4. Charging

4.1. Charging steps

  • Charging increases the battery voltage.
  • By simultaneously increasing the charging voltage, we maintain a constant charging current = constant current charging (1st charging step).
  • However, the maximum cell voltage must not be exceeded.
  • When the maximum voltage is reached (Unab = Umax), the voltage must not increase further and must stay constant = charging at constant voltage (2nd charging step, so-called saturation part).
  • The charging current decreases exponentially => the charging at constant voltage is time consuming.
  • Manufacturers often state charging time to reach 70-80% capacity.
  • Maximum cell voltage depending on technology:
    •      usually 4.2 V for Co, Ni, Mn, Al.

  • The cell charging current is usually recommended to be 0.5-1C.
  • To increase the service life, low charging currents are recommended and charging below full SoC (e.g. do not start the saturation part) → reduced voltage stress.
  • A large charging current speeds up the first charging step, but it prolongs the saturation part.



    Typical charging characteristics of a Li-ion battery


    • The aging of the battery results in a shortening of the 1st part (charging with constant current) and a prolongation of the saturation part.
    • SoC is practically impossible to determine according to the current voltage.
    • It is possible according to the OCV characteristic for the given temperature → in steady state, i.e. sufficient time from last loading.
    • Usually, the calculated charge is counted:

    • Overcharging is not possible - Li-ion will not take over another charge, charging must be stopped, otherwise safety is endangered.




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