What you'll learn:
- What type of equipment is best to use for low-voltage test of battery cells?
- What is an electronic load, and how does it work?
- 如何通过低压分离测试克服主要限制。
电池放电的时候,有时desirable to discharge the cells to a low voltage. Under normal operation, care must be taken to prevent discharging the cell below the safe operating voltage of the cell. However, when testing cells, the purpose of the test may be to characterize cell behavior when pushed below normal operating voltages.
In this article, I will focus on lithium-ion cells whenever I use examples of voltage thresholds, but the challenges and solutions to low-voltage testing are the same—independent of the cell chemistry.
指我的锂离子电池的示例，最小放电电压是当电池的开路电压（OCV）撞击与0％电荷状态（SOC）相关的电压时。虽然每种锂离子化学制剂（例如磷酸铁或镍锰钴）的最小OCV不同，但该数字通常在2.5V范围内。一些化学物质可以降低到2 V甚至更低。
Depending on the nature of the test, you may want to keep within the safe operating voltage or drive the cell down lower in voltage. For example, if you’re performing a cell cycle-life test, the goal is to properly cycle the cell between charged and discharged states. This determines how long the cell will survive before it loses its ability to adequately hold a charge.
对于此测试，您将需要将单元保持在其正常操作的OCV范围内，以防止由于过度收取或过度充电而损坏单元。相比之下，对于安全测试，您可能需要故意将细胞超出其最小OCV的表征，以表征其行为并确定何时损坏或危险。
Low-Voltage-Discharge Test Equipment Choices
要进行这些测试，您将需要能够将电池排放到低压的测试设备。在电池表征实验室中，例如在电动汽车OEM中发现的，您将测试许多细胞并进行许多测试，因此有必要专用的交钥匙细胞测试设备。
对于某些用户，例如小型物联网设备的设计团队，细胞测试可能只是他们总体设计任务的一小部分。对于这个实验室，专用的交钥匙细胞测试仪可能不值得。取而代之的是，可以将标准的台式电源和电子负载用于偶尔的细胞测试。Table 1describes various types of test equipment that can be used.
FromTable 1, you can see that there are two cases for test equipment（表2）。
For case 2, what is the cause of this limitation on low-voltage operation? Why can the more expensive equipment for case 1 operate down to 0 V but not for case 2? The answer is related to how power electronics operate to produce negative current to discharge cells.
How Does an Electronic Load Work?
从根本上讲，电池测试系统中的电子负载或单元格电路必须显示电池上的负载，以将电流从电池中拉出。以最简单的形式，该负载是电阻器。由于欧姆定律，通过负载电阻的电流将是电阻跨电阻的函数。因此，随着电池放电的排放，电池电压将下降，并且通过电阻的电流会降低，从而导致非恒定电流负载条件。
Therefore, in practice, no cell test equipment company would propose to use a resistor, as they’re not programmable, and they cannot produce a constant current load on the cell.
取而代之的是，以最简单的形式，测试设备使用晶体管，通常是场效应晶体管（FET），将其排放（图。1）。FET看起来像是可以通过对照电压变化的电阻。使用电流电阻器（即电流分流器）测量通过FET的电流。测得的电流作为对放电控制电路的反馈信号提供，以便可以将电流调节为固定的恒定值。
The discharge control circuit adjusts the resistance of the FET to maintain the required constant current out of the cell independent of the cell’s voltage, creating a variable resistor that’s controlled to draw regulated constant current from the cell during a test. The power being drawn from the cell is the product of the voltage of the cell times the current being drawn from the cell.
For some test systems that are either lower in power or less sophisticated, the power being drawn from the cell is dissipated as heat. In more sophisticated systems, the power is captured and regenerated back to the ac line.
Limitations and Solutions for Low-Voltage-Discharge Testing
While this FET load method has worked quite well for years, it suffers from a limitation. As the discharge control circuit wants to draw more current, it programs the FET to have lower and lower resistance. Eventually, the FET’s resistance cannot be “turned down” any further—it has reached its minimum resistance, known as the full-on resistance of the FET.
Once the FET reaches this minimum resistance, it just operates as a fixed, very low resistance. As the voltage of the cell goes toward 0 V, the maximum current that can be drawn drops linearly because of Ohm’s Law:
Max_Available_Load_Current = Cell_Voltage / FET_Full-on_Resistance
这种行为在循环器通道或电子负载的放电特征中可见（图2）。
如果您需要使用基本循环仪或电子负载以2 V低于2 V，则有一个解决方案。可以将增强供应与电池的输出一起串联（图3）。This boost supply adds the required voltage to keep the input voltage to the cycler or electronic load above this minimum operating voltage such that the discharge control circuit never has to push the FET to its full-on resistance state.
The boost supply can be complex to configure and adds extra cost to the test setup. But if you must test at low voltage and you’re limited to the equipment you have available, this may be a viable choice. In a future article, I will cover how to configure a boost supply to allow for low-voltage operation.
对于情况1，在测试设备可以达到0 V甚至负电压的情况下，增强电源有效地内置在测试人员体系结构中。因此，由于其内部增强供应，通过设计，单元格测试通道或SMU可以通过设计实现低压操作，而无需外部增强供应。尽管这为您提供了最大的灵活性，但这确实意味着测试人员更加复杂，并且会带来更高的成本。
Summary
If low-voltage-discharge testing is required by your test protocol, it’s recommended to buy test equipment that supports low-voltage-discharge testing below 2 V. For cycle-life testing, where your test protocol never requires low-voltage-discharge testing, you can achieve a measurable savings by selecting equipment that doesn’t support operation below 2 V and doesn’t have a built-in boost supply.
