The relationship of lithium battery voltage and power

18650 is the originator of lithium-ion battery --a standard lithium-ion battery model that Japan SONY Company set in order to save cost. Among them, 18 means diameter is 18mm, 65 means length is 65mm, 0 means cylindrical battery.

The common 18650 battery is divided into a lithium ion battery and a lithium iron phosphate battery. The lithium-ion battery voltage is 3.7V, the charge cut-off voltage is 4.2v, the lithium iron phosphate battery has a nominal voltage of 3.2V, the charge cut-off voltage is 3.6v, the capacity is usually 1200mAh-3350mAh, and the common capacity is 2200mAh-2600mAh.

18650 lithium battery use

The 18650 battery life theory is cyclic charging 1000 times. Due to the large capacity per unit density, most of them are used in laptop batteries. In addition, the stability of the 18650 is very good at work. It is widely used in various electronic fields: it is often used in high-end glare flashlights. Power, wireless data transmitters, electric warm clothing, shoes, portable instrumentation, portable lighting equipment, portable printers, industrial equipment, medical equipment, etc.

Using voltage to estimate the remaining capacity of the battery has the following instabilities:

1. The same battery, in the case of the same remaining capacity, the voltage value changes due to the magnitude of the discharge current.

The higher the discharge current is, the lower the voltage. In the absence of current, the voltage is highest.

2. The effect of ambient temperature on the battery voltage. The lower the temperature is, the lower the battery voltage of the same capacity.

3. The effect of cycling on the battery discharge platform,

As the cycle progresses, the discharge platform of the lithium ion battery tends to deteriorate. The discharge platform is lowered. Therefore, the capacity represented by the same voltage also changes accordingly.

4. Different manufacturers, different capacity lithium-ion batteries, the discharge platform is slightly different.

5. Lithium-ion batteries of different types of electrode materials have large differences in discharge platforms. The discharge platforms of cobalt lithium and manganese lithium are completely different.

All of the above will cause voltage fluctuations and voltage differences, making the battery capacity display unstable.

When using voltage to measure the battery capacity of a mobile phone, it is impossible to keep the phone in low current standby state all the time. Temporary losses of large current, such as turning on the backlight, playing a ringtone, and especially passing through, can quickly reduce the battery voltage. At this time, the capacity displayed by the mobile phone has to be reduced more than the actual capacity. When the high current is removed, the battery voltage goes back up. This will cause the unreasonable phenomenon that the display of mobile phone capacity increases instead.

One useful feature of lithium-ion batteries is that, when they are discharged, the voltage of the battery decreases with the passage of the charge and has a fairly steep slope. This gives us another approximate metering method and the method of taking the battery voltage. It's like measuring the height of the water in a tank gives you an idea of how much water is left. But actually the voltage of the lithium battery is much more complicated than measuring the height of the calm water in the tank. The voltage used to estimate the residual capacity of a battery has the following instability: for the same battery, the voltage value varies with the discharge current under the same residual capacity. The higher the discharge current is, the lower the voltage. In the absence of current, the highest voltage; The effect of ambient temperature on battery voltage, the lower the temperature, the lower the battery voltage of the same capacity; The influence of the cycle on the discharge platform of the battery, with the progress of the cycle, the discharge platform of the lithium ion battery tends to deteriorate. Discharge platform lowered. So the capacity of the same voltage changes accordingly; Different manufacturers, different capacity of lithium ion batteries, the discharge platform slightly different.

Lithium-ion batteries of different types of electrode materials have large differences in discharge platform. The discharge platforms of cobalt lithium batteries and manganese lithium batteries are completely different. All of the above will cause voltage fluctuations and voltage differences, making the battery capacity display unstable. When using a voltage to measure the battery capacity on a mobile phone, it is impossible for the mobile phone to remain in a standby state with a small current. Temporary loss of high current, such as turning on the backlight, ringing the ring, especially through, will cause the battery voltage to drop quickly. At this point, the capacity displayed by the phone is reduced more than the actual capacity. When the large current is removed, the voltage of the battery will rise. This will cause the unreasonable phenomenon that the capacity display of the mobile phone will increase instead.

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.

Generally, 18650 lithium batteries have a full voltage of 4.2V. When it is discharged to 3.7V with a current of 1C and put it for 60 minutes, then we say that the battery capacity is 2200mAh, during which time according to the characteristics of the rechargeable battery. , make a picture below, can better understand the relationship between battery capacity and voltage and current time and discharge platform

Method of 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 should be equal to plus or minus 1% accuracy. 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 will be easier to calculate the load resistance according to the capacity, discharge with the standard discharge current, then detect the voltage, as long as it can It is sufficient to reach or approach the nominal discharge time.

There are many shortcomings in the method of measuring the battery voltage, such as the relationship between the open circuit voltage and the capacity of the battery produced by different manufacturers. 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 by estimating 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.

The page contains the contents of the machine translation.