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 (Sony Vaio VGN-FZ 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 (Sony VGP-BPS8 battery).

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 (Sony VGP-BPL9 battery).

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 (Sony VGP-BPL11 battery).

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 (Toshiba PA3535U-1BRS battery).

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 (Toshiba PA2522U-1BRS battery).

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?” (ASUS A3000 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 (ASUS Eee PC 900 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 (ASUS Eee PC 1000HE Battery).

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 (IBM ThinkPad R50 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 (IBM ThinkPad R60 battery).

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 (ACER Travelmate 2300 Battery).

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 (ACER Aspire 3020 Battery).

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 (ACER Aspire 3000 Battery).

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 (ACER Aspire 5020 Battery).

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 (Dell Inspiron 1501 battery).

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 (Dell Inspiron 6400 battery).

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 (Dell Inspiron 6000 battery).

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 (Dell INSPIRON 1525 Battery).

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 (Dell Latitude D620 Battery).

‘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 (Dell Inspiron E1505 Battery).

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 (Dell INSPIRON 1420 Battery).

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.



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