THE BPMS CHARGING CONTROL
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The battery module design can continue to undergo improvements. The temperature level could be increased by the user adding insulation to the outer walls of the battery module. The cooling period could be extended to 12 hours, making a colder climate acceptable without indoor garage and charging facilities. The optimum battery temperature for the vehicle application depends upon the separator decomposition and positive plate corrosion. Both the decomposition and the plate corrosion increase by increasing temperature—50°C is considered a suitable upper limit. In addition, the battery cycle life tends to increase by increasing temperature due to increased capacity and thus decreased DOD for the same utilized capacity. In an EV application, the user can expect this effect to be rather pronounced due to the high peak discharge currents.
THE BPMS CHARGING CONTROL Specialized integrated circuits, available today, have been designed for developing a control scheme for optimization of battery charging. The circuits operate a general assumption that the battery cells share uniform charge and discharge characteristics thus limiting the treatment of the battery as a two-terminal energy storage element. As discussed in the earlier chapters, limitations in the cell manufacturing process result in no two cells being identical, which leads to uncertainties in the cell characteristics. Furthermore, two detrimental effects of this nonuniformity are that certain battery cells undergo overcharging while the useful charging capacity of the battery decreases. It is essential to minimize the effects of destructive overcharging while maintaining a uniform charge across a battery regardless of the initial cell conditions. A technique, referred to as active equalization allows for a portion of the charging current to be diverted past certain battery cells so that the cells can receive the charging current selectively. Commonly, DC-to-DC converters are used to shunt current around cells (or a group of cells) in a battery. As the string of batteries charges, each cell in the battery reaches a threshold voltage. Upon reaching the threshold voltage, 15.5 V typical for a 90 Ah VRLA battery, charging current is diverted around the battery. Thus the fully charged battery maintains a threshold terminal voltage, and the excess energy is placed back into the charging bus and appears as additional charging current. The process of recirculating the charging current via shunts allows the undercharged batteries to gain the equalization charge while the fully
