Lithium-ion battery capacity calculation - voltage method

Analysis of the corresponding relationship between the open circuit voltage and battery capacity of lithium-ion batteries

First, a table is given: as follows, the percentage is the remaining capacity of the battery, and the right side is the corresponding open circuit voltage (OCV) of the battery.
100%----4.20V
90%-----
4.06V 80%
-----3.98V
70%-----3.92V 60%-----3.87V 50%
-----3.82V
40
%-----3.79V 30%
-----3.77V 20%-----3.74V
10%-----3.68V
5%------3.45V
0%------3.00V

The following is the ins and outs of this table.

1. First, some concepts are explained:

1.OCV: abbreviation of open circuit voltage.

2.Lithium-ion battery: This article discusses the single-cell lithium-ion battery that is commonly used in mobile phones and is charged at a constant voltage of 4.2V.

3.mAh: a unit of measurement for battery capacity, which is actually the total number of electrons in the battery that can be released for external use.

The standard unit in physics is the familiar coulomb.

The SI unit of coulomb is the ampere-second, which is the current multiplied by the time.

1mAh = 0.001 ampere * 3600 seconds = 3.6 ampere seconds = 3.6 coulombs

mAh is not a standard unit, but it can be conveniently used for measurement and calculation.

For example, a 900mAh battery can provide 300mA constant current for 3 hours.

4. Fuel gauging: Fuel gauging originally meant fuel measurement, but later it was used in electrochemistry to mean fuel measurement.

The most scientific and original battery power measurement method is to count the amount of electrons flowing through it. That is, the coulomb count.

★To get the correct usage of lithium-ion batteries, you have to use a coulomb count. It is just like the working principle of the water meter used to measure the amount of water in your home. You have to calculate the amount of charge flowing through to get the usage of lithium-ion batteries.

2. Relationship between voltage and capacity

However, lithium-ion batteries have a very useful characteristic for power measurement, that is, when discharging, the battery voltage will gradually decrease with the loss of power, and there is a considerable slope. This provides us with another approximate way to measure power. The method of taking the battery voltage is just like measuring the height of the water level in a water tank to roughly estimate the remaining water volume. But in fact, the battery voltage is much more complicated than measuring the calm water level in a water tank.

There are several instabilities in using voltage to estimate the remaining capacity of the battery:

1. For the same battery, with the same remaining capacity, the voltage value varies with the size of the discharge current.

The greater the discharge current, the lower the voltage. When there is no current, the voltage is the highest.

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

3. The impact 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 decreases. Therefore, the capacity represented by the same voltage also changes accordingly.

4. Lithium-ion batteries of different manufacturers and capacities have slightly different discharge platforms.

5. The discharge platforms of lithium-ion batteries with different types of electrode materials are quite different. 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 a mobile phone uses voltage to measure the battery capacity, it is impossible for the mobile phone to be in a low current standby state all the time. Temporary high current loss, such as turning on the backlight, playing ringtones, especially passing through, will cause the battery voltage to drop quickly. At this time, the capacity displayed by the mobile phone will be reduced more than the actual capacity. When the high current is removed, the battery voltage will rise again. This will cause the unreasonable phenomenon that the mobile phone capacity display increases instead.

3. Table of battery voltage and battery capacity

Having said so much, here is a table of standard voltage and battery remaining capacity (left side)

And the high current constant current discharge is a table of battery voltage to capacity (right side)

standard conditions description:

1. Room temperature

2. New batteries

3. After fully charged, perform GSM simulated discharge

4. When measuring the battery voltage, turn off the discharge circuit and measure the battery open circuit voltage. Eliminate the influence of discharge current on voltage.

5. Choose cobalt-lithium batteries, because most mobile phones currently use cobalt-lithium batteries. Manganese-lithium batteries are rare.

High current constant current discharge conditions description:

1. Room temperature

2. New batteries

3. After fully charged, discharge at a constant current of 550mA

4. When measuring the battery voltage, do not shut down the discharge circuit and measure the actual working voltage of the battery. Carry the influence of discharge current on the voltage.

5. Choose cobalt-lithium batteries, because most mobile phones currently use cobalt-lithium batteries. Manganese-lithium batteries are rare.

★★★★★
The specific data is as follows, divided into 10% capacity intervals

OCV voltage to capacity relationship550mA constant current discharge voltage to capacity relationship100
%----4.20V 100%----4.20V
90%-----4.06V 90%-----3.97V
80%-----3.98V 80%-----3.87V
70%-----3.92V 70%-----3.79V▲
60%-----3.87V 60%-----3.73V
50%-----3.82V 50%-----3.68V
40%-----3.79V▲ 40%-----3.65V
30%-----3.77V 30
%-----3.62V 20%-----3.74V 20%-----3.58V
10%-----3.68V 10%-----3.51V
5%------3.45V 5%------3.42V
0%------3.00V 0%------3.00V
★★★★★

The above data is the statistics of my three-year lithium-ion battery test. Because there are too many factors affecting battery voltage, the data in the table is for your reference only.

Note that the battery voltage and capacity are not linearly related. There is no other formula that can be applied. The mobile phone can only prepare a corresponding table to compare the actual measured voltage to approximate the battery capacity.

The OCV discharge voltage of lithium-ion batteries has a faster decline slope between 4.20V and 3.90V.

There is a relatively gentle discharge platform around 3.8V

After falling below 3.7V, the voltage drops sharply to 3.0V as the capacity decreases.

The discharge platform of the comparative large current discharge (1C) appears at around 3.65V. As

can be seen from the comparison table.

If the phone is in a high current discharge state for a long time, when the voltage drops to 3.79V, the phone thinks that it only has 40% (refer to the left) of power. In fact, the battery should still have 70% of power (refer to the right). When the phone re-enters the low current standby mode, the battery voltage will rise back to 3.92V (70%) on the left. Then the capacity display of the phone will rebound.

4. Specific application of using voltage measurement method to measure battery capacity on mobile phones:

1. The power display designed for mobile phones is set according to the relationship between voltage and capacity.

2. Commonly used power grids have three or four grids. Let's take the four-grid power display as an example.

When designing each mobile phone, the amount of power represented by each bar is not necessarily the average 25%.

The actual situation is that the first box may represent 50% or more, and the second box represents 20%. This is an uneven distribution.

The first bar of the Bird S1200 represents more than 60% of the previous power.

The first bar of Xiaxin A8 represents the first 70% of the battery.

Other Samsung phones and Philips phones have their own grid definitions.

Of course, this principle is also used to display percentages (like the percentage above).

3. However, as long as the mobile phone uses the principle of voltage to measure capacity, the power will inevitably have the following disadvantages

The accuracy (resolution) of the battery level is not high. Three, four or 10 grids (like the percentage above).

The battery level may change repeatedly, for example, it will drop from two bars to one bar after a phone call, and then rebound to two bars after a while.

4. When sampling the battery voltage, the mobile phone will generally try to use the voltage when the current consumption is low (such as when in standby mode).

The voltage obtained at this time is more valuable for reference.

Moreover, the mobile phone software will use the "multi-point sampling and average calculation" algorithm to avoid the interference of instantaneous current on voltage.

5. The voltage threshold for setting the capacity grid in the mobile phone is fixed.

As the battery cycles or when a battery with a poor discharge platform is selected, the corresponding remaining capacity will vary greatly. For example, for a battery with a high discharge platform, the first grid represents a capacity that can be used for 3 days, while a battery that has been cycled 100 times and a poor quality battery.

The first bar of power can only be used for 1 to 2 days. There is even a strange phenomenon that the battery can still be on standby for 2 days when the battery is empty.

6. When dealing with voltage rebound, some mobile phones will use an algorithm to lock the capacity grid. That is, the capacity display will not rebound, but the usage time of the current capacity grid will be relatively extended.

7. Some early mobile phones used nickel-metal hydride batteries or nickel-cadmium batteries.

Similarly, the relationship between discharge voltage and capacity can be plotted for nickel-based batteries. The principle is the same. It will not be explained in detail here.

In short, measuring battery voltage to measure battery capacity is a simple, but slightly rough method.

Truly advanced lithium-ion battery fuel gauging only uses a coulomb counter and advanced fuel gauging algorithms.