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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
through which simple power-related chips can communicate with
the rest of the system. One wire handles the data; the second
is the clock. It uses I²C as its backbone. Defined by Philips,
the I²C is a synchronous multi-drop bi-directional communications
system, which operates at a speed of up to 100 kilohertz (kHz).
The Duracell/Intel SBS, in use today, was standardized
in 1993. In previous years, computer manufacturers had developed
their own proprietary ‘smart’ batteries. With the new SBS
specification, a broader interface standard was made possible.
This reduces the hurdles of interfering with patents and intellectual
properties.
In spite of an 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 early SMBus batteries had problems of poor
accuracy. Electronic circuits did not provide the necessary
resolution; neither was real time reporting of current, voltage
and temperature adequate. On some batteries, the specified
accuracy could only be achieved if the battery was new, operated
at room temperature and was discharged at a steady rate of
1C. Operating in adverse temperatures or discharging at uneven
loads reduced the accuracy dramatically. Most loads for portable
equipment are uneven and fluctuate with power demand. There
are power surges on a laptop at start up and refresh, high
inrush currents on biomedical equipment during certain procedures
and sharp pulse bursts on digital communications devices on
transmit.
In the absence of a reliable reporting system
on the older generation of ‘smart’ batteries, capacity estimation
was inaccurate. This resulted in powering down the equipment
before the battery was fully depleted, leaving precious energy
behind. Most batteries introduced in the late 1990s have resolved
some or all of these deficiencies. Further improvements will
be necessary.
Design— The design philosophy 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 done
out of concerns over charger quality, compatibility with new
and old battery chemistries, administration of the correct
amount of charge currents and accurate full-charge detection.
Simplifying the charging for the user is an issue that is
important 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.
The analogy of charging a ‘smart’ and ‘dumb’
battery can be made with the eating habits of an adult and
a baby. Charging a ‘smart’ battery resembles the eating choices
of a responsible adult who knows best what food to select
how much to take. The baby, in on the other hand, has limited
communications skills in expressing the type and amount of
food desired. Putting this analogy in parallel with charging
batteries, the charger servicing ‘dumb’ batteries can only
observe the approximate SoC level and avoid overcharge conditions.
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