Battery capacity of lithium batteries

We're designing electronics, and we're using lithium batteries a lot of the time. Just as cell phones or tablets use lithium batteries, familiarity with lithium-ion battery capacity may be necessary to use and design lithium-ion batteries, including battery chargers.
Now many people who pay attention to lithium battery discharge have the experience that when lithium battery discharge from 4.2V to 3.7V full voltage, it takes a long time, but after 3.7V it discharges quickly, yes, it does.
Here are some information about lithium batteries.
First of all, the battery discharge platform, that is, charged lithium batteries in the discharge, battery voltage changes.
Battery constant current discharge, battery voltage to go through three processes, namely, decline, stability, and then decline, in these three processes, the stability 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 the market positioning of each lithium battery manufacturer is different, the technical means are different, the discharge platforms controlled by them are 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 at 3.7V with a current of 1C for 60 minutes, we say that the capacity of the battery is 2200mAh. During this period of time, according to the characteristics of the recharged battery, a diagram is drawn as follows, which can better understand the relationship between the battery capacity, voltage and current time and the discharge platform. :
Schematic diagram of lithium battery capacity and discharge platform understanding
Capacity (C) = discharge current * battery discharge platform time
For 18650 lithium batteries with 2200mAh capacity of 1, 1C will be discharged to 3.7V for 1 hours.
Capacity (C) =2200mA * 1 hours =2200mAh
So the problem is, for better lithium batteries, the voltage drops quickly after 3.7V when we do the product test, so the amount of power is very small in a short time.
On the contrary, when a bad battery is discharged between 4.2V and 3.7V, the voltage drops very quickly, but after 3.7V, the voltage drops very slowly. This kind of battery is generally of poor performance and very low capacity. A good lithium battery discharge platform is 3.7V.
Generally speaking, under the condition of constant voltage, when the charge voltage is 4.2V, the current is less than 0.01C, stop charging, and then shelve for 10 minutes, at any rate of discharge current, discharge to 3.7V, the battery discharge experience a length of time, is an important indicator of battery quality.
However, do not blindly pursue high platforms, sometimes the platform voltage is high, but the capacity is reduced, because, under different magnification conditions, the platform voltage is different, therefore, the problem of the platform should be considered from many aspects. It requires high capacity and long duration of specified voltage to be a real good battery.
What is discharge rate?
Discharge rate F: [1/hour], meaning "N-hour charge (discharge) rate, often only said"number", not units; F is also known as"N-hour charge (discharge) rate ", F = 1/(N-hour), generally can be calculated as follows: I = 0.1 X [1/(N-hour)]
For example, a battery with a capacity of 2200 mAh charged at a current of 0.1C is equivalent to a clear charge current of 0.1 [1/h] X2200 [mA] = 220 [mA]
With the above lithium battery capacity and discharge platform to understand the schematic diagram, can better understand the charging battery capacity and battery discharge platform, also can be said to measure the battery high power working time, the same two batteries capacity is the same, charged assumed at the same time from 4.2V to 3.7V, but a long time, A short time, that is, a long battery platform is high, that is, a long time to work high voltage time, such as the two batteries used on the phone, standby time is the same, but two mobile phones together, long platform time battery notification time will be long, short platform time battery call time will be two, for this figure, another Meaning is also significant for understanding the power management monitoring of lithium batteries.
For example, at present, there are two ways to monitor the power of rechargeable batteries.
Method of measuring battery voltage. When the charger is charging, the battery voltage is detected. When the voltage reaches the specified voltage value, it is considered to be full. If the lithium battery voltage is charged to 4.2V, it is considered full. The accuracy of the voltmeter for detecting voltage is up to 1% accuracy. Because lithium battery is overcharged, it will be damaged. If you want to monitor battery charging and discharging, the book has a very mature circuit for reference, if only to measure it, then it is easy to do, according to the capacity of the load resistance calculation, discharge with standard discharge current, then detect voltage, as long as it can reach or close to the nominal discharge time.
There are many deficiencies in the measurement of battery voltage. For example, for different manufacturers, the relationship between open circuit voltage and capacity is different. The advantage is that the design cost is relatively low.
Another common method of monitoring residual capacity is to estimate the remaining capacity of a battery by measuring the net charge flowing in and out of the battery, where more accurate capacity is required. Integrating the total current flowing into and out of the battery, i.e. calculating the area under the curve in the diagram, the net charge number obtained is the residual capacity.
This method of calculating battery capacity is currently considered to be a more accurate method of calculating battery power. Of course, the design cost is relatively high.