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.
The recommended Raychem devices are RGE300 through RGE1400 for 12V battery systems. RUE090 through RUE900 are suitable for 24V battery systems. These are available from DigiKey and other distributors.
The device performance charts are available at the Wickmann website, Raychem website, and the Bourns website. These show the holding and trip currents vs. ambient temperature for their products.