SM Bus in Smart Battery

Smart battery, also known as Smart Battery System (SBS-Smart Battery System), is a branch and important part of modern power technology. A smart battery system consists of the following parts: system management bus (SMBus), smart battery charger and smart battery. SMBus is the abbreviation of System Magement Bus, which is translated as system management bus. It uses SMBus smart batteries. Its core concept is to transfer the charging control core from the charger to the battery, and the battery controls the charging process of the charger. This allows us to use only one universal charger to adopt a matching strategy for current and even future battery products to charge correctly.

This is only one aspect. Compared with traditional products, smart battery systems also have the following characteristics:

  • High precision

The high-precision fuel gauge in the smart battery pack is different from the common voltage estimation method. Instead, it uses an advanced algorithm based on the electrochemical impedance of the battery. Even if the ambient temperature of the battery changes and the life of the battery ages, it can always accurately monitor the battery, even when the battery is not in the system.

  • Intelligence

In the intelligent battery system, the host can obtain rich information about the battery, and the user can more accurately grasp the status of the battery (including the effective battery capacity, real-time voltage and current, historical cumulative charge and discharge times, remaining discharge time, predicted life, etc.), so as to achieve safer, more efficient and accurate battery management.In addition, it can also provide its production information, tell the system its model, manufacturer, etc.

  • Security

Smart batteries have multiple redundant safety protections, which can realize battery safety protection even under the failure conditions of some devices. At the same time, the battery will permanently record battery failure information to facilitate the analysis of undesirable causes.

  • Standardization

SBS has formed an international standard. The host designed with SBS architecture can minimize the non-repetitive engineering costs of hardware and software, and easily realize product upgrade iteration.

In fact, smart batteries have been widely used in many fields. Almost all notebook computers, as well as precision instruments, such as medical, precision measurement and aerospace technology, many devices use smart batteries to accurately control charge and discharge.

The interface of a standard smart battery is usually a 5PIN interface composed of +/C/D/T/-, where +/-is the positive and negative electrodes of the battery, which is connected to the load power supply port, and the remaining C and D are the clock lines and data lines for SMBus communication of the battery, which directly interact with the host microcontroller. Data, the T pin is the external temperature interface of the battery or the identification pin of the battery.Some specific types of tape recorders or power distributors can read the precise data of the battery in real time, and accordingly more accurately estimate the time when the equipment can continue to be used, so as to avoid accidental power outages.

What is SM Bus?

The system management bus (SMBus), defined by Intel in 1995, is a subset of I2C and defines stricter usage. One purpose of SMBus is to promote robustness and interoperability. Therefore, modern I2C systems incorporate some policies and rules from SMBus, and sometimes support both I2C and SMBus, and only need to use minimal re-configuration through commands or output pins.

As we all know, SMBus can be used in applications that require critical monitoring of parameters.Its most common applications are computer motherboards and embedded systems. It has additional monitoring specifications for temperature, power supply voltage, fan monitoring and/or control integrated chips.

SMBus is a 2-wire bus, similar to the I2C bus developed by Philips in the 1980s.The two main signals are clock (SMBCLK) and data (SMBDAT).I2C Primer and SMBus are compatible with each other, but there are obvious differences, such as:

  • The SMBus logic level threshold is fixed and is not proportional to the power supply voltage of the device.Therefore, devices with different supply voltages can run on the same primer.For example, an SMBus may have multiple devices powered by 1.8 V, 3.3V, and 5 V power supplies.
  • -They all run at the same speed of up to 100kHz, but I2C Primer is available in 400kHz and 2 MHz versions.
  • -SMBus specifies the minimum clock speed and limits the amount of clock that can be extended in a transaction.Violation of the timeout limit will cause all SMBus devices to reset their I/O logic to allow the bus to restart.This design enhances the robustness of the bus.
  • -The timeouts of the two are also different.I2C Primer does not have a timeout, while SMBus has a timeout-for the minimum clock speed of 10kHz, a timeout of 35 ms can be considered.
  • -Packet error checking (PEC) was originally defined for SMBus.Add a grouping error code byte at the end of each transaction.
  • -Some of the remaining differences involve transmission type, alarm line, pause line, shutdown or power-up.

Every time an SMBus device receives its own address, no matter what it is doing, it must answer (ACK). This is a clear requirement to ensure that the master device can accurately determine which devices on the bus are active.All SMBus transactions are executed through one of the specified SMBus protocols.SMBus also has an optional signal SMBALERT#, which can be used by the node device to quickly notify the master device or the system host. It has the information required by the master device, such as reporting a fault condition.

What is the relationship between I2C Primer, SMBus and PMBus?

SMBus was originally developed to assist in the battery management system, using I2C hardware, but a second level of software was added, which eventually allowed the device to be hot-swappable without restarting the system.PMBus extends SMBus and defines a set of device commands dedicated to managing power converters, exposing the measured voltage, current, temperature and other properties of the device.Generally speaking, I2C Primer, SMBus, and PMBus devices can share the bus without any major problems.

Advantages of I2C, SMB, and PMB

  • Use only two lines
  • -With ACK⁄NACK bit
  • -Well-known agreement
  • -Support multiple master devices and multiple nodes
  • -The hardware is not as complicated as UART
  • -Widely used method


  • Data transmission rate is slower than SPI
  • -The size of the data frame is limited to 8 bits
  • -The hardware required for implementation is more complicated than SPI