Charging: Cell charging

Charging current lower than discharging (e.g. 10x, 20x, etc.).
Lithium ions Li + must intercalate into the graphite layers → it limits the charging speed.
By charging, we transfer the cell to an unstable state.
The total charging current $I_{nab}$ is given by

Charging increases the battery voltage.
By simultaneously increasing of $U_{nab}$ , we maintain a constant charging current = constant current charging (1st charging step).
However, the maximum cell voltage must not be exceeded.
If $U_{nab}$ reaches the maximum level ( $U_{nab} = U_{max}$ ), the voltage does not increase further and is constant = charging with constant voltage (2nd charging step, so-called saturation part).
The charging current decreases exponentially => this charging is time consuming.
Manufacturers often publish charging time needed to reach 70-80% capacity.
Maximum cell voltage depends on technology:
- usually 4.2 V for Co, Ni, Mn, Al.

Step 1: constant current Step 2: constant voltage

Li-ion battery charging

The cell charging current is usually recommended to be 0.5-1C.
To increase the battery service life, small charging currents are recommended.
In parallel, do not charge to full SoC (e.g. do not start the saturation part) → reduced voltage stress.
High charging currenta speed up the first part, but prolongs the saturation part.

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