Madgardener wrote:... what model the varistor in the battery pack is.
, not "varistor".
Does anyone know what it is and where I can buy a replacement? ...
AFAIK, the exact thermistor has not been identified. Perhaps you can use the data contained in this thread to source a likely candidate: viewtopic.php?f=4&t=12750
That thread's OP refers to Roomba-500 series (a.k.a., "R3") thermistor data, but all former Roombas have used the same nominal thermistor in their batteries.
Generally speaking, it is possible to harvest the thermistor from the old pack that is being replaced.
If your replacement pack does not come with an in-line fuse assembled to it, you need also be concerned about fusing the pack. A polymer (auto resetting) type is used, and is, I think, procurable in piece-part form.