Linear Technology is a leader in power management in our industry (but you know that). What you may not know is that they have developed a wirelesss BMS using a high performance, multi-cell battery stack monitor using the LTC6811 with their SmartMesh wireless technology. This BMS device has a high stability voltage reference along with high voltage multiplexers, and a dual 16 bit delta-sigma ADC. I only write about products when I am excited about one that has some very strong impact on our industry for designers. This is one of those times with this particular product. Read on to see how they have added wireless technology to this system design.
One of these ICs is capable of measuring up to 12 series-connected battery cells voltages with more than 0.04% accuracy. The measurement can be done in 290 μsec with the ADC in its fastest mode of operation. Noise reduction is critical in most applications, especially in an automobile, so the 8 programmable, third order low-pass filter settings takes care of that aspect in such a harsh as well as sensitive environment.
Most high powered automotive systems require a large number of cells divided into packs. These packs are distributed in various places throughout the vehicle. An EV/HEV has an extraordinarily high EMI environment, so a robust communication system is critical. The isolated CAN Bus as well as Linear Technology’s isoSPI bus are both road-proven solutions in such an environment.
Using the CAN Bus
The CAN Bus is well established in automobiles to connect battery modules together reliably. If a designer wants to design an isolated CAN Bus via the LTC6811 SPI interface, they will need to add a CAN transceiver, a microprocessor, plus an isolator. This will increase cost and use up more board space with the eternal components needed. See Figure 1.
The modular BMS electronics using an isolated CAN Bus with the LTC6811s
Alternatively, the designer may want to go the route of using the Linear Technology 2-wire isoSPI interface. The LTC6811 has a basic transformer integrated on the chip. A single twisted pair can be used in this design vs. the four wires needed with the CAN Bus design. The isoSPI interface gives the design a high RF noise-immune interface and a daisy-chain interconnect for long cable runs at data rates up to 1Mbps. See Figure 2.
A modular BMS schematic with the isoSPI bus connecting multiple LTC6811s via Daisy Chain
Using a wireless BMS
Now it is possible to reliably connect each module via a wireless connection. Linear Technology has demonstrated this system in the world’s first wireless BMS in a concept car at Electronica in Munich, Germany. No CAN Bus cable and no isoSPI twisted pair wires.
The BMWi3 with its wrap at Electronica
The all-electric BMWi3 design uses the LTC6811 battery stack monitors along with the Linear Technology SmartMesh wireless networking solution to replace all the associated copper wires from an interconnect system that add weight and cost to the automobile. Reliability will certainly be enhanced as mechanical failures of connectors, cables and wiring harnesses are eliminated in this harsh high-vibration environment.
Adding an enhanced level of reliability in this Electric Vehicle (EV) design, SmartMesh employs a redundant interconnect system via the use of both path and frequency diversity that will route the wireless messages around any obstacles as well as lessen any interference. The SmartMesh reliability has proven itself to be >99.999% reliable for data transmission in harsh environments like those in mining, industrial process plants and railcar monitoring. See Figure 4.
The modular BMS electronics structure that uses SmartMesh wireless
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