THE FAST CHARGER CONFIGURATION
101
Both the charging voltage peaks are noticed when initial decline is caused by the decreasing battery resistance during the CC charge interval, with the second decline due to decreasing current. In conclusion of the fast charge it can be ascertained that: •
•
The commercially available batteries can be fast charged with a temperature rise of less than 25°C Fast charging does not exhibit detrimental effects on battery cycle life
In the first part of the fast charging process, to about 40% return, resistive heating is the major cause of the initial battery pack temperature rise. In case the battery is designed such that it meets the fiveminute-50% charge return requirement, the best strategy is to design the battery pack with low resistance batteries. The temperature distribution of batteries in the pack appears nonuniform, with possibly temporary localized hot spots. In the case of a VRLA battery, the temperature on the casing surface follows the internal temperatures more closely during charging than for immobilized electrolyte batteries. A battery with a larger heat capacity will have a lower temperature rise when it is subjected to fast charging. Assuming that all other battery pack conditions are the same. Therefore, VRLA batteries have a lower temperature rise, since sulphuric acid solutions have a large heat capacity. VRLA batteries will also have less heat production from water vapor decomposition. In addition, the battery heat transfer to the battery exterior is better. The lowest charging current should be chosen such that it takes full advantage of the time allowed to meet the charging requirement. This lowers the resulting battery temperature and saves energy. In addition, it may also return more charge to the battery in the given time.
THE FAST CHARGER CONFIGURATION The fast charger for traction batteries provides charging of batteries in 5 to 30 minutes. In order to apply this fast charge in a period of 10 to 30 minutes, the charger must be able to provide voltages up to 450 V and currents up to 500 A. Such a charger characteristic “envelope” is depicted by a maximum voltage-maximum current profile as shown in Figure 5–3. This envelope implies a peak output power of 225 kW. Thus
