Smart Battery System, SBS

- details of the Smart Battery System technology used to enable battery management systems to operate satisfactorily.

The Smart Battery System, SBS is a specification used within battery management areas. The SBS is used for determining accurate battery capacity readings and it is widely used in portable computers.

The SBS enables operating systems to perform power management operations based on remaining estimated run times.

Using the Smart Battery System, SBS, the power consuming system also controls the amount the battery is charged.

Communication is carried over an SMBus two-wire communication bus. The specification originated with the Duracell and Intel companies in 1994, but was later adapted by several battery and semiconductor makers.

Smart battery system basics

The Smart Battery System was developed by the Smart Battery System Implementers Forum, SBS-IF. This Smart Battery System specification is now managed by the System Management Interface Forum (SMIF), Inc., a non-profit industry group incorporated in Texas, USA.

The SBS itself was initiated by a number of battery companies including Duracell, Energizer, Toshiba and Varta as well as semiconductor manufacturers including Intel, Benchmarq, Linear Technology, Maxim, Mitsubishi and National Semiconductor.

The Smart Battery System specification is generic, being independent of battery chemistry, voltage and packaging.

Typically the SBS will provide a communications link from the battery to items including the equipment being powered and also the charger.

The specification contains four sections:

  • System Management Bus, SMBus   The SMBus is an established bus that is used within the overall Smart Battery System specification. Using two wires and based upon the I2C protocol it enables communication between the different elements of the overall battery system. The two lines are the data and clock. A further line on what is often termed the T-pin is often added for safety indications in case of a failure of the data and clock lines.

    Within the overall system one element within the communications element is nominated as the master, all others are given slave status. In this way different elements of the overall electronic system can interrogate each other to gain information about battery status, etc. before performing various actions. Also other actions can be initiated when the battery flags a certain status.

    There are essentially two forms of signalling mechanism that are used within the system

    • The first uses the SMBus CLOCK and DATA lines. It is the primary communication channel between the Smart Battery and other SMBus devices. The Smart Battery provides data when requested, send charging information to the Smart Battery Charger, and broadcast critical alarm information when parameters exceed predetermined limits within the particular Smart Battery.
    • The other required communication interface is the secondary signalling mechanism or 'Safety Signal' on a separate on known as the 'T-pin' on a Smart Battery pack connector. This is a variable resistance output from the Smart Battery which indicates when charging is permitted. It is meant as an alternate signalling method should the SMBus become inoperable. It is primarily used by the Smart Battery Charger to confirm correct charging. Often a thermistor is used to ensure that the battery is operating within safe limits. If the temperature rises the thermistor will provide a simple indication of a malfunction. Any malfunction of this connection which indicates a variable resistance will easily show an out of range reading - typically a high resistance of the connection is not made properly and this can be detected and any charging terminated.
    Some so-called smart batteries use a single wire system - these are not supported by the SBS Smart Battery System. However it is possible to effect communications over a single wire and this is adequate for many applications. The battery therefore uses three wires: a common positive and negative battery terminals and one single data terminal. This data link also provides the path for the clock information. For safety reasons, most battery manufacturers run a separate wire for temperature sensing.
  • Smart Battery Data, SBD specification   This element of the overall Smart Battery System defines the data that flows across the SMBus. The smart battery data specification provides definitions of not only the software, but also error detection protocols, signalling and the data protocols. It also includes specific items about battery signalling including voltage, current, temperature, device type and various stored values. The stored values are of importance because they can be used to store the latest figures on level of charge and also the cell capacities as these charge over the life of the cells and the overall battery.
  • Smart Battery Charger, SBC specification   Chargers vary considerably in their capabilities. Accordingly the Smart Battery System specification details various levels for chargers:

    • Level 1  : This is the basic level of charger than can be used within a smart battery system. It can only interpret critical warnings and turn off, for example, when a major failing is detected.

      While level 1 SBS chargers can interpret Smart Battery end-of-charge messages minimizing over-charge, they do not provide fully chemistry independent operation. As a result of this, level 1 chargers were removed from the specification.
    • Level 2  : This level of battery charger has a greater level of capability. It can adjust its output according to accommodate the charging algorithm. This algorithm is stored within the smart battery pack.

      More specifically the Level 2 Smart Battery Charger operates as a slave device and responds to the Charging Voltage and Charging Current messages sent to it by a Smart Battery by adjusting its charging characteristics.

      For a Level 2 Charger, the Smart Battery is responsible for initiating the communication. It also provides the charging algorithm to the charger as battery is optimally placed to know how it should be charged. The smart battery is able to store the algorithm that it can communicate with the charger. As a result the Level 2 Smart Battery Charger is independent of the battery chemistry as it is able to change its charging profile to meet the needs of the algorithm provided by the battery.
    • Level 3  : This type of battery charger is the most sophisticated type outlined in the Smart Battery System specification.

      A Level 3 Charger not only interprets critical warning messages from the Smart Battery, but it is also able to act as an SMBus master device. For this level, the Smart Battery is not necessarily responsible for initiating the communication of statistics including Charging-Current and Charging-Voltage or for providing the charging algorithm to the charger.

      The Level 3 Smart Battery Charger may act as a slave or it may poll the Smart Battery to determine the required charging voltage and current and then dynamically adjust its output.

      In view of its mode of operation, a Level 3 charger is chemistry independent because the Smart Battery informs the charger the details of charging.

      However, a Level 3 Smart Battery Charger is able to implement an alternative specialized charging algorithm. To achieve this it may obtain relevant data from the Smart Battery. This may include data such as the time remaining to full charge, battery temperature or other data used to control proper charging or discharge conditioning.
    The communications between the Smart Battery and the Smart Battery Charger can be split into two main areas:

    • Basic Communicating between the Smart Battery and Smart Battery Charger:   There are many elements of data that can be passed between the smart battery and its charger:

      • Instructions to enable the Smart Battery Charger to set the appropriate charge current and voltage.
      • Pass data to the charger regarding the charge algorithm for the Smart Battery.
      • Data to enable the battery to be charged as fast and safely as possible.
      • Data transfer enables new battery technologies to be installed in an equipment without the need for a completely new set of chargers, etc.
    • Critical Communicating between the Smart Battery and Smart Battery Charger:   It is obviously essential that any Smart Battery Charger is able to respond accurately and swiftly to any critical warning messages:

      • Over-charge
      • End of normal charge
      • Over temperature
      • Charging restart after alarm conditions return to normal
      • End of discharge cycle
  • SMBus/BIOS interface specification   This defines how the operating system and applications that run the powered equipment can communicate with SMBus components, through the BIOS operating layer.

The Smart Battery System, SBS specification was originally aimed at applications within portable computers where battery life and indications are of great importance. However it can also be applied to many other areas as well.

By Ian Poole

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