Hybrid test system based on LXI platform for lithium-ion battery testing

Mobile phone lithium battery testing mainly conducts functional testing on the battery protection circuit after packaging, using power supplies, electronic loads, multi-way switches, digital multimeters and other instruments to simulate various extreme overvoltage and overcurrent conditions during charging and discharging, and test whether the battery protection circuit is effective, thus preventing substandard battery products from entering the market at the source.

Testing requirements for lithium-ion batteries

In order to test whether the various protection functions of the battery protection circuit are effective, the main test steps of the battery test system include the following parts:

1) Conventional open circuit voltage test

Battery function testing requires charging the battery for a short period of time and measuring the open circuit voltage to determine whether the battery's basic charging function is effective.

2) Overcurrent protection and recovery test

Use an electronic load to discharge the mobile phone battery with a large current and test the internal current cut-off function of its protection circuit. After cutting off the discharge circuit, test whether the battery can resume normal operation.

3) Operating temperature test

Use a multimeter to read the resistance value on the temperature sensor and test the battery operating temperature.

4) Overcharge and recovery test

Use a power supply to over-voltage charge the mobile phone battery to test whether the battery protection circuit can cut off the charging circuit.

5) Over discharge and recovery test

Use an electronic load to over-discharge the mobile phone battery to test whether the battery protection circuit can cut off the discharge circuit.

6) AC internal resistance test

Test the battery AC internal resistance using an AC internal resistance meter.

Lithium-ion battery test system

A typical lithium-ion battery test system should include the following test instruments:

●Multi-channel switch is used to load different stimuli to the DUT and switch different test equipment

●High-precision digital multimeter to complete the DC voltage, current and resistance test of the battery

●Multiple power supplies with more than two channels are used to charge the DUT and power the test fixture

●Electronic load, used to discharge the device under test

●AC internal resistance tester, used to measure the AC internal resistance of lithium batteries under 1kHz AC conditions

●Industrial computer and test software, used to control the equipment to complete the test

Taking the overcurrent protection and recovery test in lithium battery testing as an example, we describe the specific testing process:

The computer in the test system sends instructions to each instrument in turn through the GPIB interface to control the test process. First, connect the electronic load to the device under test by switching the multiplexer switch. Set the electronic load to work in constant current mode so that the device under test outputs a relatively large constant current (for example, 5A). After 20ms, connect the digital multimeter to the battery output terminal through the multiplexer switch to test the output voltage of the battery. If the battery output voltage is 0, it means that the battery protection circuit is activated and the battery output circuit has been cut off, and this project test has passed. If the test result is not zero, this project target is defined as a test failure. After the test is completed, quickly turn on the switch to cut off the connection between the electronic load and the battery, because long-term high-current discharge may cause permanent damage to the battery.

Based on the above test solutions, Agilent launched a new generation of lithium-ion battery test solutions in 2007. The new solution makes extensive use of LXI (LAN Extensions for Instrumentation) system test instruments launched in recent years. Compared with traditional products, these LXI instruments can simplify the difficulty of system integration, improve the test speed, and greatly reduce the system size and assembly cost by building the system through network cables. Each instrument in the system is connected to the computer through network cables or GPIB cables to form a hybrid test system based on the LXI platform. The height of the entire system is only 14U. In the actual application of some mobile phone battery manufacturers, customers told us that compared with their original test system configuration, the new solution reduces the system size by more than 1/3, reduces the cost by more than 1/4, and the test speed is also increased by about 10%. In addition, the smaller chassis size, simpler system connection and lower power consumption brought by the switching power supply also bring many additional benefits.

In the development of test software, LXI instruments also support various mainstream software development environments. Test programs can be written in various development environments such as VB, VC++, C/C++, Labview, etc., which is exactly the same as the traditional instrument control method.

During the battery testing process, the issues that customers are most concerned about, in addition to ensuring battery quality, include how to increase battery production and reduce production costs. In addition, since different types of batteries have different electrical characteristics, the test system must have a certain degree of flexibility in software and hardware to facilitate customers to carry out secondary development on the system. Finally, under the condition that the production line is continuously turned on 24 hours a day, the quality of the system instruments and maintenance costs are also important factors in measuring the battery testing system solution.

Test system instrument selection

Selection of multi-way switches in battery testing systems:

In single-cell battery testing, multi-way switches are mainly used to connect different loads or power sources to the battery for repeated charge and discharge tests. Multi-way switches can also be used to switch measuring instruments. In the ordinary battery testing process, it takes about dozens of switch switching steps to complete all tests on a single cell. The daily production volume of the production line is usually more than 8,000 pieces, and the number of opening and closing times of a single switch in a day can reach more than 150,000 times. Therefore, good switch quality and fast switch switching rate are the key to selecting switch products.

Agilent 34980A provides a variety of high-quality switch cards. These switches provide the fastest opening and closing time and the longest switch life among similar products. The switch count function can timely report the working status of each switch so that the switches that are about to fail can be replaced in advance. In addition, its built-in digital multimeter can directly complete high-precision measurements of various DC signals. It can save investment and improve system integration and test speed.

Selection of Multi-channel Power Supply in Battery Testing System

In battery testing, there are more than two power sources loaded on the DUT. In addition to powering the fixture of the DUT, these power sources are also used to charge the battery, simulate various overvoltage and overcharge phenomena, and detect the response of the battery protection circuit. When conducting such tests, the response speed of the power supply is very critical. Usually, the battery is overcharged for no more than 1 second, otherwise it may cause the battery to burn and explode. In order to ensure the test speed, this time is generally limited to a few hundred milliseconds.

N6700 modular DC power supply, each power supply chassis can be placed in 1U height up to 4 power modules, forming independent multi-way power supply. In addition, these power modules have built-in voltage and current measurement functions, which can monitor the output status without switching to other measuring instruments. In the automatic test system, the modular power supply system can greatly reduce the size of the multi-way power supply and reduce the system cost, becoming the first choice for power supply in building test systems today.

Selection of electronic load in battery test system

The electronic load is used to test the discharge of the battery and to simulate whether the internal protection circuit can cut off the battery output in time when the battery is short-circuited and outputs a large current. In the battery test, the large current discharge time cannot exceed 30ms, otherwise it will cause accidental battery combustion or explosion. Therefore, in order to ensure the safety of the test, this type of test has very high requirements for the response speed of the electronic load and the speed of current change.

Agilent provides a variety of electronic load products, among which the fastest current change speed can reach up to 107A/S. The built-in 16-bit voltage and current readback function can measure the voltage and current values ​​in real time under working conditions without switching to other devices.

Selection of AC internal resistance tester

The internal resistance of a battery is directly related to the quality of the battery cell and the quality of the battery packaging. If the measured internal resistance of a battery is higher than the expected value, it means that there is a high probability that there is a problem with the quality of the battery. Since lithium-ion batteries are often accompanied by dynamic loads and often generate a large number of current pulses during operation, measuring the AC internal resistance of a battery at 1kHz has become the industry standard for measuring the internal resistance of a battery. The lower the internal resistance of a battery, the more stable the voltage value output by the battery. Therefore, measuring the internal resistance of a battery at 1kHz can very effectively illustrate the battery's ability to regulate its output voltage under dynamic load conditions. Agilent's dynamic internal resistance tester 4338B can help solve problems in AC internal resistance testing and provide high-speed and accurate measurement results.