Methods and Competencies - Charging Strategies
Charging electrochemical storage devices require more than simply applying a current until a certain voltage level is reached. Every storage technology requires a special charging strategy, in order to reach a maximum of available capacity, efficiency and lifetime.
- There are four major charging regimes which are used today:
- Constant Current / Constant Voltage Charge (CC/CV)
- Constant Current / Constant Voltage with varying voltage levels in the constant voltage period
- Constant Current / Constant Voltage / Constant Current with limited charging time and a small current in the second constant current phase
- Constant current charging until a certain end-of-charge criterion
It becomes more difficult, if several cells in series have to be charged. While the current is the same for all cells, it is not possible to control the voltage of each cell by controlling the string voltage by the charger. Cell technologies such as supercaps and lithium-ion batteries which are critical with regard to charging voltage have to be monitored and controlled individually to keep their voltage within safe limits. An intelligent battery and cell management is required.
For the detection of the full state of charge, the voltage and current of the storage device is monitored. Full state of charge is reached typically if the the current is not changing anymore over a period of one hour at constant temperature and voltage
For Nickel based systems like NiCd or NiMH batteries, other criteria for the full-charge-detection have to be taken into account. The typicall charging regime for those batteries is constant current, where the battery voltage and the temperature and their derivatives have to be monitored. In applications without constant power supply such as autonomous power supply systems more sophisticated full charge detection is required.