion polymer has the potential for lower cost. Reduced
control circuit offsets higher manufacturing costs.
Figure 6: Advantages and limitations of Li?ion
During the last few decades, rechargeable batteries have
made only moderate improvements in terms of higher capacity
and smaller size. Compared with the vast advancements in areas
such as microelectronics, the lack of progress in battery
technology is apparent. Consider a computer memory core of
the sixties and compare it with a modern microchip of the
same byte count. What once measured a cubic foot now sits
in a tiny chip. A comparable size reduction would literally
shrink a heavy-duty car battery to the size of a coin. Since
batteries are still based on an electrochemical process, a
car battery the size of a coin may not be possible using our
Research has brought about a variety of battery chemistries,
each offering distinct advantages but none providing a fully
satisfactory solution. With today’s increased selection, however,
better choices can be made to suit a specific user application.
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.