SLYY197A october 2020 – october 2020 BQ79600-Q1 , BQ79606A-Q1 , BQ79616-Q1 , CC2642R-Q1

TI’s wired vs. wireless BMS protocol

Going deeper into the TI protocols used in both solutions, the wired solution uses a differential, bidirectional and half-duplex interface, and therefore has a transmitter (TX) and receiver (RX) on both the high- and low-side communication interfaces to propagate information from low to high by default. These TX and RX functions are controlled automatically by the hardware based on the device’s base or stack detection, and the data is re-clocked as it propagates to each module. The RX topology of the BQ796xx devices is similar to RS-485 but with added design mechanisms to attenuate high common-mode voltages caused by the noisy conditions typical in vehicle environments. Each byte transmits at 2 MHz (250 ns per pulse or 500 ns per couplet). As shown in Figure 4, the time between each byte depends on the UART baud rate (1 Mbps in normal operation), but the byte time is always the same.

Table 1 Wired vs. Wireless BMS Considerations.

Considerations	Wired BMS	Wireless BMS
Weight	Typical reliability standard Increases overall vehicle weight and complexity	Decreases overall vehicle weight and complexity
Design flexibility and serviceability	Less flexibility with a larger overall footprint More difficult to service Modular once cables are disconnected	Smaller footprint enables design flexibility and more flexible placement within the vehicle Easier to service
Measurement	Time-synchronized measurements of voltage and current must propagate all the way up and down, causing delay between readings Delayed measurement feature can improve performance	Wireless system naturally enables time-synchronized measurements Offers the ability to add more synchronized sensing capabilities
Reliability	Wiring is reliable and meets functional safety standards but can break over time Ring Architecture features built-in redundant cables in both directions Can be more complex to repair	No wires to maintain Design must overcome harsh automotive radio-frequency environments and non-line-of-sight challenges
Security	Contained and fully secure system communication	Possible to breach poorly designed systems that lack security protocols

Figure 4 BQ796xx Byte-level Communications.

Figure 5 Wired Isolated Circuit Example.

The wired interface is designed to support capacitive or inductive isolation for robustness amid stringent automotive EMC/EMI specification limits. Figure 5 shows an example using capacitors and chokes. You would design the circuit between each battery monitor PCB, with as many as 64 devices all in one stack, to support varying sizes of vehicle battery modules.

To meet the requirements of tier-1 and original equipment manufacturers developing next-generation EVs, TI developed a proprietary wireless BMS protocol based on Bluetooth® Low Energy technology operating in the 2.4-GHz frequency band. Table 2 lists features for the TI wireless BMS protocol, including the star network configuration to support as many as 32 nodes per central unit; the ability to provide high-throughput, low-latency data transmission; and the use of a functional safety-rated protocol.

Table 2 TI’s Wireless BMS System Targets.

Function	Target
Safety-critical reaction time (latency)	Max. 100 ms (safety)
Data throughput	Up to 400 bytes per wireless device
Link reliability	99.9999%
Security	Secured and encrypted messages
Scalability	Up to 32 wireless devices and more
Multi-cluster support	Yes
Functional safety	ASIL-D / ASIL-C at the system level
Power consumption	<1 mA (avg) at primary nodes, <1 mA (avg) at secondary nodes
Link budget	>95 dB
Time for forming network	<600 ms

The key difference between the two protocols is the daisy- chain twisted pair wiring propagating the signal from the MCU all the way to the top monitor and back down; in a wireless star network configuration, each individual module can communicate independently back to the host processor. Both solutions do provide specifications that are highly important for automotive systems to deliver large amounts of pertinent battery pack data quickly, safely and reliably.