This article will use a low-power demo board based on the NXP MX7 ARM processor from Toradex Colibrii MX7 to demonstrate a lithium battery application solution.
The iMX7 employs heterogeneous multi-core processing technology with Arm Cortex-A7 and Cortex-M4 cores. An embedded Linux system runs on the application core, while the real-time operating system FreeRTOS runs on the M4 core. We will use this demonstration board to showcase lithium-ion battery applications. It's named a low-power demonstration because we can disable the Cortex-A7 core and run only the i.MX7 M4 core to read sensors and control the SPI LCD display. The block diagram is shown below.
ColibriiMX7 Low Power Demo Board Block Diagram
Our requirements for the battery system:
Charging via 5V USB BC1.2
5V to 20V variable external power supply
5V and 3.3V system voltage
Battery life exceeds 1 day
According to the Colibrii MX7 datasheet, the CPU requires up to 300mA of current under maximum load. As I previously suggested, we will use two removable batteries connected in series, rather than the more dangerous parallel connection. We will use the TIBQ2920x for voltage protection and automatic balancing for 2S lithium batteries. Battery balancing can also be achieved externally. For the demo board, autonomous active balancing could be used, as you see in many consumer devices. I still recommend using dedicated pins to control balancing charging, controlled only at a higher SoC level, to avoid over- or premature discharge. We will use LT's LTC2943 battery meter to measure current, voltage, and temperature via the I2C bus and display the actual charging status on an SPI LCD. This device uses a 14-bit delta-sigma ADC to obtain accurate coulomb values. We will only use the BC1.2 standard, maintaining the voltage at 5V. Therefore, the TIBQ24392 will be used as the battery charging detection chip. I recommend carefully reading the datasheet for this chip and thoroughly studying the charging detection block diagram. The core of the charging circuit is the ISL9237 with a buck-boost converter, so we can charge the battery using USB 5V and an external 5V to 20V DC power supply, as shown in the example below.
This is a very high-end solution that meets our needs and demonstrates possible options. However, for practical applications, I recommend using only a DC power supply with a fixed voltage, higher than the battery's maximum voltage. Alternatively, you can use only USB or a 5V DC power supply for charging, employing a simple charging circuit with a boost converter.
Toradex Colibrii MX7 Low Power Demo Board BMS Overview
I hope the solution presented is easy to understand and can serve as a reference design for your own solutions. Again, this is just one of many solutions and can be further optimized in terms of cost or performance. This case illustrates that powering embedded systems with rechargeable lithium batteries is not difficult. With our modular design, similar solutions can be easily implemented by expanding the baseboard with additional electronics. If you have any further questions or would like more information, please feel free to contact us. I'd love to speak with a battery management system expert.
ColibriiMX7 Low Power Demo Board
