Chapter 7: The ‘Smart’ Battery
A speaker at a battery seminar remarked that, “The battery is a wild animal and artificial intelligence domesticates it.” An ordinary or ‘dumb’ battery has the inherit problem of not being able to display the amount of reserve energy it holds. Neither weight, color, nor size provides any indication of the battery’s state-of-charge (SoC) and state-of-health (SoH). The user is at the mercy of the battery when pulling a freshly charged battery from the charger.
Text Box: “The battery is a wild animal and artificial intelligence domesticates it.”Help is at hand. An increasing number of today’s rechargeable batteries are made ‘smart’. Equipped with a microchip, these batteries are able to communicate with the charger and user alike to provide statistical information. Typical applications for ‘smart’ batteries are notebook computers and video cameras. Increasingly, these batteries are also used in advanced biomedical devices and defense applications.
There are several types of ‘smart’ batteries, each offering different complexities, performance and cost. The most basic ‘smart’ battery may only contain a chip to identify its chemistry and tell the charger which charge algorithm to apply. Other batteries claim to be smart simply because they provide protection from overcharging, under-discharging and short-circuiting. In the eyes of the Smart Battery System (SBS) forum, these batteries cannot be called ‘smart’.
What then makes a battery ‘smart’? Definitions still vary among organizations and manufacturers. The SBS forum states that a ‘smart’ battery must be able to provide SoC indications. Benchmarq was the first company to commercialize the concept of the battery fuel gauge technology. Early IC chips date back to 1990. Several manufacturers followed suit and produced ‘smart’ chips for batteries.
During the early nineties, numerous ‘smart’ battery architectures with a SoC read-out have emerged. They range from the single wire system, the two-wire system and the system management bus (SMBus). Most two-wire systems are based on the SMBus protocol. This book will address the single wire system and the SMBus.
The Single Wire Bus
The single wire system is the simpler of the two and does all the data communications through one wire. A battery equipped with the single wire system uses only three wires, the positive and negative battery terminals and the data terminal. For safety reasons, most battery manufacturers run a separate wire for temperature sensing. Figure 7-1 shows the layout of a single wire system.
The modern single wire system stores battery-specific data and tracks battery parameters, including temperature, voltage, current and remaining charge. Because of simplicity and relatively low hardware cost, the single wire enjoys a broad market acceptance for high-end mobile phones, two-way radios and camcorders.
Most single wire systems do not have a common form factor; neither do they lend themselves to standardized SoH measurements. This produces problems for a universal charger concept. The Benchmarq single wire solution, for example, cannot measure current directly; it must be extracted from a change in capacity over time.
In addition, the single wire bus allows battery SoH measurement only when the host is ‘married’ to a designated battery pack. Such a fixed host-battery relationship is feasible with notebook computers, mobile phones or video cameras, provided the appropriate OEM battery is used. Any discrepancy in the battery type from the original will make the system unreliable or will provide false readings. [7.1]
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.
Architecture — 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.
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 application of Level 2 is a battery that contains the charging circuit within the pack. Level 3 is reserved for full-featured external chargers.
Most external SMBus chargers are based on Level 3. Unfortunately, this level is complex and the chargers are costly to manufacture. 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 buy these economy chargers because of the lower price. [7.2]
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 specific battery and charger types is unfortunate given that the ‘smart’ battery is intended to be universal.
Among the most popular SMBus batteries for portable computers are the 35 and 202 form-factors. Manufactured by Sony, Hitachi, GP Batteries, Moltech (formerly Energizer), Moli Energy and many others, this battery works (should work) in all portable equipment designed for this system.
Figure 7-3 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, provided the common five-prong knife connector is used. [7.3]
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.
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 7-4 shows such a fuel gauge. [7.4]
While SoC information displayed on a battery or computer screen is helpful, the fuel gauge resets to 100 percent each time the battery is recharged, regardless of the battery’s SoH. A serious miscount occurs if an aged battery shows 100 percent after a full-charge, when in fact the charge acceptance has dropped to 50 percent or less. The question remains: “100 percent of what?” A user unfamiliar with this battery has little information about the runtime of the pack.
The Tri-State Fuel Gauge
The SoC information alone is incomplete without knowing the battery’s SoH. To fully evaluate the present state of a battery, three levels of information are needed. They are: SoC, SoH and the empty portion of the battery that can be replenished with a charge. (The empty portion is derived by deducting the SoC from the SoH.)
How can the three levels of a battery be measured and made visible to the user? While the SoC is relatively simple to produce, as discussed above, 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.
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 7-5 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. [7.5]
The most practical setting to place the tri-state-fuel gauge is on a charger. Only one display would be needed for a multi-bay charging unit. To view the readings of a battery, the user would simply press a button. The SoC and SoH information would be displayed within five seconds after inserting the battery into the charger bay. During charge, the gauge would reveal the charge level of each battery. This information would be handy when a functional battery is needed in a hurry. Cadex offers a series of SMBus chargers that feature the tri-state fuel gauge as an option.
The Target Capacity Selector
For users that simply need a go/no go answer and do not want to bother about other battery information, 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.
If a battery falls below target, the charger triggers the condition light. The user is prompted to press the condition button to cycle the battery. Condition consists of charge/discharge/charge and performs calibration and conditioning functions. If the battery does not recover after the conditioning service, the fail light illuminates, indicating that the battery should be replaced. A green ready light at the completion of the program assures that the battery meets the required performance level.
An SMBus charger with the above described features acts as charger, conditioner and quality control system. Figure 7‑6 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.
Some SMBus chargers can be fully automated to apply a conditioning cycle whenever the battery falls below the target setting. An override button cancels the discharge if a fast-charge is needed instead. Such a system maximizes the life of fleet batteries and assures that deadwood is identified and removed. [7.6]
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.
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.
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.
Battery SoH readings are only available with Level 3 SMBus chargers servicing valid SMBus batteries. ‘Dumb’ batteries cannot provide SoH readings, even if they share the identical footprint of the ‘smart’ battery and can use the same charger.
Fuel Gauges for Large Batteries
Most ‘smart’ battery applications today are limited to portable electronic equipment, and the electronic circuit is built right into the battery pack. In applications where larger ‘smart’ batteries are needed, such as electric wheelchairs, scooters, robots and forklifts, the electronic circuit may be placed in a box external to the battery.
The main benefit of adding intelligence to the battery is to enable the measurement of SoH and reserve energy. Most measuring devices used are based on voltage, which is known to be highly inaccurate.
Cadex is extending the ‘smart’ battery technology to wheeled applications. Called the Cadex Fuelcheck™, the device is based on the tri-state fuel gauge described earlier in this section. The system consists of a controller, current measuring device and a display unit. The Cadex Fuelcheck™ device can be added to new equipment when it is manufactured and can be installed in existing equipment as a retrofit. The installation is permanent and each wheeled appliance requires one system.
A tri-state linear bar graph consisting of colored LED’s would be the preferred display for a personal user, such as a wheelchair operator. By replacing the light display with a digital readout, additional information can be shown. For example, a display could indicate the remaining runtime based on the average power consumption logged. Corrections would be applied if the load factor changes during the course of the day. Figure 7‑7 illustrates a digital fuel gauge using the LCD panel.
To initialize the Cadex Fuelcheck™ system, a one-time setup procedure is required. A PC will prompt the technician to enter information such as battery chemistry, desired end-of-discharge voltage, Ah rating of the battery, and designated service flags. The system is then calibrated by applying a full charge, followed by a full discharge. The initial discharge assesses the battery, or learns the performance. The SoH readings will be known after the first full discharge. [7.7]
The simplest way to discharge the battery as part of calibration is to run it down through normal use in the equipment. The most accurate readings are obtained by using a load bank. Such a device may not be always available.
With a data logging option built into the system, the PC allows downloading service data that have been collected during use. Such information would benefit large equipment fleets and rental places that need to check the performance of the batteries on demand.
Similar to other ‘smart’ battery systems, calibration will be needed once every 6 to 12 months. More frequent calibrations may be required if the equipment is used for short durations between each recharge. The most practical way to calibrate the system is by occasionally allowing the battery to run down through continued use. Information relating to the date of the last calibration, or an early call for calibration if certain conditions occur, can be calculated and displayed.
Knowing the SoH of a battery at any time and scheduling timely service or replacement is a major benefit for industrial battery users. Such a system would be especially helpful for organizations in which different individuals use the equipment and no one is given maintenance responsibilities. Equipment rental places fall into this category.