Cost-effective digital control loop battery tester design case

Lithium-ion batteries can be used in small electronic devices as well as larger applications such as electric vehicles and power grids, and they can easily meet the requirements of various sizes, voltages and shapes. However, this breadth of applications means that battery manufacturers have to purchase and maintain testing solutions for each battery type, resulting in huge related capital investment, which directly accounts for 20% of the final cost of the battery.

Clearly, a cost-effective, multi-range battery testing solution is needed that can handle different voltages, capacities, and form factors. This article describes the advantages of a digitally controlled loop battery tester and provides an example of a flexible and cost-effective battery testing design.

Advantages of digital control loops

The main function of a battery tester is to control and monitor the charging and discharging of the battery. Figure 1 shows the functional block diagram of a switch-mode battery tester. The control portion can be implemented in analog or digital mode. In the analog implementation, a pulse width modulation (PWM) controller regulates the output voltage or current flowing through the high-voltage and low-voltage power supplies. Protection circuits are integrated into the PWM controller. Constant current and constant voltage feedback loops drive the reference input of the PWM controller to accurately control the output current and voltage. A 16-bit digital-to-analog converter (DAC) connected to the feedback controller sets the output current and voltage. Finally, a precision 16-bit analog-to-digital converter (ADC) monitors the battery voltage and current.

Figure 1: Battery tester block diagram

In the digital implementation, the microcontroller (MCU) performs all the functions within the red box in Figure 1. With the help of the C2000™ real-time control MCU, a 16-bit PWM is generated and its comparators are used to implement the protection algorithm. The MCU adjusts the current and voltage controllers with the data fed back by the ADC. Because the controller is in the digital domain, this architecture does not require a precision 16-bit DAC. A 12-bit on-chip ADC can achieve a control accuracy of less than ±0.05%, which is sufficient for cost-optimized battery test systems. However, if you want to achieve a control accuracy of ±0.01%, then this solution requires external 16-bit feedback accuracy. The digital solution can easily achieve accuracy and flexibility; the performance and cost differences only depend on whether you choose to use an external 16-bit ADC or an internal 12-bit ADC in the feedback. In this solution, the efficient use of the MCU can save more than 30% of the bill of materials.

Multiple voltages, capacities and sizes, one reference design.

Current test equipment is designed for specific battery types. Larger batteries require higher currents, so battery testers have multiple channels connected in parallel. But if battery manufacturers are producing smaller batteries with lower current requirements, they often use testers optimized for lower current levels, leaving the high-current battery testers idle. Using testers that can test both small and large batteries reduces such equipment redundancy and helps reduce the overall cost of battery production. Digital control loops add the flexibility to test large or small batteries with software, while analog solutions require hardware changes.

As battery technology advances, battery manufacturers require new features and test methods. Using control loops in software makes it easier for test equipment manufacturers to provide additional test capabilities.

Design of a multi-range battery tester

The digital control reference design for cost-optimized battery test systems uses multiple independently controlled, low-current battery tester channels connected in parallel to meet the needs of different levels of high-current battery testers. The reference design can be configured for multi-phase operation with simple software changes, as shown in Figure 2. In the multi-phase configuration, each phase uses an independent constant current loop connected in parallel. In constant voltage mode, one constant voltage loop is connected to all constant current loops to ensure current balance. Therefore, the same test environment can provide multiple output current ranges.

Figure 2: Feedback controller in a multiphase configuration

The block diagram of the reference design is shown in Figure 3. The TMS320F280049 MCU can control up to 8 independent channels. It generates high-resolution 16-bit PWM for the synchronous buck power stage and executes subroutines for the current and voltage control loops. The INA821 instrumentation amplifier senses the current, and the TLV07 operational amplifier senses the voltage. Both the external ADS131M08 ADC and the C2000 on-chip ADC convert the current and voltage signals into digital information. A control accuracy better than ±0.01% can be obtained based on the feedback 16-bit ADC signal. For cost-optimized systems, the ADS131M08 can be removed based on the feedback, and a control accuracy of less than ±0.05% can be achieved using the on-chip 12-bit ADC.

Implementing a multi-feedback controller allows for a smooth transition from constant current to constant voltage, where the inner loop is always in constant current mode. When a constant voltage mode condition is detected, the constant voltage loop output is connected to the constant current loop.

Figure 3: Digital control loop battery tester

in conclusion

Digital architecture enables high accuracy, high current, high speed and flexibility without having to invest a lot of money in battery test equipment. You no longer need to invest in multiple architecture testers for different current levels, so high current equipment is no longer idle while testing low current applications.

With TI’s digital control reference design, you can save on overall system costs by investing in low-current battery test equipment, increasing the ability and flexibility to test multiple current ranges without compromising accuracy.