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Battery Communications Interfaces
Many battery applications require communications with other system devices or with external equipment. These are often called Intelligent Batteries. This may simply be a data link used for performance monitoring, for data logging, for diagnostics, or for setting system parameters or it may be a communications channel carrying system control signals. The choice of the communications protocol is determined not by the battery but by the application in which it is to be used.
Apart from the SMBus which is specific to batteries, all the communications interfaces and protocols mentioned below were designed for general purpose, or other applications however they have all been called into service for various uses in particular battery applications.
RS232 connection - A standard for serial transmission of data between two external devices with cable lengths up to 50 feet. Separate transmit and receive lines provide full duplex communication. Used for data rates up to 20,000 bits per second.
EIA-485 (formerly RS485) connection - A standard for serial transmission of data between multiple devices with cable lengths up to 4000 feet. (Possible 32 channels). Normally half duplex, it uses a differential balanced line over twisted pair for noise immunity and does not specify or recommend any particular data protocol. Suitable for data rates up to 100K bits per second or 10M bits per second depending on the length of the cables.
Inter - Integrated Circuit (I2C) Bus
The (I2C) Bus was a low speed bus originally designed for use between internal modules within a system rather than for external communications. It is a bidirectional, half duplex, two wire synchronous bus. It runs with data rates up to 3.4 Mbits/s and is suitable for Master - Slave applications. Multiple slaves are possible but only a master can initiate a data transfer.
Typically used for internal communications within embedded systems such as a BMS.
See practical examples of the above 3 bus systems in the section about Battery Management Systems (BMS)
USB Universal Serial Bus
The USB standard was designed to replace the numerous proprietary serial and parallel data connections between computers and peripheral devices using a star topology. It was designed to be "hot pluggable" as well as "plug and play" with a standard 4 pin connector incorporating 2 pins connected to a twisted pair for carrying a differential data signal, a ground (earth) line and a 5 Volt power rail. The data rate specified in the original version was 1.5 Mbits/sec but this has increased with subsequent versions up to 3 Gigabits/sec. Up to 127 devices including hubs may be connected to the bus. Cable lengths are limited to 5 metres (16 feet).
The host transmits data packets to, or receives data packets from, all the devices connected to it, but each device has a unique address so that only one device can actually receive or transmit data at any one time.
In battery applications, the USB connection is used for monitoring the battery status or setting control limits.
See also USB Chargers.
The industry standard for on-board vehicle communications is the CAN (Controller Area Network) bus. It was originally designed in the 1980s by Bosch in Germany with the purpose of eliminating hundreds of wires from the wiring harness, reducing the size and weight of the wiring loom by multiplexing the signals on to a single shared broadcast data bus. The CAN bus is defined as a two wire, serial communications bus designed for networking intelligent sensors and actuators in a centralised multiplexing system. It is now accepted as an ISO standard.
The CAN Bus allows 12 Volts, or other potential, power to be distributed around the vehicle (or system) on a single power rail with local actuators connecting the power to each application as needed. Vehicle functions such as temperatures , alarms or the positions of switches are monitored by sensors connected to the bus and the controller can direct the actuators to initiate the appropriate response as required.
Two variants are available to suit different transmission rates. A high speed (1 MBaud) bus is used for rapid control devices for operations such as engine management and vehicle stability and motion control. For simple switching and control of functions such as lighting, windows, mirror adjustment and instrument displays a low speed (100 KBaud) bus is used.
The CAN bus was designed to provide secure communications in the very harsh operating environments with high levels of electrical noise which are found in automotive systems.
The communications protocol defines the following:
- Method of addressing the devices connected to the bus
- Data word format (The message)
- Priority setting
- Transmission sequence
- Control signals
- Error detection
- Error handling or correction
- Transmission speed
The message data, the control signals and the error correction are contained in a data frames of equal length which are transmitted sequentially over the transmission channel.
Because each device connected to the bus contains its own intelligence, each station in effect monitors its own output, interrupts disturbed transmissions and acknowledges correct transmissions. Thus faults can be detected and the appropriate actions taken.
Standard CAN microcontrollers are available from a variety of semiconductor manufacturers.
Automotive BMS therefore uses the CAN bus as its main communications channel.
The LIN Bus is another automotive communications standard, initiated in 1998, similar to the CAN Bus.
It is a single wire Local Interconnect Network operating at 20 KBaud using distributed multipexers and standardised Smart Connectors based on standard UART/SCI IC hardware allowing for simple, low cost
IC solutions. It uses more electronics than the CAN Bus but it is more flexible and uses less wiring.
Although the LIN Bus was conceived for automotive applications, its implementation is not confined to such uses. It can for instance be used to control complex household appliances such as dishwashers and washing machines.
The new FlexRay Bus has been developed recently in response to the demands of more complex control applications such as drive by wire, steer by wire, brake by wire and ever more sophisticated engine management systems planned for future automotive use. It provides a fault tolerant 10Mbit/sec data rate on each of two channels enabling both synchronous and asynchronous data transfer. The FlexRay data payload per frame is 20 times greater than the CAN Bus.
Unlike event-triggered systems such as the CAN system, FlexRay is based on a time-triggered architecture known as Time Division Multiple Access (TDMA) where communication is organized into repetitive, predefined time slots. TDMA thus allows high priority signals to be guaranteed synchronous access to a channel in predetermined, cyclic time slots, while low priority signals, which are not needed continuously, are transmitted asynchronously and only gain access to the bus as required when the bus is free.
The FlexRay Bus can therefore support fast responding dynamic control systems rather than just the simpler sensors and actuators permitted with the CAN Bus.
The SMBus (System Management Bus) is a two wire, 100 KHz, serial bus designed for use with low power Smart Battery Systems (SBS) with the limited objectives of interconnecting Smart Batteries which have built in intelligence, with their associated chargers. Sometimes found in simple vehicle applications, it does not have the range of capabilities for controlling devices connected to the power lines which the CAN and LIN buses have. See Intelligent Batteries for details of Smart Battery Systems.