Li-ion batteries promise dramatic improvements in electric and hybrid-electric vehicles. But these batteries require specialized semiconductor devices that are built to monitor and manage Li-ion cells correctly so the batteries perform as desired. The monitoring devices must detect battery faults reliably by watching each Li-ion cell, with the ability to keep track of the monitoring system itself.
There are four important criteria for a reliable Li-ion monitoring system. First is accuracy. The system must measure each cell’s state to ensure optimal performance of the battery array. The precise level of accuracy required depends on the type of cells used.
The second criterion is comprehensive diagnostics. The system must consistently check its own functionality to make certain each part operates constantly at the required accuracy.
Third is robust communication. The monitoring system coordinates operations, and to do so must communicate reliably. This is a difficult requirement; most existing communication methods have not done well with this requirement, because of the noisy environment typical of battery arrays.
Finally, there is safety. Managing the Li-ion cells lets the system avoid failures and safety issues. If faults occur, the system must take the appropriate action. Equally important: It must not act if it receives a false alarm.
The semiconductor monitoring devices must constantly check factors such as cell over- and under-voltage, and over-temperature. They also have to monitor the system’s power-supply levels, voltage, and temperature.
The backup monitoring devices must communicate fault detections among themselves to shut down the entire system, if necessary. Standard CAN or SPI interfaces enable reliable communications with the microcontroller, but using such an interface between all the monitoring devices adds expense. Instead, using a CAN interface along with an isolator or using a specialized interface can do the job at minimal added cost.
The HEV/EV solution from Intersil is a new generation of Li-ion cell monitoring and balancing products that satisfies the requirements of today’s electric and hybrid-electric vehicles. The ISL78600 is a Li-ion battery manager IC that supervises six to 12 series connected cells. The automotive grade (AEC-Q100) part performs accurate monitoring, cell balancing, and system diagnostics functions. With an integrated 14-bit analog-to-digital converter (ADC) and next-generation input design, the device provides the accuracy needed to maximize performance from state-of-the-art Li-ion batteries—extending vehicle range and battery life.
The ADC scans 12 channels in less than 250 µsec. Average mid-size HEV vehicles today utilize 126 to 168 cells, which can be addressed with 10 to 14 ISL78600s per vehicle. When combined with four external temperature sensors, the ISL78600 provides high accuracy for state-of-charge (SOC) measurements across the full battery operating temperature range.
The device’s integrated system monitoring and diagnostics functions emphasize safety and reliability while filtering fault-detection inputs to reject false alarms. These safety-related capabilities allow the ISL78600-based HEV/EV solution to approach Automotive Safety Integrity Level (ASIL) C compatibility as a stand-alone part.
For the battery electronics, the specification requires an ability to detect and report any fault that compromises the function of the electronics in maintaining a safe state. For example, the monitoring devices must assure that cell voltage measurements remain accurate to within predefined limits and detect shorts in components that could cause damage through excess heat dissipation.
To achieve the highest possible reliability for inter-system communication, the ISL78600 utilizes a high noise immunity and transient tolerant communication scheme. This fully differential daisy-chain architecture allows the use of low cost twisted pair wiring to stack multiple battery packs together while protecting against hot plugging and high voltage transients.
The ISL78600 allows for easy connection to microcontrollers via either a 2.5 MHz SPI or 400 kHz I2C interface, and is specified for -40 to 105C operation. The device can also be adapted to industrial and battery back-up applications.
If the monitoring devices themselves develop a fault there is the ISO26262 specification, which attempts to cover this requirement using a statistical analysis of single-point and multipoint faults to reduce the incidence of residual faults (faults that go undetected) to an acceptable level. This built-in fault detection guards major internal functions, as well as detection of external faults such as open wire, over and under voltage, and temperature and cell balancing faults.
As an example, to check voltage measurement accuracy, the ISL78600 integrates two separate voltage references that can be compared for accuracy. The ISL78600 also works with a backup device, the ISL78601, that provides cell over-voltage and under-voltage detection for redundant monitoring systems.Pricing and availability
The ISL78600 is available now
in a 10 x 10 mm 64-lead TQFP package. Pricing starts at $6.50 each in 10,000-piece quantities. The ISL78601 is available now in a 38-lead TSSOP package. Pricing starts at $1.85 each in 10,000-piece quantities.