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We are now designing electronic products and often use lithium batteries for power supply, just like smartphones or tablets that use lithium batteries for power supply. Familiarity with the knowledge of lithium battery capacity may be necessary for the use and design of lithium battery power supply, including the design of battery chargers.

Many people who pay attention to the discharge of lithium batteries now have the experience that when a lithium battery discharges from a full charge voltage of 4.2V to 3.7V, the time is very long, but once it exceeds 3.7V, it discharges quickly. That's right, it's true. Below, I will review some information about lithium batteries and give you a summary.

1、 Let me first talk about the discharge platform of a battery, which refers to the voltage change state of a fully charged lithium battery during discharge. The constant current discharge of a battery undergoes three processes: decrease, stabilization, and further decrease. Among these three processes, the stabilization period is the longest. The longer the stability time, the higher the discharge platform of the battery. The height of the discharge platform is closely related to the battery manufacturing process. Because different lithium battery manufacturers have different market positioning and technological processes, their controlled discharge platforms and quality vary greatly.

Generally, a 18650 lithium battery has a full voltage of 4.2V. When discharged at 3.7V with a current of 1C for 60 minutes, we can say that the battery's operating capacity is 2200mAh. During this period, based on the characteristics of the rechargeable battery, a graph is made as follows to better understand the relationship between battery capacity, voltage and current time, and discharge platform:

Understanding diagram of lithium battery capacity and discharge platform

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

For a 18650 lithium battery with a capacity of 2200mAh, it takes 1 hour to discharge from 1C to 3.7V,

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

So here's the problem. For better lithium batteries, the voltage usually drops rapidly after 3.7V during product testing, resulting in very little discharge in a short period of time. On the contrary, when discharging from 4.2V to 3.7V, batteries with poor performance experience a rapid decrease in voltage, while after 3.7V, the voltage drops slowly. These batteries have poor performance and generally have very low capacity. The discharge platform for a good lithium battery is 3.7V.

Generally speaking, under constant voltage conditions, when the voltage is charged to 4.2V and the current is less than 0.01C, stop charging, and then let it stand for 10 minutes. At any rate of discharge current, the length of time the battery undergoes discharge to 3.7V is an important indicator of the quality of the battery. However, do not blindly pursue a high platform. Sometimes, when the platform voltage is high, the capacity decreases because the platform voltage varies under different magnification conditions. Therefore, platform issues should be considered from multiple perspectives. A truly good battery requires both high capacity and a long duration of specified voltage.

What is discharge rate?

Discharge rate F: [1/hour], which means the charging (discharging) rate in N hours, often only referring to "number" rather than units; F is also known as the N-hour charging (discharging) rate, F=1/(N hours),

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

For example, when the battery capacity is 2200mAh and charged at a current of 0.1C, it is equivalent to specifying that the charging current for the battery is I=0.1 [1/hour] X2200 [milliampere hour]=220 [milliampere hour]

With the help of the schematic diagram of lithium battery capacity and discharge platform above, it is possible to better understand the principle of charging battery capacity and battery discharge platform. It can also be said that it is a measure of the high-power working time of batteries. Similarly, two batteries with the same capacity are assumed to increase from 4.2V to 3.7V at the same time after being fully charged. However, if one time is long and the other time is short, the longer the battery platform is, the longer the working time of the high voltage is. For example, when these two batteries are used on a mobile phone, the standby time is the same, but when two phones make a call together, the battery with the longer platform time will have a longer notification time, while the battery with the shorter platform time will have a shorter call time

2、 For this diagram, another meaning is also meaningful for understanding the power management and monitoring of lithium batteries.

For example, currently, there are generally two methods for monitoring the battery level of rechargeable batteries.

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. When the voltage of a lithium battery reaches 4.2V, it is considered fully charged. The accuracy of the voltage meter for detecting voltage should reach plus or minus 1%. Damage caused by overcharging of lithium batteries. If you want to monitor battery charging and discharging, there are mature circuits in the book for reference. If it's just a measurement, it's easy to do. Calculate the load resistance based on the capacity, discharge with standard discharge current, and then test the voltage. As long as it can reach or approach the nominal discharge time, it's enough.

The method of measuring battery voltage has many shortcomings, such as the different relationship between open circuit voltage and capacity for batteries produced by different manufacturers. The advantage is that the design cost is relatively low.

Another commonly used method for monitoring remaining battery capacity is to estimate the remaining battery capacity by measuring the net charge flowing into/out of the battery, and to estimate the remaining battery capacity in areas where precise battery capacity is required. Integrate the total current flowing into/out of the battery, i.e. calculate the area under the curve in the graph, and the net charge obtained is the remaining capacity. The method for calculating battery capacity is currently considered a more accurate method for calculating battery power. Of course, the design cost is also relatively high.

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