7: Discharge characteristics Part 1

Discharge characteristics of lithium-ion batteries change greatly depending on current, temperature, and charge and discharge.

Discharge characteristics

It is a graph in which the horizontal axis is the discharge capacity and the vertical axis is the voltage,with the discharge current with constant temperature as a parameter.

As per a graph commonly displayed, it is common to display a graph of each discharge rate at a discharge temperature of 20 °C.

1C is the current value at which the constant discharge of the cell with the nominal capacity value is stopped in 1 hour at the end of the discharge. In the cell with the nominal capacity value of 3.4Ah, 1C = 3.4A, 0.2C is the current value in which the capacity of the nominal capacity value is stopped in 5 hours at the end of the discharge, and in the cell with the nominal capacity value of 3.4Ah it becomes 0.68A.

The capacity of a battery depends on its size, and the battery characteristics do not depend on the absolute value of the current but on the relative value to the capacity value.

リチウムイオン電池の放電温度特性

Therefore, the relative display using the discharge rate is used to indicate the battery characteristics.

It is a graph in which the horizontal axis of discharge temperature characteristics is the discharge capacity, the vertical axis is the cell voltage, and the discharge current is constant, with the environmental temperature during discharge as a parameter.
In general, it is common to display with the upper and lower limit values ​​of the discharge temperature range specified in the specification with the discharge rate as 0.2C and the temperature as 20°C.
The cell voltage decreases when the temperature goes low, and the high temperature side is almost the same as the data of 20°C.

The reason why the voltage drops at the low temperature side is that the movement of lithium-ion becomes difficult to move, and this is because the internal resistance of the cell increases, the voltage drop increases,and the voltage drops.

Lithium-ion batteries tend to have large voltage drops during the high current discharge at low temperature, and normally the discharge end voltage is usually set at 2.5V in a single cell, but there is no leeway up to 2.3V when the lithium-ion battery pack prohibits and stops discharge from 2.5V, and it will stop the discharge in a very short time if being used.

Therefore, use of the lithium-ion battery is assessed that the remaining battery level is 0 when it is at the set side and the voltage of the battery pack is about 3V (on mobile phones, 3.2V is often seen as the discharge end voltage), and therefore the battery capacity that can be used will decrease greatly when the overall cell voltage at the low temperature side decreases.

To select a cell, if the margin is increased due to the necessity of determining the minimum voltage to be used, the required maximum current, and the battery capacity, the battery will become larger and the cost will also increase.

The characteristics of the lithium-ion cell are not disclosed in much detail publicly, but it is necessary to acquire special environmental temperature and discharge current data.

Particularly, with respect to current, normally it is not common to discharge with constant current when in use.
It is also possible to estimate the usable time of the battery by simulating the current pattern.

The discharge pattern at -10°C shows a behavior in which the voltage sharply drops immediately after the start of discharge and then picks up for a while, but the lower the temperature and the larger the current the more remarkable it is, and the reason why a drop in voltage stops is self-heating due to resistance loss inside the cell.

This is because the voltage value rises as the temperature of the lithium-ion battery rises.
Therefore, the characteristics when continuous discharge is performed are shown.

Discharge at -10°C or less is not a general situation and it can be said that it is a very improbable environmental temperature, but the reason why low temperature characteristics draw attention is because low temperature characteristics differ depending on battery manufacturers, and so generally it is said that the better low temperature characteristics are the better cell characteristics are.

That is, the fact that a drop in voltage at the time of discharge at low temperature is small indicates that it is a cell in which continuous large current discharge can be performed due to small resistance value and small temperature rise of the cell at the time of large current discharge.

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