With the development of electric vehicle technology, as well as the government’s policy encouragement and support, electric vehicles (hybrid + pure electric) are growing rapidly at a rate of more than 50% per year. There has also been a corresponding rapid growth. This article will analyze the memory requirements of battery management systems.
The battery management system (BMS) mainly realizes three core functions: prediction and calculation of battery charge and discharge status (ie SOC), balance management of single cells, and battery health status logging and diagnosis. The functional block diagram is as follows:
In the whole battery management system, the prediction and calculation of the battery state of charge (ie SOC) is its most important function, because with the accurate prediction/calculation of the battery charge/discharge state, effective balance management can be performed. Therefore, the requirement of SOC accuracy is as high as possible.
In order to improve the accuracy of the SOC, in addition to collecting the voltage and current parameters of the battery, it is also necessary to provide various parameters such as impedance, temperature, ambient temperature, and charge and discharge time. The inherent parameters of the battery will establish a software model through mathematical modeling, and the dynamic parameters will collect data in real time through the data acquisition card, and transmit the data to the MCU unit for storage in real time. Get an accurate battery state of charge.
Therefore, the SOC function will store the models of different batteries in the memory, which needs to have low power consumption, fast read and write, simple interface and data retention time of 20 years; the SOC function requires the acquisition card to keep real-time data acquisition. The battery voltage/current data is stored in the memory. If an MCU unit is connected to the acquisition data of 10 single batteries, the acquisition data card generally uses a 1MB isoSPI bus for communication, that is, for the memory of the MCU unit, the interface speed is required to be high and almost A data write operation is performed every second; and the battery life is required to be at least 10 years. If the running time of a car is 8 hours, the data write operation of the memory of the MCU unit in the battery pack life cycle The number of writes is 105 million times.
From the above analysis, it can be seen that the SOC function in the BMS is very critical, so its performance and reliability of the memory are also very high: it must be a non-volatile memory, the number of times of erasing and writing must be at least 110 million times, the interface speed is greater than 8MHz, low Power consumption and data can be reliably stored for 20 years, it needs to comply with AECQ-100, and it needs to pass functional safety certification in the future, with at least ASILB level.
The current mainstream non-volatile memories include EEPROM, Flash and F-RAM. The interface of EEPROM has SPI interface, the speed can reach 10Mhz, but each write has a 5ms write wait time, the number of erasing and writing is 1 million times, the power consumption is medium, there are automotive-grade devices, but no functional safety is currently implemented. Certification, data retention capabilities can also be achieved for 20 years.
The read and write speed of Flash is slow, and each write operation must be erased and written. Therefore, it takes at least several hundred milliseconds to complete a write operation, and the number of times of erasing and writing can only support 100,000 times, far less than 110 million times. Requirements, the data retention capacity is between 10 and 20 years.
F-RAM uses a special material such as ferroelectricity as a storage medium. It has high reliability, data retention time of 100 years, and high read and write efficiency that is completely random and does not require write waiting. The SPI interface rate can support up to 50Mhz or 108MHz QSPI, and has very low power consumption; due to its special ferroelectric material, the number of erasing and writing of this type of memory can be as high as 10 billion times. As shown below:
As shown in the figure above, F-RAM, as a unique non-volatile memory, is currently the best memory choice for implementing high-reliability BMS systems in terms of writing speed, endurance, power consumption and reliability. .
Cypress semiconductor Inc., as the world’s leading F-RAM core supplier, provides a very complete range of ferroelectric random access memory F-RAM products, with capacities ranging from 4Kb to 8Mb, and the interface is I2C/SPI interface. With almost unlimited read and write times (10 billion read and write cycles), the QSPI interface rate is up to 108Mhz, no write wait time is required, and the operating current is as low as 0.6mA. It is an automotive-grade chip solution that can withstand high temperatures of 125 degrees, and ASIL-B compliant.