Ibex Battery Systems
The term "PolyFuse" is a registered trademark of the Wickmann company for a resettable overcurrent protection device.
Similar units made by Raychem Co. are called "Polyswitch" while those made by Bourns are called "MultiFuse".
These are small polymeric devices that switch from a conductive state to a high resistance state when the current passing through them causes sufficient self-heating. Their advantage over fuses is that they self-reset when they are allowed to cool.
Like fuses, these devices have a maximum current carrying capacity before they "trip", that is dependent on the ambient temperature. As the ambient temperature rises, these devices trip at a lower current.
The recommended Raychem devices are RUE090 through RUE900 or equivalent. These are available from DigiKey and other distributors.