I'm certain that the PHEV has a 12V battery and I'm pretty sure that the HEV does too, although I've read that in older models, it's integral with the high voltage battery. So maybe a battery tender wouldn't be a bad idea if you can identify a safe way to connect it. But if it was me, I'd be tempted to do what was described in this thread: https://www.kianiroforum.com/forum/9...ge-update.html
Another thing I'd be tempted to do (unless you are 110% certain that rodents aren't a concern) is fill an onion bag or an old nylon stocking with about 1 cup of mothballs and then look for some place under the hood to hang it. Just remember to remove it before you start the car in the spring. Some people have reported damage to their car wiring from rodents when the car is just parked overnight, but in my experience, all the people I know who have had this problem have experienced it after leaving the car parked for an extended period. It's a cheap precaution and it can head off a very expensive repair bill. I did this for the last two winters that I owned (but wasn't driving) my Toyota Tacoma, and there were no ill effects.
If the car is parked outside in the sun and you don't expect to put enough miles on it to wear out the tires in four years or so, you might consider putting plastic trash bags over the tires to protect them from degradation. People who own trailers and only put 1000 miles a year on their trailer tires usually have to replace the tires not because the tread wore out, but because the sidewalls got "dry rot" due to UV and ozone exposure.
One last, "old school" thought on preparing a car for an extended period of downtime: it used to be recommended to remove the spark plugs and squirt "fogging oil" in the cylinders, and then crank the engine a few times to coat the cylinder walls with it. The idea was to prevent corrosion. I've seen really old cars and tractors that were taken care of with these kinds of techniques, and they ran like new. But I'd be reluctant to attempt this with a Niro because you can't easily crank the engine a couple of times with the spark plugs removed because the computer decides when to start and stop the ICE and if it does try to start it, I have no way to predict how it would behave when it noticed that the spark plugs weren't installed and it didn't start... might do more harm than good. Also, even if you could solve that problem, I have concerns about what the impact might be in the spring when you burn off that fogging oil and modern high-tech exhaust components like Oxygen sensors get s a whiff of it.
I've had a few occasions to ponder the question of what is best when a car is going to sit: do you just leave it, or do you start it once every couple of weeks? Where I come out is that if it is in a garage (or parked on pavement), then once it has sat for a week, you might was well let it sit for the season because the oil has probably already largely drained down as much as it will. There's more wear on the engine to start it when the oil has had a lot of time to drain out, so best to reduce the number of occasions when you expose it to that kind of torture. But this is just my opinion, I've never seen anyone study this in a scientific way.
But if the car is parked outside on dirt or gravel, then my observation has been that it benefits from being started and driven every couple of weeks. I've seen cars that were left parked on dirt driveways and not driven develop corrosion problems at a much faster rate than other cars in the same driveway that were being driven every day.