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

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Negatives of the SMBus — Like any good invention, the SMBus battery has some serious downsides that must be addressed. For starters, the ‘smart’ battery 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 procedure 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 in correcting 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. This ensures that the tracking error is less than one percent per cycle. However, 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 are 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 SMBus. Unlike other tightly regulated standards, such as the long play record introduced in the late 1950s, the audiocassette in the 1960s, the VCR in the 1970s, ISDN and GSM in the 1980s and the USB in the 1990s, some variations are permitted in the SMBus protocol. These are: adding a check bid to halt the service if the circuit crashes, counting the number of discharges to advise on calibration and disallowing a charge if a certain fault condition has occurred. Unfortunately, these variations cause problems with some existing chargers. As a result, a given SMBus battery should be checked for compatibility with the designated charger before use to assure reliable service. Ironically, the more features that are added to the SMBus charger and battery, the higher the likelihood of incompatibilities.

‘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.

The forecast in consumer acceptance of the ‘smart’ battery has been too optimistic. In the early 1990s when the SMBus battery was conceived, price many not have been as critical an issue as it is now. Then, the design engineer would include many wonderful options. Today, we look for scaled down products that are economically priced and perform the function intended. When looking at the wireless communications market, adding high-level intelligence to the battery is simply too expensive for most consumers. In the competitive mobile phone market, for example, the features offered by the SMBus would be considered overkill.

SMBus battery technology is mainly used by higher-level industrial applications and battery manufacturers are constantly searching for avenues to achieve a wider utilization of the ‘smart’ battery. According to a survey in Japan, about 30 percent of all mobile computing devices are equipped with a ‘smart’ battery.

Improvements in the ‘smart’ battery system, such as better compatibilities, improved error-checking functions and higher accuracies 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.

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