Inner workings of a smart battery
A speaker at a battery conference said, “The battery is a wild animal and artificial intelligence domesticates it.” Domesticating requires knowing the temperament of a battery, because an ordinary or “dumb” battery has an uncommunicative manner.
Weight, color and size do not reveal its state-of-charge (SoC) and state-of-health (SoH). The user is at the mercy of the battery, and simply charging a battery does not guarantee the expected runtime.
Most batteries for laptops and similar devices are “smart,” meaning that some communication occurs between the battery, the equipment and the user. The definition of “smart” varies among manufacturers and regulatory authorities. Some call their batteries smart by simply adding a chip that sets the charger to the correct charge algorithm. The Smart Battery System (SBS) forum states that a smart battery must provide state-of-charge (SoC) indications.
An increasing number of rechargeable batteries are made smart. Smart means that the battery pack includes some level of intelligence. Equipped with a microchip, these batteries talk to the charger and inform the user of its status, such as indicating state-of-charge.
Most smart batteries work on the principle of coulomb counting, a theory that goes back 250 years when Charles-Augustin de Coulomb first established the “Coulomb Rule.”
There are several types of smart batteries, each offering different complexities and cost variants. The most basic smart battery may contain nothing more than a chip that sets the charger to the correct charge algorithm.
In the eyes of the Smart Battery System (SBS) forum, these batteries cannot be called smart. What then makes a battery intelligent?
Definitions vary among organizations and manufacturers. The SBS forum states that a smart battery must provide SoC indications, and in 1990, Benchmarq was the first company to offer fuel-gauge technology.
Today, several manufacturers offer integrated circuit (IC) chips in single-wire and two-wire systems, also known as System Management Bus (SMBus).
The single-wire system delivers communications through one wire. A closer look reveals, however, that the battery still uses three wires. They consist of the data line that also provides the clock information, and the positive and negative battery terminals.
For safety reasons, most battery manufacturers also run a separate wire for temperature sensing. Figure 2 shows the layout of a single-wire system.
The single-wire system stores the battery code and tracks battery readings that typically include voltage, current, temperature and state-of-charge information. Because of the relatively low hardware cost, the single-wire system is used for less complex and more price-sensitive products such as two-way radios, cameras and portable computing devices.
Most single-wire systems do not use a common form factor and this makes standardized state-of-health measurements impossible. Deviating from a set standard poses a further problem with attempting to charge diverse batteries with a universal charger.
The Benchmarq single-wire solution, for example, cannot measure the current directly; this information must be extracted from a change in capacity over time. In addition, the single-wire bus only allows battery SoH measurement when “marrying” the host to a designated battery pack, and this requires a designated battery. Any deviation from the original battery will make the system unreliable or incompatible.
System Management Bus
The System Management Bus (SMBus) represents a concerted effort from the electronics industry to standardize on one communications protocol and one set of data. The Duracell/Intel smart battery system in use today was standardized in 1993 and consists of two separate lines for data and clock. Figure 3 shows the layout of the two-wire SMBus system.
An SMBus battery contains permanent and temporary data. The manufacturer programs the permanent data into the battery, which includes battery ID, battery type, manufacturer’s name, serial number and date of manufacture. The temporary data is being added during use and consists of cycle count, user pattern and maintenance requirements. Some of the information is kept for record, while other data is being renewed throughout the life of the battery.
The SMBus is divided into Level 1, 2 and 3. Level 1 has been discontinued because it does not provide chemistry-independent charging. It supported only one chemistry.
Level 2 works with in-circuit charging, and a laptop servicing the battery is a typical example. Another application is a battery containing the charging circuit within the pack. Battery and support circuit in Level 2 are married to each other. Level 3 supports external SMBus chargers.
Most external SMBus chargers are Level 3 and are expensive to manufacture. Some lower-cost chargers have emerged that accommodate SMBus batteries, but they may not be fully SBS compliant. Manufacturers of SMBus batteries do not completely endorse this shortcut because of safety concerns, but pricing dictates the purchase decisions. Applications such as biomedical instruments, data collection devices and survey equipment lean towards Level 3 chargers with full-fledged charge protocols.
The original design philosophy behind the SMBus battery was 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 slave that follows the dictates of the battery.
This is done out of concerns over charger quality and compatibility with new battery chemistries in applying the correct charge and managing full-charge detection. Such a system makes charging fully transparent to the user, regardless of what chemistry is used.
Offering a charger in which the command is embedded in the battery makes sense because the universal charger can charge all compatible batteries. Algorithms of future battery chemistries convert the charger to the correct settings and the charger will never be obsolete. During the 1990s, several SMBus battery packs emerged, including the 35 and 202 (Figure 4). Manufactured by Sony, Hitachi, GP Batteries and others, these batteries work (or should work) in all portable equipment designed for this system.
The idea was good but the desired standardization did not take hold and most manufacturers went their own way by offering proprietary packs. The reasons are to optimize the form factor and to ensure performance and safety, which can only be guaranteed with the manufacturers’ own battery brands. This makes good sense, but the leading motive behind this may be pricing policies.
In the absence of competition, the batteries can be sold at a premium price. To assure sole ownership, many manufacturers protect the battery with a code that is difficult to break.
Twenty years after introducing the smart battery, the battery industry has still not solved key battery problems and this keeps haunting the users. I asked a hospital technician in the USA about the use of smart batteries and he provided me with his frank opinion. Let’s examine why the smart battery does not fulfill all the promises made in the 1990s.
There is a notion that a battery indicating 100 percent SoC is good. This is not always the case because the user has no knowledge of the capacity level. The readout can be deceiving because the actual r
untime is a product of capacity and SoC. Technicians also fret over the lack of standardization between manufactures, and there is little compatibility among packs.
Other issues with SMBus batteries are logic problems, memory errors and glitches on low-voltage recovery. Custom-designed systems are said to be the most reliable.
Compliance among SMBus batteries and chargers is not improving. Unlike other tightly regulated standard formats, such as the long-play record introduced in the late 1950s, the audiocassette of the 1960s, the VCR of the 1970s, ISDN and GSM of the 1980s, or USB and MP3 in the 1990s, the SMBus protocol permits variations that include adding check bids to halt service if the circuit crashes, counting the number of discharges to advise on calibration and disallowing a charge if a certain fault condition occurs. While these additions are good by themselves, they cause compatibility problems with some chargers.
Ironically, the more features that are added to the SMBus battery and charger, the higher the likelihood of incompatibilities. Before implementing a system, SMBus batteries and chargers should be checked for proper function. The need to approve the marriage between battery and charger is unfortunate, given the assurance that SMBus technology would simplify life and not make it more complex.
Can this be the reason why the smart battery has not received the acceptance battery manufacturers had hoped for? When the SMBus battery was conceived in the early 1990s, cost was not as critical as it is now. Today, customers want products that are economically priced. Adding high-level intelligence to the battery may simply be too expensive for the purpose it serves. Some engineers go so far as to say that the SMBus battery is a “misguided principle.”
An SMBus battery costs about 25% more than the “dumb” equivalent, and this is also reflected in the charger. Instead of simplification, a full-fledged Level 3 charger must work as a hybrid by providing full charging function when charging “dumb” batteries and becoming a slave to obey the dictates of the battery on an SMBus-controlled charge. A large part of the cost is making the two systems compatible, and progress is being made in standardizing.
Electrical compliance is not the only issue. Battery shapes are variable and nowhere is this more visible than with laptop packs. Large-scale batteries used in electric powertrains may one day establish new standards, but for now everyone works on their own.
Simple Guidelines for Using Smart Batteries
* Calibrate a smart battery by applying a full discharge and charge every three months or after every 40 partial cycles.
* A 100 percent fuel gauge does not assure a good battery, nor does an inaccurate fuel gauge tell for certain that the battery is bad.
* Not all chargers are compatible with a smart battery, nor can all batteries be serviced on a given charger. Replace the battery with the same brand, or use an equivalent that is fully compatible. Always test the battery and the charger before use.
* Exercise caution when using a smart battery that does not indicate state-of-charge correctly. This battery may be faulty or not fully compatible with the equipment.