Fluoride-Based Batteries Seek to Surpass Lithium Cells
It is one thing to provide battery management for one or more cells. It’s a far more difficult challenge when you have to do this on a number of stacked cells, such as B. due to common mode voltage (CMV), multiplexing, data time offset, offsets and errors per channel and more.
As batteries improve in energy density (by weight and volume), decrease costs, and provide other benefits, they are increasingly used in such serial arrangements, with the electric vehicle (EV) being the most visible example. Other uses include electric bikes, battery backup systems, supercapacitor based systems, and even battery powered tools. Although these cells are in a series configuration, it is imperative that each cell be monitored independently of the others.
Providing cell voltage sensing in these applications is the role of the Maxim MAX17852, a 14-channel high voltage data acquisition system (cells) that the manufacturer maintains and is the first company to achieve ASIL-D compliance for voltage, current, and temperature and communications in a battery management system (BMS) (Fig. 1). Maxim also claims that using this AEC-Q100 Grade 1 qualified IC results in a design that is up to 16% smaller than a discrete solution and has a cost savings of up to 20%
(ASIL refers to the vehicle safety integrity level, a risk classification system defined by the ISO 26262 standard for the functional safety of road vehicles with four ASIL classes A, B, C and D. ASIL A stands for the lowest grade and ASIL D. for the lowest grade systems like airbags, anti-lock braking systems and power steering require an ASIL-D class.)
The system can measure up to 14 cell voltages, a current and a combination of four temperatures or system voltages with fully redundant measuring motors in just 263 µs, or make all entries exclusively with the ADC measuring machine (analog-digital converter) in 156 µs. Such small time shifts from channel to channel are of decisive importance for effective batch management.
Oversampling is available for increased interference immunity, in which up to 128 measurements per channel can be averaged internally with a resolution of 14 bits and then combined with digital IIR post-processing filtering. The 14 internal switches for cell balancing are designed for over 300 mA.
Cell and busbar voltages from – –2.5 to +5 V are measured differentially over a 65 V common mode range with a typical measurement accuracy of the cell voltage of ± 0.45 mV at room temperature and a maximum error of ± 2 mV (5 to 40)°C) and 4.5 mV (– –40 to +125°C). Since many systems have more than “just” 14 cells, up to 32 devices can be daisy-chained to manage 448 cells and monitor 128 temperatures.
Both Hall-effect sensors and shunt resistors can be used as sensor components with the MAX17852. The integrated low-noise current sense amplifier with a resolution of 5 mA (with a gain of 256) eliminates the need for an external component and ensures that current information is recorded simultaneously with the cell voltage and temperature (again problems with time offset). It uses Maxim’s UART or SPI battery management protocol for robust communication and supports an I2C master interface for controlling external devices.
As an ASIL-D device, it includes comprehensive monitoring and diagnosis of system performance, including over-voltage, under-voltage, under and over temperature conditions, as well as cell conditions such as over temperature and even single cell mismatch alarm. The MAX17852 contains a factory-trimmed oscillator (therefore no external crystal is required) and comes in a 64-pin (10 × 10 mm) LQFP package. It’s priced at $ 7.52 (1000 pieces) and is backed by an impressive 370-page data sheet detailing performance, specifications, and operational issues.
To improve familiarity with the IC and speed design-in, Maxim offers this too MAX17852EVKIT# $ 250 evaluation kit (Fig. 2). The kit’s graphical user interface enables initialization, operation and data analysis. All of this is explained in the 46-page data sheet, which also includes circuit diagrams, parts lists, and circuit board layout details.
The kit includes Force and Sense pin headers for precision measurements. Instead of the user having to use batteries first (which can get tricky on the breadboard and bench) there is a built-in resistor stack for battery emulation.