锂离子细胞的一个特征是一种称为自我放牧的现象。自我放电的概念很简单：拿一个单元格，对其进行充电，测量其开路电压（OCV），并让其与之无关。一段时间后回到单元，您会发现该单元格的OCV较低，表明该单元格在较低的电荷状态（SOC）状态。如果SOC较低，则意味着在细胞中储存的能量的量（即存储的AMP*小时）较低。
The loss of stored energy wasn’t caused by an external load (such as a light bulb or motor), but rather by self-discharge, where the energy was lost as a tiny amount of heat to an internal load inside the cell. This self-discharge, which is sometimes called leakage, reduced the SoC and ultimately reduced OCV. Note that self-discharge is a continuous process, constantly draining the cell and reducing the SoC and OCV.
自我放松的量取决于许多因素：细胞材料/化学，细胞的SOC，细胞温度和细胞的能力。通常，自放电在微型的顺序上。100 mAh的小细胞可能表现出少于10 µA的自分泌物，而100 AH的大细胞可能会表现出500 µA的自分泌物。
发现自我解雇
对于设计由锂离子电池提供动力的电池供电设备的工程师来说，要知道预期的自我放电。如果将电池供电的设备储存很长时间，例如紧急除颤器，则设计师必须能够预测细胞何时会自我递送到他们将不再能够支持生活的地步保存事件。
如果电池供电的设备是电动汽车，则设计师将希望建立一个电池管理系统（BMS），该系统非常准确地了解电池的SOC，以便将最大范围传递给驾驶员。如果细胞由于自我解脱而失去SOC，则需要将这种损失纳入范围计算中。此外，如果自我放电对制造电池供电的工程师很重要，那么对细胞制造商来说也很重要 - 他们必须向其客户提供此信息，这些工程师是设计电池供电设备的工程师。
衡量自我释放概念也很简单（图。1）。使用电压表测量单元的OCV（称为OCV1）。然后，等待了一段时间（调用此t），再次测量OCV（调用该OCV2）。OCV1-OCV2之间的差异使您可以确定SOC变更的量。OCV1和OCV2之间的时间t的差异越长，SOC的变化越大，因为它已被允许在更长的时间内进行自我释放。
1. Step by step description of Delta-OCV method, which includes the measurement method (a), and calculating the equivalent self-discharge current (b).
为了确定自我释放，您首先通过查看OCV1在细胞的OCV与SOC曲线上的位置来确定细胞的SOC。称此SOC1。然后，您查看OCV2，它将告诉您新的SOC。称之为SOC2。SOC = SOC1-SOC2的变化。SOC的这种变化将是少数数字，以AMP*小时为单位（SOC以AMP*小时为单位）。SOC的变化除以测量之间的时间t将是自放电电流。
Delta-OCV方法
The challenges in using this method, called the Delta-OCV method, include accounting for cell relaxation and voltmeter measurement error. Another consideration is that OCV will change due to temperature—this topic will be reserved for a future article, as it has the lowest impact on the Delta-OCV measurement method.
Cell relaxation (resulting from charging or discharging) is another characteristic of the lithium-ion cell. To charge the cell, you use a charger to apply an external voltage that’s higher than the OCV of the cell. The applied external voltage charges the cell by forcing current into the cell, increasing its SoC and therefore its OCV. When you stop charging the cell and remove the external charge voltage, the cell OCV drops slightly as the cell relaxes.
Conversely, to discharge the cell, you use a discharger to apply an external voltage that’s lower than the OCV of the cell. This discharges the cell by drawing current from the cell, lowering its SoC and, therefore, its OCV. When you stop discharging the cell and remove the external discharge voltage, the cell OCV rises slightly at the cell relaxes.
The Delta-OCV method uses the change in OCV to determine self-discharge and fundamentally assumes all of the change in OCV is caused by self-discharge. However, OCV can also change due to cell relaxation as just described. Therefore, to ensure a good Delta-OCV measurement, you must wait until the cell is done relaxing before making the first OCV measurement. If you don’t wait long enough, the change in OCV will be the sum of the effects of cell relaxation and self-discharge, causing the calculation of self-discharge current to be wrong.
A Relaxed State
The challenge is to know how long to wait for the cell to become fully relaxed. This needs to be characterized to be certain. During the first few minutes or hours after charge or discharge, the cell may experience several hundred millivolts of OCV change（图2）。This will interfere with a self-discharge measurement. It could take 10 days for a cell to fully relax and have no change in OCV from relaxation.
确定等待多长时间的经验法则将是将由于自我放松而导致的OCV的预期变化与由于放松而导致的OCV的变化。如果您期望由于自我放电而导致OCV的3 mV变化，那么您应该等到由于放松而导致的OCV变化降至30 µV，或者由于自分会值而导致的预期OCV变化的1％。这可能需要一天或更长时间。特定的放松时间是通过在电荷或放电操作后立即记录其OCV与时间来确定的。然后查看OCV中的变化何时足够小，可以说细胞放松（又称足够休息）。
精确点好细胞
现在，我们知道如何使用Delta-OCV方法进行自放电测量，我们是否可以使用此方法来找到低自我释放的好细胞，并以高自闭症除草不良细胞？
在上面使用的示例中，由于自我放电而引起的三角洲OCV为3 mV。不良细胞将测量3 mV以上，好细胞将为3 mV或更少。这意味着测量精度要求不仅要准确测量3 mV，而且能够准确地测量略多于3 mV和小于3 mV的差异。让我们看看一个例子。
For a 2.4-AH 18650 cylindrical cell, at 80% SoC, we let a good cell rest for eight days. We then measured the change in OCV vs. time, which was 10.4 mV per month, or 431 µV per day. This corresponded to a change of 0.0288 AH per month, or 1.15% SoC per month. The equivalent self-discharge current is 40 µA (= 0.0288 AH/720 hours). We performed a Delta-OCV measurement for seven days, resulting in a change of 3 mV.
We made the same measurements on a bad cell with approximately 100 µA of self-discharge. In this case, the Delta-OCV measurement was 4.3 mV.
这里的测量挑战是，我们正在尝试测量很小的变化（几毫伏）（4.1 V）。因此，要测量OCV，电压表必须在大量范围内进行测量4.1V。通常，这将是5或10-V范围，具体取决于电压表的范围。因此，虽然电压表似乎很容易区分3 mV和4.3 mV，但在10-V范围内准确地进行此操作可能并不那么简单。
A handheld DMM will have around 1 or 2 mV of measurement error when measuring 4 V. This means the handheld DMM isn’t accurate enough to measure the 3-mV Delta-OCV and certainly not accurate enough to accurately observe the 1.3 mV difference between the good and bad cell.
转向更准确的台式DMM，6.5位数的DMM将具有约200 µV的误差，而7.5位数的DMM在测量4 V时将具有约100 µV的误差。of the 4-V cell, it’s still marginal to definitively observe the difference between the good and bad cell.
尽管这些DMM今天用于DELTA-OCV测量值，但您不应对它们对基于或等级的基于单元格值进行分类的能力过多信心。需要的是利用漫长的等待时间（例如14天或更长时间）。这将使细胞能够排出更多，从而导致SOC的更高变化和不同级别细胞之间的OCV差异更大。
3. KeysightBT2152A自解散分析仪requires less than one hour to directly measure the self-discharge current of lithium-ion cells, thus eliminating the need to wait days or weeks for a change in the cell OCV.
总而言之，在概念上确定自我解雇非常容易，因为它仅涉及进行两个OCV测量。但是，如果您的目标是根据其自我放电价值对细胞进行分级，则有必要考虑放松和电压表精度。虽然Delta-OCV是一种可信赖的方法，但有一些较新的方法，例如使用自放电分析仪直接测量自放电电流（图3），这不依赖OCV测量来确定自排放电流。
