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In the era of smart phones, the batteries are not very powerful, so you must take a look before going out. If half of them are not enough, it is very likely that the phone will automatically shut down a few hours later and become a missing person. Therefore, the power function of mobile phones is extremely important. On the one hand, it provides us with power reminder, and on the other hand, it also plays a role in protecting the battery, because excessive discharge and overcharge are harmful to the battery.

How does the phone know how much battery power is left? In fact, there is a power meter inside the battery, which is used to indicate the remaining power in the rechargeable battery and the time when the battery can continue to supply power under specific operating conditions.

There are about three types of electricity meters in the equipment:

Voltage test method

That is to say, the battery power is obtained by simply monitoring the battery voltage. This method is relatively simple, but we can see from the figure below that the battery power and voltage are not linear, so this test method is not accurate, and the power measurement accuracy is only more than 20%. Especially when the battery power is lower than 50%, the power calculation of the mobile phone will become very inaccurate. Therefore, the protection of this method to the battery is very limited.

Discharge curve of lithium battery

Battery modeling method

This method is to establish a data table according to the discharge curve of the battery. The data table will indicate the power values under different voltages. This method can effectively improve the measurement accuracy. However, it is not easy to obtain an accurate data sheet, because the relationship between voltage and power also involves factors such as battery temperature, self discharge, aging, etc. Only by combining many factors to correct can we get a more satisfactory electricity measurement.

Coulometer

The coulometer is used to check the resistance at the positive and negative poles of the battery as a current. When a current flows through the resistance, Vsense will be generated. The current flowing through the battery can be calculated by detecting Vsense. Therefore, it can accurately track the change of battery power, with an accuracy of 1%. In addition, by cooperating with battery voltage and temperature, it can greatly reduce the influence of battery aging and other factors on the measurement results. This method is used in iPhone.

From the above three methods, we can see that the accuracy of the coulometer is the highest. It can reduce the discharge cut-off voltage of the battery without damaging the battery, so that the capacity of the battery can be maximized, especially for low-voltage systems and batteries that have been used for many times. In addition, the warning message of low battery power can be sent with higher accuracy to avoid data loss during use.