On our road to achieve zero emissions in traffic, hybrid electric vehicles (HEV) are a natural transition from internal combustion engines (ICE) to pure electric vehicles (BEV). This transitional period will last for several years, and several types of hybrid electric vehicles will be classified according to the level of electrification. One is a light hybrid vehicle, which is usually equipped with a 48V battery to support limited electric propulsion. The other is a plug-in hybrid vehicle with pure electric propulsion and an on-board charger (OBC).
The latest forecast shows that 48V light hybrid electric vehicles will dominate the HEV/BEV market, as shown in Figure 1. Consumer preferences will drive this demand. Therefore, automakers must be able to change the existing vehicle architecture to meet emission regulations and avoid the cost and time of complete redesign.
Figure 1: Global xEV market trends; Source: Strategy Analytics
Facing fierce competition, HEV manufacturers seek ideal solutions while balancing cost and performance. In this article, we will discuss how choosing a smart battery monitor with integrated functions can help you achieve design advantages, such as high-precision battery monitoring, high-level functional safety, and BOM savings.
48V HEV battery management system
Today’s light hybrid electric vehicles usually have a reduced size ICE and a 48V battery that provides limited electric propulsion and supports high-power loads such as Electronic torque assist. This 48V battery requires the use of a battery management system (BMS) to achieve monitoring, protection, power distribution and other auxiliary functions. For safety reasons, traditional low-voltage 12V batteries are still in use.
The 48V BMS consists of a 12V side battery control unit (BCU) and a 48V side battery monitoring unit (CSU) combined with a battery distribution unit (BDU), as shown in Figure 2. The engine control unit (ECU) is separated from the BMS and controls the BMS through the CAN interface. In order to improve safety, BCU and CSU are usually isolated.
Figure 2: Typical BMS of 48V HEV
Common BMS functions are:
·Monitor the voltage of each battery cell and achieve power balance
·Voltage and current measurement of the entire battery pack
·Battery temperature monitoring
·Power switching and power distribution
Obviously, the core part of BMS is battery monitoring and balancing IC. However, not all BMS functions must be performed by the battery monitor. For example, if there is no battery monitor for current measurement, an additional current monitor is required, as well as an ADC and digital isolator (such as ISO6721-Q1). In order to reduce system cost, it is best to integrate this function into the battery monitor.
Reduce solution size, save time and BOM
If all BMS functions can be executed in the battery monitor, it can significantly save development time, reduce solution size and reduce BOM cost. Figure 3 shows a 48V BMS system based on BQ75614-Q1, a 14-cell series automotive precision battery monitor, balancer, and protector that meets ASIL-D requirements and has an integrated current detection function.
Figure 3: 48V HEV BMS based on BQ75614-Q1
Compared with Figure 2, we can see that the number of components is significantly reduced. The integrated current detection, cell balancing, LDO and fuse/switch monitoring functions can save the cost of other external components. Flexible general-purpose input/output (GPIO) pins can be extended by providing an I2C interface or expanding the number of ADC inputs, such as using an NTC thermistor to measure battery temperature.
High voltage accuracy is essential for the increasingly widespread use of lithium iron phosphate (LFP) batteries. In terms of accuracy, BQ75614-Q1 achieves a high voltage accuracy of 2mV. The high current accuracy of 0.3% and the inherent voltage synchronization function enable a more accurate state of charge (SoC) and operating state (SoH) estimation, thereby extending battery life.
BQ79614-Q1 has built-in redundant paths for voltage, temperature, and current diagnostics to achieve functional safety compliance. Documents can be provided to assist in system design that meets the ISO 26262 standard, and achieve up to ASIL D (in cell voltage, current and temperature measurement and communication) and ASIL B (in overvoltage/undervoltage and overtemperature/under Temperature protection) functional safety system requirements. Through multiple built-in diagnostic functions, the device can also achieve a fault detection time interval (FDTI) of 100ms, thereby freeing up the MCU to complete other tasks.
The BQ75614-Q1 has many of the same features as the stackable BQ7961x-Q1 series for high-voltage BMS, including packaging, pinout, function control, and register mapping. Therefore, the hardware and software developed for the BQ75614-Q1 can be easily ported to other devices in the series, thereby saving development time.
Obviously, the fast-growing HEV market requires more competitive BMS solutions in terms of cost and performance. Only by providing innovative devices with stronger performance, more functions and higher integration to realize smarter BMS can we meet this challenge.
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