The mechanism that calculates and communicates the charge and discharge amount of the lithium-ion battery and so on is called gas gauge (FG).
FG has simplified type and precise type, and the simplified type is adopted in mobile phones and so on and it measures the voltage of the pack and determines the remaining charge of the battery from the voltage.
The relationship between the voltage and the remaining amount of lithium-ion batteries
1: Voltage drops due to resistance loss because of battery impedance during discharge.
2: Since impedance becomes large when the battery deteriorates, the voltage drop becomes large.
3: At low temperature,battery impedance becomes large and the voltage drop also tends to become large.
As you can see from here, the method of calculating the remaining capacity with voltage is a guideline.
However, since the cost required for FG is low, it is enough for practical use with simple remaining amount display, and it will be used when the current value in a battery pack with large capacity is relatively small.
In order to accurately calculate the remaining battery charge, it is necessary to measure and integrate the charge and discharge of the battery pack.
Because there are cycle deterioration and change with temperature, the charge and discharge cycle is counted, and the full charge capacity of the battery is reduced accordingly, and the increase and decrease and so on of the full charge capacity according to the temperature inside the battery pack are assessed.
These will be controlled by the IC control.
In addition, capacity data of lithium-ion batteries at the time of full charge is rewritten.
For purposes when discharging is performed in a short time after charging in large capacity lithium-ion battery packs, the power consumption of the FG itself is not a problem, but the power consumption of FG becomes a big problem in small battery packs such as mobile phones.
If a single cell pack is shipped to the market with 50% state of charge, the pack may become over discharged when it reaches the end user after the stock period.
Moreover, the charge amount necessary for operation when using for the first time must be left.
Although there is a problem of how long the stock period should be, the skills of the battery pack makers and design engineers to realize low power consumption FG will be the challenging part.