How often should Prius C be started/run in winter?

Have your friend take it on a drive for at least 20 miles every couple weeks. If you don't have one, pick up a "battery tender" type charger at Wal-Mart or auto part store. It'll keep your 12v battery charged up.

If you skip the battery tender charger, make sure your friend is aware that if the car doesn't start, do not continually try to start it. You don't want to run the High Voltage battery down.

 

There's an assumption there, maybe not without merit, it's debatable, but anyway: that with a very occasional drive, the car will be fine. Also, that that's the only way to deal with a car in medium term storage.

A question: what's your parking situation? Is the car in a garage, driveway, out on the street? If in a garage, or possibly even just in a driveway:

Fill the tank.
Bring the tires up to max sidewall pressure.
Connect the 12 volt battery to a 3~4 amp smart charger, one that is capable of being left on indefinitely.

 

I suggest asking them to start it for 10 minutes once a week. That's a little more than the car needs, but it's easy to remember and even if they miss one there won't be any problems for either of you.

 

welcome!
i'm leaving 01/16 and returning 03/22. pump tires to max sidewall pressure and adios.

same thing every year since 2005, 3 different prius, no prolemo