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ELECTRIC VEHICLE BATTERY FAST CHARGING proportional to the battery overpotential. This is due to the charge transfer process. The third component is the resistive heating, which is proportional to the square of the current and to the cell resistance. In addition, audible hissing is caused along with loss of electrolyte before the battery overpeak voltage is even detected. In the VRLA battery, heat produced during the charge is determined using the DS value as 350 cal/Ahr. In this case, the sign of the heat generated is positive or exothermic in nature. The unit activity of H2SO4 occurs at a specific gravity of 1.24 and even at full charge the activities differ from unity by a negligible amount. Most of the sulphate is in the form of bisulphate and only about one-third of the H2SO4 is ionized to sulphate ions. Using the bisulphate as part of the reaction, the amount of heat calculated is 53 cal/Ahr — smaller — but again as an exothermic reaction. Reversible heat production occurs at the negative plate, with some cooling at the positive plate. The heat production is exothermic in nature with about 5 mW resistance for a six-cell VRLA or AGM battery. The reversible heat produced during the charge is negligible in comparison with the resistive heating. Thus the fast charging process must avoid overcharge and minimize the internal resistance of the batteries. A fast charger on the other hand will charge even very cold batteries safely. Using a constant resistance-free voltage approach, at a low battery temperature, the entire battery charge acceptance is reduced. The fast charger senses the lower battery charge acceptance and adjusts the charge rate accordingly. The current rises for a few minutes as the battery electrodes come back to life with increase in the cell temperature. In case the charging is interrupted accidentally the battery charge algorithm once again senses the current SOC and reapplies the adjusted charge current. A temperature sensor monitors the battery pack temperature and applies battery temperature compensation over the operating range of the battery charger.
FAST CHARGING STRATEGIES A large number of charging approaches have been discussed previously. One of the preferred methods by most electrochemists is constant current-constant voltage (CV) interval. This requires some knowledge of the electrochemical processes in order to make a good choice between CV and constant-current (CC) methods. Using the CC-CV charging method for a fixed high current limit, Ilimit and for two choices of CV, the current curve for each of the two choices of CV is defined for charging to a low voltage limit, VL. This is a lower
