6: Charging method

Lithium-ion batteries may cause incidents such as fire and explosion if the charging method is incorrect.
In general, the charging method is that the voltage is up to 4.2V per cell and the current is less than 1C.

There are cases where the voltage is 4.1V, but setting it as 4.2V is common.
If the charging voltage is low, the capacity will decrease but the life of lithium-ion batteries will be improved.
If the charging current is excessively higher than the specified value, lithium gas is generated in the negative electrode.
Lithium gas is extremely dangerous and causes a likelihood of incidents such as fire to occur.

Charging of lithium-ion batteries is called CCCV method that manages voltage and current.

For Ni-MH batteries and Ni-Cd batteries,constant current charging with only current is performed.
However, with Ni-MH batteries and Ni-Cd batteries, it is difficult to assess the end of charging and it is necessary to have a mechanism to detect the changing point of voltage and the places where the temperature of the cell suddenly rises, and there fore lithium-ion batteries are simpler as a charger.

There is a case that a thermistor is put inside a battery pack to detect the temperature of the battery cell.

For lithium-ion batteries, it is not absolutely necessary to insert a thermistor, but since the battery manufacturer regulates the charge temperature range, there is a reason to insert a thermistor to detect that it is within that temperature range.

When lithium-ion batteries are charged at low temperature, lithium-ion from the positive electrode is less likely to be absorbed by the negative electrode, and lithium metal precipitates and accidents are more likely to occur.

In addition, if it is high temperature, the battery may become hot due to temperature rise because of charging current.
This is thereason why for lithium-ion batteries, the temperature range during charging is specified.
The charging temperature range of lithium-ion batteries is generally specified to be around from 0°C to 45°C.
It can be said that low temperature or high temperature environment beyond the range is not practical.
For example, we do not recommend charging under an environment of 0°C or below, or 45°C or above.

In the explanation, the unit called C that shows the relative current amount appears. This is an expression commonly used in the battery industry, and 1C means is the current value with which a cell having a capacity of a nominal capacity value is constantly discharged of its current and the discharge is completed in an hour.

For example, for a cell with a nominal capacity value of 3.4Ah, 1C=3.4A.

As per CCCV charging, when charging starts from the discharging state, the initial voltage is low and so it becomes constant current charging, but when the charging amount gradually increases and the cell voltage reaches 4.2V, it becomes constant voltage charge and the current amount is restricted so as not to exceed 4.2V.

Full charge is judged by the state of decrease in charging time or charging current.
When regulating by charging time, it seems that charging time of 2.5 hours is often defined as full charge with 4.2V and 1C in CCCV charging.

In constant current constant voltage charge with current 0.5C and voltage 4.2V, 3.5 hours is fully charged.
One of the drawbacks of lithium-ion batteries is that it is difficult to perform so-called rapid charging, but in the case of 1C charging, 90% is charged one hour after starting charging, and in the case of 0.5C charging, 90% or more is charged after 120 minutes.

You can see that fast charging can be done if you can come to comprehend that the battery capacity is 90% of full charge.
The charging amount is determined by [current × time].
Charging in the CCCV method becoming lengthy will lower the current value when the voltage rises.

The pulse charging method shortens the charging time by increasing the current value beyond the specified voltage only for a very short time during charging, but it is not recommended as it places a burden on lithium-ion batteries.



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