The safety of portable energy storage systems, such as rechargeable battery packs, is a primary concern among reputable manufacturers. A significant portion of the cost of a typical battery pack results from the inclusion of safety mechanisms. One such safety mechanism is a polyswitch. The use of polyswitches is well known in the art, and they are very effective at preventing high current short circuit events, commonly referred to as a hard short circuit, across battery terminals. However, loads applied to the terminals which do not draw enough current to trip the polyswitch may occur, and may produce a safety hazard. Such short circuit conditions are referred to as a soft short circuit.
Since battery packs for cellular phones and two way hand held radios will often fit within one's pocket, the potential for shoring such batteries exist due to the fact that conductive members such as coins and keys are often carded in pockets as well. These conductive members are generally flat. To minimize the occurrence of shorts circuits from flat conductive members, the battery contacts are often recessed from the surface of the battery housing. The corresponding contacts to which the battery pack attaches generally protrude from the device, and are located such that when the battery pack is attached, they are hidden from view between the device and the battery pack.
However, it may be the case that the surface to which the battery pack attaches is visible during a portion of its operation time. This would be true, for example, with a cellular phone normally powered through a power cord, such as in an automobile. It would be of great convenience to a user of such a device if the device could be taken from the automobile under battery power. Since the major surfaces of the device are normally visible, having battery contacts protrude from the device would detract from its ornamental appearance. This suggests that the opposite situation described above be implemented: recessed contacts on the device, and protruding contacts from the battery pack. A polyswitch can still be used to prevent hard short circuits, but protruding contacts present a soft short circuit issue.
Therefore, there exists a need in a battery pack having protruding contacts, which attaches to a portable device, for a means by which to prevent soft short circuits from occurring when the battery contacts are exposed.