Low-cost Battery Measurement System for 7/13-Cell Applications Darius Rydahl Introduction Standard ATA6870 Implementation The standard battery measurement system using the Atmel® ATA6870 circuit can measure the voltage of up to six battery cells. Several of these ICs can be stacked in series to measure the voltage of up to 96 battery cells simultaneously. For most applications, the “stacked” battery measurement IC approach is sufficient since the number of cells measured in these applications is a multiple of three, four or six cells. In some instances, e.g., an e-bike application, the cell count of the battery may be of an odd number: 7 or 13 cells. With these applications, the use of multiple, stacked ATA6870 circuits combined with a standard microcontroller (MCU) may not be the most cost-effective solution for the end application. A more practical, lower cost implementation is to use one ATA6870 chip in conjunction with an Atmel battery management microcontroller. The standard implementation of an ATA6870 battery management system consists of at least one ATA6870 battery measurement IC (maximum sixteen, connected in series) plus a general-purpose MCU for control and data processing. In Figure 1, the MCU is powered by the lower ATA6870 IC’s on-board 3.3V voltage regulator (VDDHVM). Communication occurs via SPI where data is transferred serially between multiple ATA6870 circuits, one IC to the next, to/from the MCU. As shown in Figure 1, a common ground reference is shared between the bottom ATA6870 device and the MCU. In this instance, there is no voltage offset between the MCU and the ATA6870 circuit, thus eliminating the need for additional interface circuitry between the CLK and SPI pins of the two ICs. © 2012 / www.atmel.com VBMS This three-chip battery measurement architecture for sevencell applications is easy to implement, but can increase the system cost due to the use of multiple ATA6870 ICs. A more practical implementation is the two-chip1 approach where a single ATA6870 (six cells) circuit is coupled with a battery measurement MCU (one cell). + + + + - Atmel ATA6870 + + - 1 VBAT The 7-cell application can be expanded to accommodate 13 or more cells by including additional ATA6870 ICs into the stack. SPI + + + + - Atmel ATA6870 VDDHVM CLK + + - MCU SPI Atmel offers two possible solutions for the seven-cell application using a battery measurement MCU as shown in Figure 3. In this example, the ATA68670 IC can be paired with either the Atmel megaAVR® ATmega32HVE2 or megaAVR ATmega32HVB MCU. Both MCUs have battery voltage and current measurement capabilities. The feature sets2 and peripheral offerings (number of cell measurement inputs, LIN bus interface, etc.) of two MCUs are slightly different, so the specific requirements of the end application must be taken into consideration before selecting the MCU. Figure 1. Standard Twelve-cell ATA6870 Configuration (Common GND) 2 Please refer to the specific datasheet for each device for more detailed information. Seven/Thirteen-cell ATA6870 Implementation In applications where the total cell count is a multiple of 7 or 13, the designer can simply add additional ATA6870 ICs to the battery stack as shown in Figure 2. The seven battery cells must be split between the ICs to maintain the minimum operating voltage of 6.7V for each ATA6870 IC. VBMS + + + - VBMS + - Atmel ATA6870 + + - Atmel ATA6870 VBAT + - + + - Level Shifter GNDBMS VBAT SPI VMCU + - + - + - Atmel ATA6870 + + - CLK VDDHVM CLK MCU SPI Figure 2. Seven-cell Battery Measurement System Using Two ATA6870 ICs Automotive Compilation Vol. 9 SPI ATmega32HVE2 or ATmega32HVB GNDMCU Figure 3. Two-chip Battery Measurement Solution for 7 or 13 Cells Ground Offset Down-converting Level Shifter Regardless of the MCU used, additional circuitry must be inserted between the digital I/O pins that connect the MCU to the ATA6870 IC to compensate for the ground offset between these two devices. This offset/voltage difference arises since the ATA6870 ground is no longer referenced to the same ground as the MCU. The battery management MCU must now measure the voltage of the bottom-most cell in the battery stack, VMCU (see Figure 3). The MCU also derives its supply voltage from VMCU, unlike the situation in the standard measurement implementation where the MCU is supplied by the ATA68670 voltage regulator (VDDHVM). Due to this configuration, VMCU is now the ground reference for the bottommost ATA6870 IC in the stack. Therefore, isolation between the two devices and level shifting of the digital I/O interconnections is required. Down-converting the voltage from the ATA6870 to the MCU is accomplished by the use of the circuit shown in Figure 5. All ATA6870 SPI output signals (MISO and IRQ) must use this circuit. The level shifter utilizes a P-channel MOSFET and a voltage divider to down-convert the ATA6870 IC's output signal to the input voltage required by the MCU. As with the upconverted inputs, the same resistor divider rules apply. Six digital I/O lines are affected by the ground offset and must be level shifted. They include the SPI pins (MOSI, MISO, SCK and CS), the ADC reference clock (CLK) and the interrupt request line (IRQ). VDDHVM IN OUT Up-converting Level Shifter Up-converting from the MCU to the ATA6870 is accomplished through the use of the circuit shown in Figure 4. All ATA6870 SPI input signals (SCK, MOSI, CS_N) and the ADC reference clock (CLK) must use this circuit. The level shifter utilizes a high-speed switching NPN transistor and voltage divider to up-convert the low-level MCU output voltage to the regulated voltage supplied by the ATA6870 IC, VDDHVM (3.3V + VMCU). It should also be noted that resistor divider values are dependent upon the voltage offset between the MCU ground, GNDMCU, and the ATA6870 ground, GNDBMS. VDDHVM OUT GND MCU Figure 5. Output Level Shifter Conclusion Since cost is the primary concern for any application, it is important to consider the type of architecture to be used. The 7/13-cell battery management application is a perfect example when it is useful to modify the system architecture in a way that completely eliminates one IC. In this case of a seven-cell application, the total IC count can be easily reduced from three ICs to two by replacing one ATA6870 circuit and the MCU with one Atmel battery measurement MCU. In making this architecture change to the system, the MCU and ATA6870 IC have different ground potentials, and the digital I/O pins between the two devices are offset by this difference in voltage. This offset can be easily eliminated by adding several low-cost transistors and resistors to the digital I/O lines that connect the two devices. The resulting cost-effective solution fulfills all end application requirements. IN GND MCU Figure 4. Input Level Shifter © 2012 / www.atmel.com Atmel Corporation 1600 Technology Drive, San Jose, CA 95110 USA T: (+1)(408) 441-0311 F: (+1)(408) 487-2600 | www.atmel.com © 2012 Atmel Corporation. All rights reserved. / Rev.: Article-AC9-Low-Cost-Battery-Measurement-System_V2_042015 Atmel®, Atmel logo and combinations thereof, and others are registered trademarks or trademarks of Atmel Corporation or its subsidiaries. Other terms and product names may be trademarks of others. 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