The meaning of lithium battery capacity

We now design electronic products, many times also use lithium battery power supply, the same as the phone or tablet computer with lithium battery power supply. To be familiar with the knowledge of lithium battery capacity, perhaps for the use and design of lithium battery power supply, including the design of battery charger, is very necessary.

Nowadays, many people who pay attention to the discharge of lithium batteries have the experience that when the lithium battery is discharged from the full voltage of 4.2V to 3.7V, the time is very long, but once it passes 3.7V, the discharge is very fast, yes, indeed. Let's take a look at some of the lithium battery information for a summary.

First, let's talk about the discharge platform of the battery, which means that the voltage of the battery changes when the fully charged lithium battery is discharged. The battery has a constant current discharge, and the battery voltage has to go through three processes, namely, falling, stabilizing, and then falling. In these three processes, the stable period is the longest. The longer the stabilization time is, the higher the discharge platform of the battery. The level of the discharge platform is closely related to the battery manufacturing process. It is because the market positioning of each lithium battery manufacturer is different, the technical process is different, the discharge platform controlled by it is different, and the quality is very different.

Capacity (C) = discharge current × battery discharge platform time

For a 18650 lithium battery with a capacity of 2200mAh, 1C discharges to 3.7V for 1 hour.

Capacity (C) = 2200mA × 1 hour = 2200mAh

Then the problem comes. For a better lithium battery, the voltage will drop very quickly after 3.7V when we do the product test, so the power will be discharged in a short time. On the contrary, when the battery is discharged from 4.2V to 3.7V, the voltage drops rapidly, and after 3.7V, the voltage drops slowly. The battery is not good in performance and the general capacity is very low. The discharge platform of a good lithium battery is 3.7V.

In general, under constant voltage conditions, when the charge is 4.2V, the current is less than 0.01C the battery is stopped, and then left for 10 minutes. At any rate of discharge current, when discharged to 3.7V, the battery is discharged. A length of time is an important indicator of how good or bad a battery is. However, do not blindly pursue high platforms. Sometimes the platform voltage is high and the capacity is reduced. Because the platform voltage is different under different magnification conditions, the problem of the platform should be considered from many sources. It is a good battery if it has a high capacity and a long duration of the specified voltage.

What is the discharge rate?

Discharge rate F: [1/hour], meaning "N hour charge (discharge) rate, often only say "number", not the unit; F also called "N hour charge (discharge) rate", F =1/(N hours),

Generally can be calculated as follows: I = 0.1X [1/ (N hours)] XC

For example, the battery capacity is 2200mAh, and the current is charged at 0.1C, which is equivalent to: the charging current of the battery I=0.1[1/hour]*2200[MAH]=220[MA]

With the above lithium battery capacity and discharge platform understanding diagram, it is better to understand the capacity of the rechargeable battery and the discharge platform of the battery. It can also be said to measure the high power of the battery. The same two battery capacities are the same. After fully charged, it is assumed to be put from 4.6V to 3.7V at the same time, but one time is long, one time is short, that is, the battery platform with long time is high, that is, the time of high voltage is long, such as these two batteries are used on mobile phones, standby The time is the same, but the two mobile phones are calling together, the battery notification time of the platform is long, and the battery talk time of the platform time is short.

Second, another meaning is also very meaningful for understanding the battery management monitoring of lithium batteries.

For example, at present, there are generally two methods for monitoring the power of a rechargeable battery.

Method for measuring battery voltage, when the charger detects the battery voltage while charging, it is considered full when the voltage reaches the specified voltage value. If the voltage of the lithium battery is charged to 4.2V, it is considered full. The accuracy of the voltmeter for detecting voltage is to achieve an accuracy of plus or minus 1%. The lithium battery is damaged due to overcharging. If you want to monitor the battery charge and discharge, there are very mature circuits in the book for reference. If you just measure it, then it is easy to handle. Calculate the load resistance according to the capacity and discharge it with the standard discharge current. At this time, check the voltage, as long as it can It is enough to reach or approach the nominal discharge time.

There are many shortcomings in the method of measuring the battery voltage. For example, for batteries produced by different manufacturers, the relationship between the open circuit voltage and the capacity is different. The advantage is that the design cost is relatively low.

Another common method of monitoring the remaining capacity is to estimate the remaining battery capacity in some places where more accurate battery capacity is required. Estimate the battery life and the remaining battery capacity by measuring the net charge flowing into/out of the battery. The total current flowing into/out of the battery is integrated, that is, the area under the curve in the figure is obtained, and the obtained net charge number is the remaining capacity. This method of calculating the battery capacity is currently considered to be a more accurate method of calculating the battery power. Of course, the design cost is also relatively high.

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