BU-601: How does a Smart Battery Work?

Learn about the different bus systems and where the limitations lay.

A speaker at a battery conference once said, “The battery is a wild animal and artificial intelligence domesticates it.” A battery does not exhibit visible changes as part of usage; it looks the same when fully charged or empty, new or old and in need of replacement. A car tire, in comparison, distorts when low on air and indicates end-of-life when the treads are worn.

Battery users imagine a battery pack as being an energy storage device that resembles a fuel tank dispensing liquid fuel. For simplicity reasons, a battery can be seen as such; however, measuring stored energy from an electrochemical device is far more complex and cannot be done well.

While an ordinary fuel gauge measures in-and-out-flowing liquid from a tank of known size, a battery fuel gauge has unconfirmed definitions and only reveals the open circuit voltage (OCV), a reflection of state-of-charge (SoC). To compound the difficulty, a battery is a shrinking vessel that takes on less energy with each charge and with age. This results in the specified Ah rating no longer holding true. Nor can the fuel gauge by itself assess the capacity; the reading always shows full after recharge even if the capacity has dropped to half the specified Ah.

The most simplistic method to measure state-of-charge is reading voltage, but this can be inaccurate. Load currents pull the voltage down during discharge, but the largest challenge is the flat discharge voltage curve on most lithium and nickel-based batteries. Temperature also plays a role; heat raises the voltage and a cold ambient lowers it. Agitation by a previous charge or discharge causes further errors and the battery needs a few hours rest to neutralize.

Most batteries for medical, military and computing devices are “smart.” This means that some level of communication occurs between the battery, the equipment and the user. The definitions of “smart” vary among manufacturers and regulatory authorities and 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. The SBS forum states that a smart battery must provide state-of-charge (SoC) indications.

Safety is a key design objective and the concept behind the SBS specifications is to place system intelligence inside the battery pack. The SBS battery thus communicates with the charge management IC in a closed loop to only service authorized batteries. In spite of this digital supervision, most SMS still rely on analog signals from the chemical battery and separate temperature sensing to override system errors or outside interference.

Benchmarq was the first company to offer fuel-gauge technology in 1990 and today, many manufacturers offer integrated circuit (IC) chips in single-wire and two-wire systems, also known as System Management Bus (SMBus).

State-of-charge typically includes coulomb counting, a theory that goes back 250 years when Charles-Augustin de Coulomb first established the “Coulomb Rule.” The principle of coulomb counting is illustrated in Figure 1 on hand of a fuel gauge that measures in-and-out flowing energies, the stored energy that represents state-of-charge.

Principle of and fuel gauge based on coulomb counting


Figure 1: Principle of and fuel gauge based on coulomb counting

A circuit measures the in-and-out flowing energy; the stored energy represents state-of-charge. 1A charge passes 1 coulomb of charge or discharge per second.

Courtesy of Cadex

Coulomb counting should be flawless but nothing is. If, for example, a battery was charged for one hour at one ampere, the same amount of energy should be available on discharge, and this is not the case. Inefficiencies in charge acceptance, especially towards the end of charge, as well as losses during discharge and storage reduce the total energy delivered and skew the readings. The available energy is always less than what had been fed into the battery.

Single-wire Bus

The single-wire system, also known as 1-Wire, communicates through one wire at low speed. A closer look reveals that the battery still uses three wires. Designed by Dallas Semiconductor Crop., the 1-Wire consists of the data line that also provides the clock information, 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.

Single-wire system of a “smart” battery



Figure 2: Single-wire system of a “smart” battery

A single wire provides data communication. For safety reasons, most batteries also feature a separate wire for temperature sensing.

Courtesy of Cadex

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 price-sensitive devices such as measuring instruments, mobile phones, two-way radios, cameras and scanners.

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 when trying 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 selected battery, and this requires a designated pack. 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. Derived from I2C, the Duracell/Intel smart battery system was standardized in 1995 and consists of two separate lines for data and clock. I2C (Inter-Integrated Circuit) is a multi-master, multi-slave, single-ended, serial computer bus invented by Philips Semiconductor. Figure 3 shows the layout of the two-wire SMBus system.

Two-wire SMBus system



Figure 3: Two-wire SMBus system

The SMBus works on a two-wire system using a standardized communications protocol. This system lends itself to standardized state-of-charge and state-of-health measurements.

Courtesy of Cadex

The 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. Offering a charger in which the command is embedded in the battery makes sense as the battery can set the correct charge algorithm for future chemistries.

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 solid but the desired standardization did not take hold as most manufacturers built their own proprietary packs. To prevent unauthorized batteries from infiltrating the market, some manufacturers further add a code that will only enable their own packs to work with their devices. The universal SMBus packs did not materialize.

35 and 202 series batteries featuring SMBus

Figure 4: 35 and 202 series batteries featuring SMBus

Available in nickel- and lithium-based chemistries, these batteries power laptops, biomedical instruments and survey equipment. Non-SMBus (dumb) versions with same footprint are also available.

Courtesy of Cadex

A SMBus battery contains permanent and temporary data. The battery 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 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 renewed throughout the life of the battery. The resolution for measuring the voltage is typically 1mV; current flow can also be read at a resolution of 0.5mA. Temperature accuracy is about ±3ºC.

Smart battery chargers are divided into Level-1, 2 and 3. Level-1 has been discontinued because it does not provide chemistry-independent charging and it supported a single chemistry only.

A Level-2 charger is completely controlled by the Smart Battery.  The charger acts as a SMBus slave and responds to voltage and current charging messages that are received from the Smart Battery. Level-2 also serves as in-circuit charging commonly seen in a laptop environment. Another use is a battery with a built-in charging circuit. In Level-2, battery and circuit are married to each other.

Depending on the battery to be charged, the Level-3 charger can interpret commands from a Smart Battery, as is done with Level-2, and also act as master. In other words, the Level 3 charger can request charging information from the Smart Battery but can also impose its own charging algorithm. Most industrial smart chargers are the hybrid type Level-3.

Some lower-cost chargers have emerged that accommodate SMBus batteries that may not be fully SBS compliant. Manufacturers of SMBus batteries do not endorse this shortcut because of safety concerns. Applications such as biomedical instruments, data collection devices and survey equipment lean towards Level-3 chargers with full-fledged charge protocols.

System management can also be built on the I2C platform that is installed in the battery. Furthermore, there are hybrid systems in which the charger provides data to a resident EEPROM in the battery. An EEPROM is an electrically erasable programmable read-only memory that is non-volatile.

The electronic protection circuit, which all Li-ion batteries include, is independent of the SMBus. This ensures safe operation if SMBus communication fails. Some SMBus batteries allow reading the individual cell voltages from the protection circuit to verify cell balance.


Provides state-of-charge status

Records battery history such as cycle count, user pattern, maintenance requirements, etc.

Reminds user of periodic service

Protects battery from unauthorized use


Adds 25% to the cost of a battery

Complicates charger; most chargers for intelligent battery are hybrid and also service non-intelligent batteries

Requires periodic calibration

Readout only shows state-of-charge and not actual runtime

Table 6-8: Advantages and limitations of the smart battery. 

Simple Guidelines for Using Smart Batteries

Last Updated 2015-07-14


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On March 22, 2011 at 8:13pm
David wrote:

It is grate to see that there are companies that are willing to bring forward this technology.
Do these batteries sulfate like a standard lead acid battery?
If so can they be de-sulfated?
Does the charging system prevent sulfation?

Are you offering any classes to attend in order to better understand the system?
I would like to attend, thank you. David

On November 15, 2011 at 5:08pm
Robert h Price wrote:

I am dealing with much of same data your site
Is researching, Auto s R 2000 Audi s4
quattro B5 fbw. Also 2002 vw Gti 18t 20 v
Awp motor both are turbo charged, I have had mult.
Issues , concerns , and strange Documented
Occurrences with both. However, no dealership
Will confirm that I am not far off on what I believe may or is
a “Smart” issue. I love VAG cars when they love me, I can be reached via txt at 9044023207 if interested in assist. / possib. Hlping one another.

On November 28, 2011 at 1:45am
Andrew wrote:
On February 26, 2012 at 12:43pm
Kristina Cramer wrote:

I teach a high school video production class with a limited budget.  We have a really hard time understanding how to charge our batteries.  I’ve been reading that the Dyson batteries are the same way in that if you just put them on the charger to insure a full charge when the next user takes it out - it only works for a very short time. I have always thought it was reading the sensor was reading the top of the charge - can we get the camera to go past that and read the rest of the charge that is underneath?

So far I’m gathering that the battery must be fully discharged before we recharge it. If I just took all the batteries and drained them - then recharged them would that fix the problem?  Also I have lithium ion batteries and chargers that indicate if they are full or not - how do I know if the chargers and the batteries are compatiable?

On November 26, 2012 at 5:24pm
Siva Ganesh Malla wrote:

Dear, Sir.

I have a 5Ah lead-acid battery. lets a my load is suddenly increases to 100A, now please tell me how much time it will take to discharge and how much long it can deliver power to load. And also tell me, can battery dicharge to AC loads with in one cycle through Inverter? means, my AC load frequency is 50Hz, if suddenly load power increses, then can my battery give power to load with in one cycle (20 ms.)?. or please tell me how much time it will take to discharge for required load?

On March 11, 2013 at 5:15am
FUOYE wrote:

what is the simplest way and simplest meter to test a weak battery from a parallel configuration

On March 11, 2013 at 6:04am
FUOYE wrote:

The batteries we use in the charging bays of the Federal University Oye , fuoye,is connected in parrallel , but at times they do remove one out of it that is weak. i now want to know a simple way and simple meter to test a faulty or weak battery from the parallel battery configuration. You can send me mail through [email protected]   <a >FUOYE</a>

On March 15, 2013 at 5:31am
FUOYE wrote:

sorry, the charging bay batteries are connected in series, federal university OYE,  www.fuoye.edu.ng

On July 18, 2013 at 12:15pm
adonis ugarte wrote:

investigación sobre la parte inteligente de la bateria.

On February 28, 2014 at 7:31am
Frank wrote:

To test a battery is called a load test.Load test meters at E-Bey

On February 28, 2014 at 7:49am
Frank wrote:

The best battery you can buy is when you bye the cells lose and you build up your own battery pack like 6 2V cells =12V then it is easy to maintain your battery pack you replace just the weakest cells which is easy to check.

On January 10, 2015 at 5:08am
kosoko luke wrote:

Sir its real imformative.

On July 31, 2015 at 10:54pm
patel vikas wrote:

Xolo q1010i mobile bettery new