ADI Li-Ion Battery Formation/Grading Equipment Solution

Application Introduction

Energy consumption is a common problem faced by the world, and many industries are working to achieve safer, cleaner, efficient, low cost power solutions to overcome it. The increasing popularity of hybrid and electric vehicles, solar PV energy, and wind energy is a result of this trend. All of these solutions share one trait in common: Li-Ion batteries. Because of the rapid growth of these fields, Li-Ion batteries will play a more important role in energy conversation.

Li-Ion battery manufacturing is a complicated procedure, which includes electrode production, stack construction, and cell assembly. After this process, an electrical test is then done to grade battery capacity and performance. For these electrical tests, high power, efficient, and high accurate test equipment for Li-Ion battery manufacturing is required. This is the highlight of ADI's solution based on AD8450/1 and ADP1972.

System Design Considerations

Efficiency：

The capacity of Li-Ion batteries in laptops, cell phones, and similar portable devices is usually small, typically several ampere hours. However, Li-Ion batteries for vehicles or energy storage have much higher capacity, typically in tens or even hundreds of ampere hours. The linear test equipment for small capacity batteries will consume and dissipate a lot of power during the charge phase if it is also used for high capacity battery testing; it is inefficient and also a considerable thermal issue for equipment hardware design. The ADI AD8450/1 and ADP1972 solution is based on PWM architecture, which can help to resolve this problem.

The ADI PWM architecture can also help boost battery energy back to the grid or other channels for charging. This is an environmentally friendly and efficient solution compared to the linear architecture, which discharges battery energy to a resistive load.

Accuracy：

To achieve accurate Li-Ion battery capacity, precise measurements for current and voltage in both the charge and discharge mode is required. ADI's solution based on AD8450/1 and ADP1972 can deliver highly accurate measurements and settings with precise ADCs, DACs, and other components in the system.

Low System Cost：

• Higher switching frequency enables smaller size and cheaper power components, such as inductors and capacitors
• Energy recycling enables lower operational costs
• AD8450/1’s higher accuracy enables lower cost heat management and simplifies the control loop’s design
• AD8450/1’s unique in-amp design enables half the calibration time in manufacturing and longer warranty time
• An integrated solution leads to smaller size, which enables lower cost equipment and maintenance

ADI Solution

System Block Diagram：

Below is the system block diagram from dc bus to battery, including microcontroller, analog front end and controller, PWM controller, high voltage MOSFET driver, power stage (MOSFET, inductor, capacitor, shunt resistor), voltage/current reading (ADC), and voltage/current setting (DAC).

System Theory of Operation：

There are two main functions of the diagram above: one is to charge the battery, the other is to discharge the battery, which is determined by the mode signal of AD8450/1 and ADP1972. For each function, there are two modes: constant current (CC) mode and constant voltage (CV) mode. Two DAC channels control the CC and CV setpoints. The CC setpoints determine how much current is in the loop in CC mode in both charge and discharge functions. CV setpoints determine the battery potential when the loop goes from CC to CV, also for both charge and discharge functions.

The AD8450/1 precision analog front end and controller measure the battery voltage by internal difference amplifier PGDA and current by internal instrumentation amplifier PGIA with external shunt resistor (RS). Then it compares the current and voltage to the DAC setpoints with internal error amplifier and external compensation network which is used to determine the loop function—CC or CV. After this block, the output of error amplifier goes to PWM controller ADP1972 to determine the duty cycle of the MOSFET power stage. The loop completes with an inductor and capacitor. The descriptions in this section are for both charge and discharge functions, since ADP1972 is a buck and boost PWM controller.

In this implementation, the ADC gets the readings for voltage and current for the loop, but it’s not part of the control loop. The scan rate is unrelated to the control loop's performance, so a single ADC can measure current and voltage on a large number of channels in multichannel systems. This is true for the DAC as well, so a low cost DAC can be used for multiple channels. In addition, a single processor only needs to control the CV and CC setpoints, mode of operation, and housekeeping functions, so it can interface with many channels.

System Performance：

The ADP1972 and AD8450 demo board is made to verify efficiency and accuracy by following graph. The dc bus input is 12 V and the maximum charging/discharging current is 20 A for this asynchronous buck and boost power system.

Efficiency：

the efficiency of the demo board is ~90% in maximum rating, with 20 A CC mode for both charge and discharge with a 3.3 V load. To achieve this number, the external diode, shunt resistor, inductor, and MOSFETs are optimized.

Accuracy：

after initial accuracy has been calibrated, the accuracy of the current includes drift in temperature, linearity over full current range (0 A to 20 A), short term stability (noise), and CMRR over full voltage range (0 V to 3.6 V). The typical current accuracy of the ADI solution verified on a demo board is less than 0.01% under 25°C ± 10°C. The similar analysis can be done for voltage accuracy, which is also less than 0.01% verified on this demo board.