A standard (automotive) battery measurement system using Atmel’s ATA6870 is capable of measuring the voltage of up to 6 battery cells. Several of these ICs can be stacked in series to measure the voltage of up to 96 battery cells simultaneously. For the majority of applications, the “stacked” battery measurement IC approach is sufficient, as the number of cells measured in these applications is a multiple of three, four or six.
“In some instances, such as an e-bike application, the cell count of the battery may be of an odd number: 7 or 13 cells,” Atmel engineering rep Darius Rydahl told Bits & Pieces. “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.”
According to Rydahl, 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,” Rydahl continued. “As you can see in the image above (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, neatly eliminating the need for additional interface circuitry between the CLK and SPI pins of the two ICs.
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. However, the 7 battery cells must be split between the ICs to maintain the minimum operating voltage of 6.7V for each ATA6870 IC.
“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 ATmega32HVE2, or ATmega32HVB MCU,” said Rydahl.
“Both MCUs have battery voltage and current measurement capabilities. The feature sets 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.”