Batteries in a Portable World 2^nd Ed.
       A Handbook on Rechargeable Batteries for Non-Engineers

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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 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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