Is the ‘smart’ battery help or deterrent?
Isidor Buchmann
Cadex Electronics Inc.
isidor.buchmann@cadex.com
www.buchmann.ca
April 2001
A speaker at a battery seminar remarked that, “The battery
is a wild animal and artificial intelligence domesticates
it.” An ordinary or ‘dumb’ battery has the inherit problem
of not displaying the amount of reserve energy it holds. Neither
weight, color, nor size provide any indication of the battery’s
state-of-charge (SoC) and state-of-health (SoH). The user
is at the mercy of the battery when pulling a freshly charged
battery from the charger.
Help is at hand. An increasing number of today’s rechargeable
batteries are made ‘smart’. Equipped with a microchip, these
batteries are able to communicate with the charger and user
alike to provide statistical information. Typical applications
for ‘smart’ batteries are notebook computers and video cameras.
Increasingly, these batteries are also used in advanced biomedical
devices and defense applications.
 There are several types of ‘smart’ batteries, each offering
different complexities, performance and cost. The most basic
‘smart’ battery may only contain a chip to identify its chemistry
and tell the charger which charge algorithm to apply. Other
batteries claim to be smart simply because they provide protection
from overcharging, under-discharging and short-circuiting.
In the eyes of the Smart Battery System (SBS) forum, these
batteries cannot be called ‘smart’.
What then makes a battery ‘smart’? Definitions still vary
among organizations and manufacturers. The SBS forum states
that a ‘smart’ battery must be able to provide SoC indications.
In 1990, Benchmarq was the first company to commercialize
the concept of the battery fuel gauge technology. Today, several
manufacturers produce chips to make the battery ‘smart’.
During the early nineties, numerous ‘smart’ battery architectures
emerged. They range from the single wire system, the two-wire
system and the system management bus (SMBus). Most two-wire
systems are based on the SMBus protocol. Let’s look at the
single wire system and the SMBus.
The Single Wire Bus
The single wire system is the simpler of the two and delivers
the data communications through one wire. A battery equipped
with the single wire system uses only three wires: the positive
and negative battery terminals and the data terminal. For
safety reasons, most battery manufacturers run a separate
wire for temperature sensing. Figure 1 shows the layout of
a single wire system.
The modern single wire system stores battery-specific data
and tracks battery parameters, including temperature, voltage,
current and remaining charge. Because of simplicity and relatively
low hardware cost, the single wire enjoys a broad market acceptance
for high-end mobile phones, two-way radios and camcorders.
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Figure 1: Single wire system of a ‘smart’
battery.
Only one wire is needed for
data communications. Rather than supplying the clock
signal from the outside, the battery includes an embedded
clock generator. For safety reasons, most battery manufacturers
run a separate wire for temperature sensing.
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Most single wire systems do not have a common form factor;
neither do they lend themselves to standardized SoH measurements.
This produces problems for a universal charger concept. The
Benchmarq single wire solution, for example, cannot measure
current directly; it must be extracted from a change in capacity
over time.
On a further drawback, the single wire bus allows battery
SoH measurement only when the host is ‘married’ to a designated
battery pack. Such a fixed host-battery relationship is feasible
with notebook computers, mobile phones or video cameras, provided
the appropriate OEM battery is used. Any discrepancy in the
battery type from the original will make the system unreliable
or will provide false readings.
The SMBus
The SMBus is the most complete of all systems. It represents
a large effort from the portable electronic industry to standardize
to one communications protocol and one set of data. The SMBus
is a two-wire interface system; one wire handles the data;
the second is the clock. It uses the I²C defined by Philips
as its backbone.
The Duracell/Intel SBS, which is in use today, was standardized
in 1993. In previous years, computer manufacturers developed
their own proprietary ‘smart’ batteries. With the new SBS
specification, a broader interface standard is made possible.
This reduces the hurdles of interfering with patents and intellectual
properties. Figure 2 shows the layout of the two-wire SMBus
system.
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Figure 2: Two-wire SMBus system.
The SMBus is based on a two-wire
system using a standardized communications protocol.
This system lends itself to standardized state-of-charge
and state-of-health measurements.
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In spite of the agreed standard, many large computer manufacturers,
such as IBM, Compaq and Toshiba, have retained their proprietary
batteries. The reason for going their own way is partly due
to safety, performance and form factor. Manufacturers claim
that they cannot guarantee safe and enduring performance if
a non-brand battery is used. To make the equipment as compact
as possible, the manufacturers explain that the common form
factor battery does not optimally fit their available space.
Perhaps the leading motive for using their proprietary batteries
is pricing. In the absence of competition, these batteries
can be sold for a premium price.
The objective behind the SMBus battery is to remove the charge
control from the charger and assign it to the battery. With
a true SMBus system, the battery becomes the master and the
charger serves as a slave that must follow the dictates of
the battery. This is based on concerns over charger quality,
compatibility with new and old battery chemistries, administration
of the correct amount of charge currents and accurate full-charge
detection. Controlled charging makes sense when considering
that some battery packs share the same footprint but contain
radically different chemistries.
The SMBus system allows new battery chemistries to be introduced
without the charger becoming obsolete. Because the battery
controls the charger, the battery manages the voltage and
current levels, as well as cut-off thresholds. The user does
not need to know which battery chemistry is being used.
An SMBus battery contains permanent and temporary data. The
permanent data is programmed into the battery at the time
of manufacturing and include battery ID number, battery type,
serial number, manufacturer’s name and date of manufacture.
The temporary data is acquired during use and consists of
cycle count, user pattern and maintenance requirements. Some
of the temporary data is being replaced and renewed during
the life of the battery.
The SMBus is divided into Level 1, 2 and 3. Level 1 has been
eliminated because it does not provide chemistry independent
charging. Level 2 is designed for in-circuit charging. A laptop
that charges its battery within the unit is a typical example
of Level 2. Another Level 2 application is a battery that
contains the charging circuit within the pack. Level 3 is
reserved for full-featured external chargers.
External Level 3 chargers are complex and expensive. Some
lower cost chargers have emerged that accommodate SMBus batteries
but are not fully SBS compliant. Manufacturers of SMBus batteries
do not readily endorse this shortcut. Safety is always a concern,
but customers prefer these economy chargers because of lower
price.
Serious industrial battery users operating biomedical instruments,
data collection devices and survey equipment use Level 3 chargers
with full-fledged charge protocol. No shortcuts are applied.
To assure compatibility, the charger and battery are matched
and only approved packs are used. The need to test and approve
the marriage between a specific battery and charger is unfortunate
given that the ‘smart’ battery is intended to be universal.
Among the most popular SMBus batteries for portable computers
are the 35 and 202 form-factors. Manufactured by Sony, Hitachi,
GP Batteries, Moltech, Moli Energy and many others, this battery
works (should work) in all portable equipment designed for
this system. Figure 3 illustrates the 35 and 202 series ‘smart'
batteries. Although the ‘35’ has a smaller footprint compared
to the ‘202’, most chargers are designed to accommodate all
sizes. A non-SMBus (‘dumb’) version with same footprint is
also available.
Figure 3: 35 and 202 series 'smart'
batteries featuring SMBus.
Available in NiCd, NiMH and Li‑ion
chemistries, these batteries are used for laptops, biomedical
instruments and survey equipment. A non-SMBus (‘dumb’) version
with same footprint is also available.
Negatives of the ‘smart’ battery
Like any good invention, the ‘smart’ battery has some serious
downsides. For starters, the ‘smart’ battery, in particular
the SMBus, costs about 25 percent more than the ‘dumb’ equivalent.
In addition, the ‘smart’ battery was intended to simplify
the charger, but a full-fledged Level 3 charger costs substantially
more than a regular dumb model.
A more serious issue is maintenance requirements, better
known as capacity re-learning. This is needed on a regular
basis to calibrate the battery. The Engineering Manager of
Moli Energy, a large Li‑ion cell manufacturer commented,
“With the Li‑ion battery we have eliminated the memory
effect, but are we introducing digital memory with the SMBus
battery?”
Why is calibration needed? The answer is to correct the
tracking errors that occur between the battery and the digital
sensing circuit during use. The most ideal battery use, as
far as fuel-gauge accuracy is concerned, is a full charge
followed by a full discharge at a constant 1C rate. In such
a case, the tracking error to less than one percent per cycle.
In real life, a battery may be discharged for only a few minutes
at a time and commonly at a lower C‑rate than 1C. Worst
of all, the load may be uneven and vary drastically. Eventually,
the true capacity of the battery no longer synchronizes with
the fuel gauge and a full charge and discharge is needed to
‘re-learn’ or calibrate the battery.
How often is calibration needed? The answer lies in the type
of battery application. For practical purposes, a calibration
is recommended once every three months or after every 40 short
cycles. Long storage also contributes to errors because the
circuit cannot accurately compensate for self-discharge. After
extensive storage, a calibration cycle is recommended prior
to use.
Many applications apply a full discharge as part of regular
use. If this occurs regularly, no additional calibration is
needed. If a full discharge has not occurred for a few months
and the user notices the fuel gauge losing accuracy, a deliberate
full discharge on the equipment is recommended. Some intelligent
equipment advises the user when a calibrating discharge is
needed. This is done by measuring the tracking error and estimating
the discrepancy between the fuel gauge reading and that of
the chemical battery.
What happens if the battery is not calibrated regularly?
Can such a battery be used in confidence? Most ‘smart’ battery
chargers obey the dictates of the cells rather than the electronic
circuit. In this case, the battery will be fully charged regardless
of the fuel gauge setting. Such a battery is able to function
normally, but the digital readout will be inaccurate. If not
corrected, the fuel gauge information simply becomes a nuisance.
The level of non-compliance is another problem with the ‘smart’
battery, in particular the SMBus. Unlike other tightly regulated
standards, the SMBus protocol allows some variations. This
may cause problems with existing chargers and the SMBus battery
should be checked for compatibility before use. Ironically,
the more features that are added to the SMBus charger and
battery, the higher the likelihood of incompatibilities.
The state-of-charge indicator
Most SMBus batteries are equipped with a charge level indicator.
When pressing a SoC button on a battery that is fully charged,
all signal lights illuminate. On a partially discharged battery,
half the lights illuminate, and on an empty battery, all lights
remain dark. Figure 4 shows such a fuel gauge.
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Figure 4: State-of-charge readout
of a ‘smart’ battery.
Although the state-of-charge
is displayed, the state-of-health and its predicted
runtime are unknown.
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While SoC information displayed on a battery or computer
screen is helpful, the fuel gauge resets to 100 percent each
time the battery is recharged, regardless of the battery’s
SoH. A serious miscount occurs if an aged battery shows 100
percent after a full-charge, when in fact the charge acceptance
has dropped to 50 percent or less. The question remains: “100
percent of what?” A user unfamiliar with this battery has
little information about the runtime of the pack.
The tri-state fuel gauge
The battery cannot be evaluated without knowing its state-of-health.
Three information levels are needed, which are: SoC, SoH and
the empty portion that can be refilled. Figure 5 illustrates
the three imaginary sections consisting of the empty zone,
available energy and rock content.
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Figure 5: 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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How can the three levels of a battery be measured and made
visible to the user? While the SoC is relatively simple to
produce, measuring the SoH is more complex. Here is how it
works:
At time of manufacture, each SMBus battery is given its specified
SoH status, which is 100 percent by default. This information
is permanently programmed into the pack and does not change.
With each charge, the battery resets to the full-charge status.
During discharge, the energy units (coulombs) are counted
and compared against the 100 percent setting. A perfect battery
would indicate 100 percent on a calibrated fuel gauge. As
the battery ages and the charge acceptance drops, the SoH
begins to indicate lower readings. The discrepancy between
the factory set 100 percent and the actual delivered coulombs
is used to calculate the SoH.
Knowing the SoC and SoH, a simple linear display can be made.
The SoC is indicated with green LED’s; the empty part remains
dark; and the unusable part is shown with red LED’s. Figure
6 shows such a tri-state fuel gauge. As an alternative, the
colored bar display may be replaced with a numeric display
indicating SoH and SoC. The practical location to place the
tri-state-fuel gauge is on the charger.
Figure 6: Tri-state fuel gauge.
The Battery Health Gauge reads the
‘learned’ battery information available on the SMBus and displays
it on a multi-colored LED bar. The illustration shows a partially
discharged battery of 50% SoC with a 20% empty portion and
an unusable portion of 30%
The target capacity selector
For users that simply need a go/no go answer, chargers are
available that feature a target capacity selector. Adjustable
to 60, 70 or 80 percent, the target capacity selector acts
as a performance check and flags batteries that do not meet
set requirements.
If a battery falls below target, the charger triggers the
condition light. The user is prompted to press the condition
button to calibrate and condition the battery by applying
a charge/discharge/charge cycle. If the battery does not recover,
the fail light illuminates, indicating that the battery should
be replaced. A green ready light assures that the battery
meets the required performance level. Figure 7 illustrates
a two-bay Cadex charger featuring the target capacity selector
and discharge circuit. This unit is based on Level 3 and services
both SMBus and ‘dumb’ batteries. SoH readings are only available
when servicing SMBus batteries.
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Figure 7: The Cadex SM2+ charger
This Level 3 charger serves
as charger, conditioner and quality control system.
It reads the battery’s true state-of-health and flags
those that fall below the set target capacity. Each
bay operates independently and charges NiCd, NiMH and
Li‑ion chemistries in approximately three hours.
‘Dumb’ batteries can also be charged but no SoH information
is available.
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By allowing the user to set the desired battery performance
level, the question is raised as to what level to select.
The answer is governed by the applications, reliability standards
and cost policies.
A practical target capacity setting for most applications
is 80 percent. Decreasing the threshold to 70 percent will
lower the performance standard but pass more batteries. A
direct cost saving will result. The 60 percent level may suit
those users who run a low budget operation, have ready access
to replacement batteries and can live with shorter, less predictable
runtimes. It should be noted that the batteries are always
charged to 100 percent, regardless of the target setting.
The target capacity simply refers to the amount of charge
the battery has delivered on the last discharge.
Summary
SMBus battery technology is predominantly used for higher-level
industrial applications. Improvements in the ‘smart’ battery
system, such as higher accuracies and self-calibration and
will likely increase the appeal of the ‘smart’ battery. Endorsement
by large software manufacturers such as Microsoft will entice
PC manufacturers to make full use of these powerful features.
‘Smart’ battery technology has not received the widespread
acceptance that battery manufacturers had hoped. Some engineers
go so far as to suggest that the SMBus battery is a ‘misguided
principal’. Design engineers may not have fully understood
the complexity of charging batteries in the incubation period
of the ‘smart’ battery. Manufacturers of SMBus chargers are
left to clean up the mess.
One main drawback of the ‘smart; battery is high price.
In the early 1990s when the SMBus battery was conceived, price
many not have been as critical as it is today. Now, buyers
want scaled down products that are economically priced and
perform the function intended. In the competitive mobile phone
market, for example, the features offered by the SMBus would
be considered overkill.
In spite teething problems and relative high costs, the ‘smart’
battery will continue to fill a critical market segment. Unless
innovative improvements are made and manufacturing costs are
drastically reduced, this market will be reserved for high-level
industrial applications only.
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 book@cadex.com,
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
the world.
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
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