Fast Charging Strategies

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 350cal/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 53cal/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 5mW 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

end-of-charge and ensures safe, efficient recharge without significant gas generation. It is assumed that the battery has been subjected to a deep discharge, or heavily sulphated, owing to a high initial resistance in comparison to a fully charged lead-acid battery. Under these conditions, the initial charging current will be less than the current limit set because a substantial portion of the applied overvoltage (Vapp) will appear as ohmic drop, IR, and relatively little will be available to drive the electrochemical conversion reactions. The voltage will rapidly rise from the initial open-circuit voltage (OCV). If IR is sufficiently large, VAPP (lower VAPP curve) will quickly reach the low value of limiting voltage chosen, VL, and the current will continue to rise until it reaches the selected current limit. At constant current, the applied voltage may fall and pass through a minimum limit depending on the value of ILIMIT and whether the resistance is still decreasing. When nonohmic polariztion increases sufficiently, the applied voltage will rise again to maintain the current at ILIMIT until VAPP reaches VL again. Then the current decreases at constant applied voltage to a very, nearly zero, value, as the equilibrium potential increases and the overvoltage is no longer large enough to drive the electrode reactions as shown in Figure 5–2.

Figure 5–2 Fast charge voltage/current profile for VRLA batteries.

Applied Voltage (V) or Current (I) 350

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High Voltage (VH) Low Voltage (VL) High Current (IH) Low Current (IL)

When the applied voltage limit is raised to a high value, VH, the time at which the battery accepts the maximum current begins earlier and extends longer. When the final current decline occurs, at constant VH, the gassing reactions remain appreciable, so that the current declines towards a nonzero limiting value. At the low VL, the charging efficiency is very high, but it may take an impractically long time to return all the charge because of the very low current. This phenomenon is illustrated by the already low current when 80% SOC is reached. At the high voltage, the current remains high. The average rate of charging is faster but less efficient, and a high proportion of the charge goes into gassing reactions—particularly after about 80% SOC. Eighty percent SOC is sufficient to reduce the value of the applied voltage to VL if 100% return is not needed. In order to attain the intermediate value, the voltage is tracked using artificial intelligence. For automatic charging, a computer algorithm determines the SOC with user specified voltage and current limits.

As an alternative, feedback control is employed so that the ohmic component of the voltage decreases. The user must still specify the current limit and a resistance free voltage, VFR—but this, instead of the applied voltage, is maintained constant.

When a constant resistance-free voltage is applied during the fast charging, the initially applied voltage rises to a value limited only by the power supply. After the first few seconds, most of the overpotential is ohmic, and the current reaches ILIMIT. As the resistance decreases, the applied voltage decreases until other nonohmic polarizations (hNOP) become important at about a 40 or 50% SOC. The applied voltage VAPP, rises and maintains VAPP = VRF + IR from the point the current begins to fall. Then VAPP must also fall since VRF is constant. By choosing the applied voltage suitably, optimum charging rate is obtained, since the current remains at its maximum value for the longest time consistent with the desired charging efficiency and temperature rises.

The optimum value of VRF is not truly a constant but varies according to the acid concentration, kinetic and thermal characteristics of each battery in the pack. It is therefore a function of the SOC of the battery. The principle of charge control used here is an approximation to the ideal battery-charging algorithm. The algorithm accounts for the factors influencing the battery pack performance and compensates for them.

For an 85 to 90Ahr battery pack, a fast charge is applied at 8C to 9C. The current limit is set to 450A and is somewhat lower, and the value of VRF is 2.50V. The temperature compensation for the VRF is set to 6mV/°C/cell.