Why 3.7V lithium batteries? You know what?
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2022/05/27 15:37:08
We now design electronic products, many times also use lithium battery power, the same as mobile phones or tablet computers with lithium battery power, familiar with the knowledge of lithium battery capacity, perhaps for the use and design of lithium battery power including the design of battery charger, it is necessary.
Now a lot of people who pay attention to the discharge of lithium battery, have such experience, that is, when the lithium battery from the full voltage of 4.2V discharge to 3.7V, the time is very long, but once the 3.7V discharge is very fast, yes, indeed. Read some of the following lithium battery information, to make a summary.
First, the discharge platform of the battery, which refers to the voltage change of the battery when the lithium battery is full of electricity is discharged. Battery constant discharge, battery voltage to go through three processes, namely drop, stability, and then drop, in these three processes, the stability period is the longest. The longer the stability time, the higher the discharge platform of the battery. The level of discharge platform is closely related to battery manufacturing technology. It is because the market positioning of various lithium battery manufacturers is not the same, the technical process means are different, the discharge platform is not the same, the quality is very different.
Generally, a 18650 lithium battery with a full voltage of 4.2V, when discharging at 1C current to 3.7V for 60 minutes, then we can say that the capacity of the battery is 2200mAh. During this period of time, according to the characteristics of the rechargeable battery, we can make a figure as follows to better understand the relationship between battery capacity, voltage and current time and discharge platform:
Schematic diagram of understanding lithium battery capacity and discharge platform
Capacity (C) = Discharge current x battery discharge platform time
For an 18650 lithium battery with a capacity of 2200mAh, discharging from 1C to 3.7V takes an hour,
Capacity (C) =2200mA x 1 hour =2200mAh
Then the problem comes, for better lithium batteries, generally when we do product testing after 3.7V voltage drop quickly, so in a short period of time put very little power. On the contrary, when bad batteries discharge from 4.2V to 3.7V, the voltage drops quickly, and after 3.7V, the voltage drops slowly, this kind of battery is not good performance and the general capacity is very low. So a good lithium battery discharge platform is 3.7V.
In general, under constant voltage conditions, charge to 4.2V voltage, current is less than 0.01c stop charging, and then use for 10 minutes, at any rate of discharge current, discharge to 3.7V, the length of time experienced by the battery discharge, is an important indicator to measure the quality of the battery. However, do not blindly pursue the high platform, sometimes the platform voltage is high, but the capacity is reduced, because, under different conditions, the platform voltage is different, therefore, the platform problem should be considered from multiple aspects. A really good battery requires both a high capacity and a long duration at a specified voltage.
What is the discharge rate?
Discharge rate F: [1/ hour], meaning "N hour charge (discharge) rate, often only said" number ", rather than say unit; F also called "N hour charge (discharge) rate", F=1/ (N hour),
I=0.1X [1/ (N hours)] XC
For example, when the battery capacity is 2200mAh and the current is 0.1c, it is equivalent to clear: the charging current of the battery I=0.1 [1/ hour] X2200 [milliampere hour] =220 [milliampere]
With the aid of the lithium batteries battery capacity and discharge platform above understanding schematic diagram, can better understand the rechargeable battery capacity and battery discharge platform, also can saying is to measure high power battery working time, two battery capacity is the same, the same after a full charge assumptions at the same time from 4.2 V to 3.7 V, but a long time, a short time, For example, if these two batteries are used in mobile phones, the standby time is the same. However, if two mobile phones make a call together, the battery notification time will be long if the platform time is long, and the battery talk time will be short if the platform time is short
Second, for this diagram, another meaning is also significant for understanding the power management monitoring of lithium batteries.
For example, at present, there are two kinds of monitoring methods for rechargeable batteries.
Method for measuring battery voltage. When the charger detects the battery voltage while charging, it is considered full when the voltage reaches the specified value. If the voltage of lithium battery is charged to 4.2V, it is considered full. The accuracy of the voltage meter should reach plus or minus 1%. Lithium battery damage due to overcharging. If you want to monitor the battery charge and discharge, the book has a very mature circuit can refer to, if you just measure it, it will be easy to do, according to the capacity to calculate the load resistance, discharge current discharge with the standard, then detect the voltage, as long as it can reach or close to the nominal discharge time.
This method of measuring battery voltage has many disadvantages, such as the relationship between open circuit voltage and capacity varies from manufacturer to manufacturer. The advantage is that the design cost is relatively low.
Another common way to monitor battery capacity is to estimate the remaining capacity of a battery by estimating the life of the battery by measuring the net charge flowing in/out of the battery in places where precise battery capacity is required. By integrating the total current flowing into/out of the battery, which is the area under the curve in the figure, the net charge is the remaining capacity. This method of battery capacity calculation is currently considered to be more accurate method of battery power calculation. Of course, the design cost is also relatively high.
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