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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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