Read the concept of lithium battery capacity in one article
Time：2020.08.27
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We design electronic products now, and they often use lithium batteries to power them. Just as mobile phones or tablets use lithium batteries to power them, we are familiar with the knowledge of lithium battery capacity. Perhaps for the use and design of lithium battery power, including the design of battery chargers, Needed.
Many people who are concerned about the discharge of lithium batteries now have the experience that when the lithium battery is discharged from a full voltage of 4.2V to 3.7V, it takes a long time, but once it passes 3.7V, it will discharge quickly, yes, indeed such.
Let's go through some information about some lithium batteries and give you a summary.
Let me talk about the discharge platform of the battery first, which refers to the state of the voltage change 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, falling, stable, and then falling. Among these three processes, the stable period is the longest. The longer the stabilization time, 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 it controls is different, and the quality is also very different.
Generally, a 18650 lithium battery has a full voltage of 4.2V. When discharged with a current of 1C, it is placed at 3.7V for 60 minutes. Then we say that the battery's capacity is 2200mAh. During this time, according to the characteristics of the rechargeable battery , Make a graph as follows, you can better understand the relationship between battery capacity and voltage and current time and discharge platform:
Lithium battery battery capacity and discharge platform understanding diagram
Lithium battery battery capacity and discharge platform understanding diagram
Capacity (C) = discharge current × battery discharge platform time
For an 18650 lithium battery with a capacity of 2200mAh, it takes 1 hour to discharge from 1C to 3.7V.
Capacity (C) = 2200mA × 1 hour = 2200mAh
Then the question is coming. For a better lithium battery, the voltage drops rapidly after 3.7V when we do product testing, so the amount of electricity discharged in a short time is very little.
On the contrary, when a battery that is not good is discharged from 4.2V to 3.7V, the voltage drops quickly, but after 3.7V, the voltage drops slowly. The general capacity of this battery is not good, and its capacity is very low. The discharge platform of a good lithium battery is 3.7V.
Generally speaking, under a constant voltage condition, when the voltage is 4.2V and the current is less than 0.01C, the charging is stopped, and then set aside for 10 minutes. At any rate of discharge current, when the battery is discharged to 3.7V, the battery experience A length of time is an important indicator to measure the quality of the battery.
However, do not blindly pursue a high platform, sometimes the platform voltage is high, but the capacity has dropped, because, under different magnification conditions, the platform voltage is different, so the issue of the platform should be considered from many sides. Only a high capacity and a long duration of the specified voltage can be considered a really good battery.
What is the discharge rate?
Discharge rate F: [1/hour], meaning "N-hour charge (discharge) rate, often just say "number", not unit; F is also called "N-hour charge (discharge) rate", F =1/(N hours), generally can be calculated like this: I=0.1 X [1/(N hours)] XC
For example, when the battery capacity is 2200mAh, charging with a current of 0.1C is equivalent to clear: the charging current for the battery I=0.1[1/hour]X2200[mA time]=220[mA]
With the help of the lithium battery battery capacity and discharge platform understanding diagram above, you can better understand the principle of rechargeable battery capacity and battery discharge platform. It can also be said to measure the high-power working time of the battery. The same two battery capacities are the same. After being fully charged, it is assumed that it will be changed from 4.2V to 3.7V at the same time, but one long time and one short time, that is, the long battery platform is high, that is, the high voltage time is long working time, such as these two batteries are used on mobile phones, standby The time is the same, but two phones make a call together, the battery notification time for a long platform time will be longer, and the battery talk time for a shorter platform time will be two. For this figure, another meaning is to understand the power management monitoring of lithium batteries. It also makes sense.
For example, at present, there are generally two methods for monitoring the power of rechargeable batteries.
Method for measuring battery voltage. When the charger detects the battery voltage during charging, it is considered to be fully charged when the voltage reaches the specified voltage value. For example, when the voltage of the lithium battery reaches 4.2V, it is considered fully charged. The accuracy of the voltmeter that detects the voltage should reach an accuracy of plus or minus 1%. The lithium battery is damaged due to overcharge. If you want to monitor the battery charge and discharge, there is a very mature circuit in the book for reference. If it is just a measurement, it is easy to handle. Calculate the load resistance according to the capacity and discharge it with a standard discharge current. Then check the voltage again, as long as it can Just reach or approach the nominal discharge time.
This method of measuring battery voltage has many shortcomings. For example, for batteries produced by different manufacturers, the relationship between open circuit voltage and capacity varies. The advantage is that the design cost is relatively low.
There is also a commonly used method for monitoring the remaining battery capacity. In some places where more accurate battery capacity is required, the remaining battery capacity is used to estimate the battery usage time, and the remaining battery capacity is estimated 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 obtained is the remaining capacity.
This method of calculating battery capacity is currently considered to be a relatively accurate method of calculating battery capacity. Of course, the design cost is relatively high.
