Cell Configuration

Just as there are series and parallel connections for dry cell batteries, there are also multiple ways to bundle the cells in Li-ion batteries (cell configurations).
It is expressed as XsYp, and each has the following meaning:
X = Number of batteries in series "X" x Voltage of a single cell = Battery pack voltage
Y = Number of parallel batteries "Y" x Capacity of a single cell = Capacity of the battery pack
s = serial
p = parallel

In the example of a battery pack using 3.6V cells in Figure 1, two cells are connected in series and three cells in parallel, so it is expressed as 2s3p (2 in series, 3 in parallel) and the battery pack voltage is 7.2V.

Battery type

As shown in Figure 2, there are various types of batteries, and they are used according to the purpose and the shape of the product in which they are used.

Li-ion battery characteristics

Battery capacity and voltage are available in two types: nominal and rated.

Nominal capacity: The center capacity designated by the battery manufacturer.
Rated capacity: Battery capacity in accordance with the Electrical Appliance and Material Safety Law
Nominal voltage: The voltage that is the standard for battery use specified by the battery manufacturer.
Rated voltage: Maximum voltage for safe use

*Currently, the method of displaying rated capacity has been unified according to international standards.

Charge/Discharge Rate

The C rate is a way to express the rate at which a battery is charged and discharged.
1C refers to the current value at which a battery of a certain capacity is discharged completely in one hour when discharged at a constant current.
For example, for a battery with a capacity of 1Ah, 1C represents 1A.

Charge/discharge characteristics

The characteristics of Li-ion batteries vary depending on the discharge current, ambient temperature, and number of charge/discharge cycles.

Differences due to discharge current

Figure 4 shows three types of discharge current, and we can see that the higher the discharge current, the greater the drop in battery voltage.

Differences due to ambient temperature

Figure 5 shows that the lower the ambient temperature, the greater the drop in battery voltage.

Differences due to the number of times the battery is charged and discharged

Figure 6 shows how the battery capacity changes as the battery is repeatedly charged and discharged.
As the number of cycles increases, the battery capacity decreases.

Batteries are usually represented as shown on the left in Figure 7, but in reality they have characteristics that look like resistors connected in series, as shown on the right.
This resistance is not always constant. It differs from battery to battery, and also depends on the remaining charge and the degree of deterioration.

As mentioned above, the resistance of a battery changes depending on various conditions. Battery remaining capacity is one of them, and managing the battery remaining capacity is very important to maintain the battery voltage. In the next section, we will explain how to predict the remaining battery capacity.

How to determine from voltage

Let's start by considering the battery as a container of water: by monitoring the settled water level, we can determine how much water is in the glass.

If we replace this with a battery, it becomes possible to determine how much capacity remains in the battery by measuring the stable battery voltage without charging or discharging.
Generally, as shown in Figure 9, even if the battery capacity changes, there are times when the change in battery voltage is small, making it difficult to determine the exact capacity value.

Also, when the battery capacity is low, the voltage drops suddenly, which may make it appear as if the capacity has decreased suddenly.

The method for measuring voltage is very simple, since it only requires measuring the battery voltage. However, you can see that the accuracy is not very good.

How to determine from the current

Here too, we will consider a container filled with water as an example of a battery.
It is possible to determine how much water is in the glass based on the amount of water flowing in and out.

If we replace this with a battery, it becomes possible to determine how much capacity remains in the battery by integrating the charge and discharge current (coulomb counter).
However, it cannot tell you how much capacity the battery has at the start.

Now let's consider the battery capacity at the start in Figure 11.
The ideal battery voltage is shown in red. Due to the internal resistance of the battery mentioned earlier, the actual voltage will be as shown by the blue line. If the battery voltage drops below the ideal voltage, the End of Discharge Voltage (EDV) will be reached sooner, so the actual usable capacity (FCC) will be less than the original battery capacity (Qmax). It is important to accurately understand the actual usable capacity (FCC) to avoid any discrepancies with the expected capacity as described above.

How to determine the actual usable capacity (FCC)

Once the battery is fully charged, the FCC value is read and the amount of discharge is accumulated during use. Then, when the battery is fully discharged, the FCC value is updated from the amount of discharge accumulated during use. This allows the system to learn the battery condition and respond to battery deterioration.

However, the timing of the FCC update is too late when the battery is completely discharged, so some ingenuity is required, such as setting a fixed threshold value, such as 10% remaining when a certain voltage (EDV1 in Figure 12) is reached.

The current integration measurement method requires that the current be measured and integrated in advance, and is affected by minute currents such as self-discharge and standby current. However, if capacity learning is possible, it can also handle cell degradation and can provide more accurate measurements than the voltage measurement method.