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Energy Storage for Hybrid Military Vehicles

Battery Pack Reliability & Overcharge

The estimated probability for fabricating a functional battery pack, where the cells are configured in a series string, is the product of the probabilities that any single cell is functional. Certain inferences arise from this observation. The battery pack reliability decreases with the number of cells connected in series. Also, the battery pack behavior is controlled largely by the poorest performing cell in a string.

From this, it follows that the reliability of the battery is improved if the number of cells connected in series is reduced. That is, the cell string voltage is kept low. Doing this implies that the DC to DC conversion requires a substantial step up of the string voltage. Parallel strings could be used to increase the total energy stored by installing additional strings. Switching failed strings out of the power source increases the battery pack reliability. Also, the power demand on any particular string configured in a parallel arrangement of strings has a reduced current demand. Although there is a need for more DC/DC converters, their individual current-handling capability is reduced. It is evident that additional control circuitry would be required to support this architecture.

During battery discharge, the heat generated is the sum of the Joule and enthalpic (chemical) heating effects. Conversely, during battery charging, the heat generated is the Joule minus the enthalpic heating. If the conditions are carefully selected, one can observe a net battery cooling during charging.

However, an interesting phenomenon takes place during overcharge. Those cells designed as sealed recombinant systems develop a significant heat production on overcharge. Flooded designs do not exhibit this effect. The reason is that the electrical energy stored in the cells generates energetic reaction products as a consequence of the electrochemical reaction. This is an energy absorbing process. The gasses produced during overcharge are then vented into the environment. Since the sealed cells undergo a closed recombination cycle, i.e., no material is exchanged with the environment, the rate of heat generated is determined by the power input to the cell. Remember that the power dissipation is the current passing through the cell multiplied by the voltage gradient, which is the cell voltage. Essentially, the cell is behaving in the manner of a heat-dissipating resistor.