[Repost] Understand the concept of lithium battery capacity in one article
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Nowadays, when we design electronic products, we often use lithium batteries for power supply. Just like mobile phones or tablets use lithium batteries for power supply, it may be necessary to be familiar with the knowledge of lithium battery capacity for the use and design of lithium battery power supply, including the design of battery chargers. Now many people who pay attention to the discharge of lithium batteries have such an experience, that is, when the lithium battery is discharged from the full voltage of 4.2V to 3.7V, it takes a long time, but once it exceeds 3.7V, it discharges very quickly. Yes, it is true. The following is a summary of some lithium battery materials. First, let's talk about the discharge platform of the battery, which refers to the voltage change state of the battery when the fully charged lithium battery is discharged. When the battery is discharged at a constant current, the battery voltage goes through three processes, namely, drop, stabilize, and drop again. Among these three processes, the stabilization period is the longest. The longer the stabilization time, the higher the discharge platform of the battery. The height of the discharge platform is closely related to the battery manufacturing process. Because the market positioning of various lithium battery manufacturers is different, and the technical process means are different, the discharge platform they control is different, and the quality is also very different. Generally speaking, a 18650 lithium battery has a full voltage of 4.2V. When it is discharged to 3.7V with a current of 1C for 60 minutes, we say that the battery capacity is 2200mAh. During this period, according to the characteristics of the rechargeable battery, a graph is made as follows to better understand the relationship between battery capacity, voltage, current, time and discharge platform: http://5b0988e595225.cdn.sohucs.com/images/20180828/f2480a7603634d09ad1408d7c7737a9d.jpeg [Microsoft YaHei] Schematic diagram of understanding lithium battery capacity and discharge platform Capacity (C) = discharge current × battery discharge platform time For a 18650 lithium battery with a capacity of 2200mAh, it takes 1 hour to discharge 1C to 3.7V, Capacity (C) = 2200mA × 1 hour = 2200mAh Then the problem arises, for better lithium batteries, generally when we do product testing, the voltage will drop quickly after 3.7V, so very little power will be discharged in a short time. On the contrary, when a bad battery is discharged from 4.2V to 3.7V, the voltage drops very quickly, and after 3.7V, the voltage drops very slowly. This kind of battery has poor performance and generally has very low capacity. Then the discharge platform of a good lithium battery is 3.7V. Generally speaking, under constant voltage conditions, stop charging when the voltage is 4.2V and the current is less than 0.01C, and then leave it for 10 minutes. At any rate of discharge current, when the battery is discharged to 3.7V, the length of time it takes for the battery to discharge is an important indicator for measuring the quality of the battery. However, don't blindly pursue a high platform. Sometimes the platform voltage is high, but the capacity is reduced, because the platform voltage is different under different rate conditions. Therefore, the platform problem should be considered from many aspects. A truly good battery must have both high capacity and long duration at a specified voltage. What is the discharge rate? Discharge rate F: [1/hour], which means "N-hour charge (discharge) rate, often only "number" is mentioned, without the unit; F is also called "N-hour charge (discharge) rate", F = 1/(N hours), generally it can be calculated as follows: I = 0.1 X [1/(N hours)] XC For example, the battery capacity is 2200mAh, and it is charged with a current of 0.1C, which is equivalent to: The charging current of the battery is I = 0.1 [1/hour] X2200 [mAh] = 220 [mAh] With the help of the above lithium battery capacity and discharge platform understanding diagram, we can better understand the principle of charging battery capacity and battery discharge platform, which can also be said to measure the high-power working time of the battery. Similarly, the two batteries have the same capacity. After being fully charged, they are assumed to be discharged from 4.2V to 3.7V at the same time, but one is long and the other is short. The battery with a long time has a high platform, which means that the high-voltage time has a long working time. For example, these two batteries are used in mobile phones, and the standby time is the same, but when the two mobile phones make calls together, the battery with a long platform time will have a longer notification time, and the battery with a short platform time will have a shorter talk time. For this diagram, another meaning is also very meaningful for understanding the power management and monitoring of lithium batteries. For example, at present, there are generally two methods for monitoring the power of rechargeable batteries. [/al ign] Method for measuring battery voltage. When the charger detects the battery voltage during charging, it is considered fully charged when the voltage reaches the specified voltage value. For example, when the voltage of a lithium battery is charged to 4.2V, it is considered fully charged. The accuracy of the voltage meter for detecting voltage must reach a precision of plus or minus 1%. Lithium batteries will be damaged due to overcharging. If you want to monitor the charging and discharging of the battery, there are very mature circuits in the book for reference. If you just want to measure it, it is easy to do. Calculate the load resistance according to the capacity, discharge with the standard discharge current, and then detect the voltage. As long as it can reach or approach the nominal discharge time, it will be fine. This method of measuring battery voltage has many shortcomings. For example, for batteries produced by different manufacturers, the relationship between their open circuit voltage and capacity is different. The advantage is that the design cost is relatively low. Another commonly used method for monitoring the remaining power is to estimate the battery usage time in some places where more accurate battery capacity is required to understand the remaining battery capacity and estimate the battery remaining capacity by measuring the net charge flowing into/out of the battery. Integrate the total current flowing into/out of the battery, that is, find the area under the curve in the figure, and the net charge number obtained is the remaining capacity. This method of calculating battery capacity is currently considered to be a relatively accurate method for calculating battery power. Of course, the design cost is also relatively high.
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