What is lithium-ion battery charge discharge test?
The charge and discharge tester is the most commonly used test equipment for power lithium batteries. New batteries need to be assembled and screened for consistency; in the process of battery pack design and finalization, multiple tests need to be charged and discharged; the performance of the battery pack is inspected, and the working condition test requires the assistance of a charge and discharge tester; old batteries, charge and discharge test health status; some certifications, spot checks and tests at the request of buyers require charge and discharge.
Main parameters of lithium battery
Lithium battery charge and discharge test equipment needs to be able to monitor the relevant parameters of battery cells, modules and battery packs in real time during the charge and discharge process. These parameters include the following:
Capacity: the battery is discharged from the fully charged state to the discharge cut-off condition, the total amount of electricity released, the unit is Ah. The capacity is greatly affected by the discharge current, ambient temperature, etc. Therefore, when it comes to capacity, it is necessary to talk about the capacity under the temperature and discharge current.
State of charge (SOC): The ratio of the current battery capacity to the overall available capacity, expressed as a percentage.
Depth of discharge (DOD): The ratio of the discharged power to the total available capacity of the battery from full charge to the present, is also expressed in percent, and the relationship with SOC is DOD=1-SOC;
Open circuit voltage (VOC): The voltage between the two poles of the battery measured by disconnecting the external circuit, is numerically equal to the electromotive force of the battery;
Working voltage: After connecting the external circuit, measure the voltage between the two poles of the battery, which is equal to the battery potential minus the internal resistance of the battery (taking the discharge process as an example);
The charging cut-off voltage: The battery management system sets the maximum voltage that can be reached during the charging process. After reaching this voltage, the battery management system requires the charging process to end. The charging cut-off voltage is generally slightly lower than the maximum open circuit voltage allowed by the battery;
Discharge cut-off voltage: The minimum voltage of the battery allowed during the discharge process. When the discharge process reaches this value and exceeds a certain delay time, the battery management system requires to disconnect the discharge circuit.
Internal resistance: the inherent characteristic of the battery's own electrochemical reaction, is expressed in the form of loop impedance during charging and discharging. It is mainly composed of two parts, ohmic internal resistance and polarization internal resistance. On the charge-discharge curve, the instantaneous drop of the battery terminal voltage at the moment of current loading is the impact of ohmic internal resistance; the rise of the voltage within a period of time when the charge is terminated, the current disappears, and the terminal voltage is stable is the embodiment of the influence of the polarization resistance.
What are the functions of the general charge and discharge tester?
- It has the function of constant current and constant voltage charge and discharge, which can realize automatic life cycle, and automatically test the standard working conditions or artificially set working conditions; the cycle test can realize the nesting of cycles;
- It has the function of recording real-time current, voltage, temperature, charge and other related test data and fault data;
- Different charging and discharging termination conditions can be set, such as total voltage, single voltage, battery state of charge, etc.;
- Safety monitoring function, dealing with over-current, over-voltage, over-temperature, under-voltage, under-current, short-circuit, power-down protection, and other fault conditions;
- According to the test records, draw time-voltage, time-current, time-stage capacity, time-charge cumulative capacity, time-discharge cumulative capacity, time-total capacity, time-power, time-resistance, time-energy, time -Single battery voltage, number of cycles-capacity at a specified stage (cycle capacity decay curve) and other curves;
- Screen display, upper computer display, sound and light alarm, screen input, selection, upper computer input, selection and other human-computer interaction functions.
Basic working principle of charge and discharge tester
1. Charging process
The charge and discharge tester can realize various forms of charging process, such as constant voltage charging, constant current charging, constant current first and then constant voltage charging, positive pulse charging, positive and negative pulse charging and so on. Different forms of charging processes can be completed according to the needs of battery performance.
Constant voltage charging:The charging and discharging equipment is adjusted to the constant voltage source mode. Since the set charging voltage must be a value near the full battery voltage, the current value is the largest at the beginning of charging. As the battery terminal voltage increases, the voltage difference between the charger and the battery becomes smaller and smaller, and the charging current also gradually decreases. When the charging current decreases to a certain value, the charging ends. Constant voltage charging, the charging current is relatively large in the initial stage, which is not good for the life of the battery cell.
Constant voltage charging: The charging current is relatively large in the initial stage, which is not good for the life of the battery cell.
Constant current charging: The charging and discharging equipment is adjusted to constant current mode, the current remains constant throughout the charging process, and the battery terminal voltage gradually increases with time until it reaches the charging cut-off voltage, and the charging process ends. Constant current charging, if the current setting is relatively small, it will take a long time to charge; if the current is relatively high, the polarization of the battery will be more obvious, and the battery voltage will drop significantly after the charging circuit is removed.
First constant current and then constant voltage: The advantages of constant current charging and constant voltage charging, first set a relatively large current constant current charging, the purpose is to improve charging efficiency; when the power reaches a certain value, it will be converted to constant voltage charging, and the charging current will be Gradually decrease. The purpose is to charge the battery with more power.
Pulse charging: Charging with a large current for a period of time, interrupted by a period of zero current time, this period of time can depolarize the battery part, reduce the power loss during the charging process, and can charge more batteries electricity.
Pulse charging can have many forms, variable current pulse charging, variable voltage pulse charging, and positive and negative pulse charging. Taking variable current pulse charging as an example, the charging current is used as the adjustment object, and the current remains unchanged for a long time, and is interrupted for a short period of time after this period, followed by a period of continuous constant current charging, and so on. In a large trend, the current follows a gradually decreasing law. Variable voltage pulse charging is similar to variable current pulse charging, except that the object of regulation is replaced by voltage. During the charging process, the current decreases according to the law, while the voltage increases according to the law. For positive and negative pulse charging, the interval of zero current time is replaced by negative current. It is said that this is done for better depolarization.
2. Discharge working process
The charge-discharge tester tests battery discharge performance, mainly for different scenarios such as life test, working condition simulation test, capacity test, consistency screening, and other battery parameter tests and safety tests. Different test purposes determine the changing law of current and voltage during the discharge process. The current and voltage requirements can be input into the host computer, and the tester will adjust the power output on demand according to the requirements of the control system.
3. Working principle of charge and discharge tester
The specific implementation forms of the charge and discharge tester vary according to different functional requirements, but basically include the following parts: display or upper computer, controller (including communication function), programmable power module, electronic load, various sensors and other auxiliary components.
The host computer inputs the test intention of the test personnel, displays the test result data and the curve graph drawn according to the result.
The controller, according to the instructions from the host computer, assigns specific tasks such as power supply module, communication module, signal acquisition module, etc., accepts the data uploaded by each part, and processes the data, typical processing such as calculating the SOC of battery charge and discharge;
Programmable power supply module, generally a group of AC-DC power supply devices, adjusts the output current, voltage and power according to the requirements of charging and discharging input data;
Electronic load, during the discharge test, the electric energy released from the battery needs to be consumed through the electronic load;
Sensors generally include temperature sensors, voltage sensors, and current sensors. The sensor accuracy of the charge and discharge tester must be higher than the sensor accuracy selected in the battery pack, otherwise it cannot be used as a device for calibrating the level of the battery management system.
Accuracy and calibration method of charge and discharge tester
Resolution is the percentage of the smallest number (other than zero) that the meter can display to the largest number. For example, the minimum number that can be displayed by a general 3 1/2-digit digital multimeter is 1, and the maximum number can be 1999, so the resolution is equal to 1/1999≈0.05%. The degree to which the measured value is close to the true value is called precision. In addition to being affected by resolution, accuracy is also related to many factors, such as measurement method, ambient temperature, etc.
The general calibration method uses a multimeter, a standard power supply, and a standard resistor with higher precision than the equipment being calibrated for detection, and compares the output value of the equipment with the detection value of the instrument. If the error is within the declared accuracy range, it is qualified.
The current accuracy of the charge and discharge tester distinguishes between high current and low current. For small current test equipment, the method of directly measuring the terminal voltage of the system is adopted; for the large current test system, the method of measuring the terminal voltage of the standard resistor connected to the loop is used for calibration.
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