BU-601: How does a Smart Battery Work?

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

A speaker at a battery conference once said, “The battery is a wild animal and artificial intelligence domesticates it.” A battery is illusive and 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.

While an ordinary fuel gauge measures in-and-out-flowing liquid from a tank of a known size with minimal losses, a battery fuel gauge has unconfirmed definitions and only reveals the open circuit voltage (OCV), a fickle reflection of state-of-charge (SoC). To compound the complexity, a battery is a leaky and shrinking vessel that takes on less energy with each charge. As the capacity fades, the specified Ah (ampere-hours) rating no longer holds 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. (See BU-903: How to Measure State-charge.)

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 indications.

Safety is a key design objective and the concept behind SBS is to place system intelligence inside the battery pack. The SBS battery thus communicates with the charge management chip in a closed loop. In spite of this digital supervision, most SBS chargers also rely on analog signals from the chemical battery to terminate the charge when the battery is full. Furthermore, redundant temperature sensing is added for safety reasons.

Benchmarq was the first company to offer fuel-gauge technology in 1990. 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 estimations in a smart battery commonly includes coulomb counting, a theory that goes back 250 years when Charles-Augustin de Coulomb first established the “Coulomb Rule.” Figure 1 illustrates the principle of coulomb counting measuring in-and-out flowing energy. One coulomb (C) equals one ampere flowing for one second (1A x 1s = 1C). Discharging a battery at 1A for one hour equates to 3,600C. (Not to be confused with C-rate.)

Principle of and fuel gauge based on coulomb counting

Figure 1: Principle of a 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 it is not perfect. 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. Designed by Dallas Semiconductor Crop., the 1-Wire combines data and clock into one line for transmission and the Manchester code separates the data at the other end. For safety reasons, most batteries 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 attractive for price-sensitive devices such as measuring instruments, mobile phones, two-way radios, cameras and scanners.

Most single-wire systems have their own protocol and use a customized charger. The Benchmarq single-wire solution, for example, cannot measure the current directly; state-of-health (SoH) measurement is only possible when “marrying” the host to a designated battery.

System Management Bus

The System Management Bus (SMBus) represents a concerted effort to agree 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 the slave that obeys the command of the battery. This enables a universal charger to service present and future battery chemistries without concern of correct settings.

During the 1990s, several standardized SMBus battery packs emerged, including the 35 and 202 (Figure 4). Manufactured by Sony, Hitachi, GP Batteries and others, these interchangeable batteries were designed to power a broad range of portable devices, such as laptops and medical instruments. The idea was solid but standardization did not take hold as most manufacturers began building their own packs.

To prevent unauthorized batteries from infiltrating the market, some manufacturers add a code to exclude other pack vendors. Some manufacturers go as far as to invalidate the battery when a given cycle count is reached. To avoid surprises, most of these systems inform the user of the pending end-of-life.

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 the 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, while other data is renewed throughout the life of the battery. The voltage is typically measured 1mV; the resolution of current is 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 fully controlled by the Smart Battery and acts as a SMBus slave, responding to voltage and current commands from the Smart Battery. Level 2 also serves as in-circuit charging, a practice that is common in laptops. 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 a 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 based on 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. Table 5 lists the advantages and limitations of the smart battery.


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

Requires periodic calibration

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

Table 5: Advantages and limitations of the smart battery. 

Simple Guidelines for Using Smart Batteries

Last Updated 2015-11-26


*** Please Read Regarding Comments ***

Comments are intended for "commenting," an open discussion amongst site visitors. Battery University monitors the comments and understands the importance of expressing perspectives and opinions in a shared forum. However, all communication must be done with the use of appropriate language and the avoidance of spam and discrimination.

If you have a question, require further information, have a suggestion or would like to report an error, use the "contact us" form or email us at: BatteryU@cadex.com. While we make all efforts to answer your questions accurately, we cannot guarantee results. Neither can we take responsibility for any damages or injuries that may result as a consequence of the information provided. Please accept our advice as a free public support rather than an engineering or professional service.

Or Jump To A Different Article

Basics You Should Know
The Battery and You
Batteries as Power Source


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 sola.afolabi@fuoye.edu.ng   <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

On October 6, 2015 at 1:14pm
Tom Lietha wrote:

Can anyone explain how current sense resistors are used in batteries, or in cojunction with batteries in cell phones or other electronic communications devices?