Due to the (for me) new type of battery design of the Niro, where KIA mixes the use of the two batteries (the litium and the boot battery) I am a bit concerned about the condition of the latter, when it is cold up here in Sweden - sometimes down to -30C!
As smax100 writes, you can´t start the car with the 12V battery drained or even empty. The Niro needs to do some initial tests driven by the 12V battery before it orders the high voltage battery to start the car.
So....... when its really cold I always have my CTEK XS 0.8- charger connected to the 12V-battery - just in case - even though I most of the time have my Niro in a garage (a cold garage).
As you might know the CTEK-chargers are considered to be the worlds best and they can be connected to the battery all the time if you want.
I also have a CTEK connected to the battery of my Snowmobile the whole off-season and is starts at once when I decide to go for a ride even if I haven´t touched it for 8-9 months!
You know that the CTEKs are a Swedish construction, don´t you?
http://ctek.se/de/de