You are concerned about your lead acid battery? Disconnect it, and/or connect to a trickle charger. Your traction battery will stay charged if lead acid battery is disconnected or hooked to a trickle charger. Otherwise, it might discharge slightly: if your 12 V lead acid battery discharges from car load while off keeping computers, door systems, and alarm working, the traction battery will recharge the 12 V battery. As I understand it, three times is the limit so it won't significantly drain the traction battery recharging the low capacity 12 V battery.
You may also be able to keep the traction battery hooked up to a charger. As I understand it (again), charging the traction battery also charges the 12 V battery at the same time. I don't know if this continues after a full charge on the traction battery is obtained.
First, I am concerned about the 270 volt drive battery. I don’t think that using a 12 volt battery charger is a solution for keeping a 270 volt battery optimized for long term storage. Keeping the 120v Wall charger plugged in would make more sense, but dealer has no idea if this is a solution. I sent an email to Kia, so far no response.
My experience with lithium ion batteries in my RC planes is that they should be stored at 50% charge and, even then, they will eventually discharge and fail after being stored for longer periods of time.
Dealer “verified” the Niro doesn’t have a 12 volt lead acid battery (said there was a separate 12 volt Lion battery), but my experiences with dealer accuracy is poor. Also, I sometimes get a “Aux battery” has been recharged message on startup...which really has me concerned about long term storage.
Surprisingly, Tesla has a 12 volt lead acid battery, go figure. It is there to power the infotainment system from what they say.
The point I was trying to make is that if you disconnect the 12 V battery (which should be a lead acid in a PHEV), your traction battery now has zero load. Zero load means zero (or very close to zero) discharge. Like a lithium watch battery new never opened. Huge shelf life.
I'd be interested to see what they say about staying plugged in, but they certainly didn't design it for your scenario. In any case, your 12 V battery will likely discharge and that is not good either.
If my Niro is anything like my Tesla (which does have a 12 volt battery with charge points at the front of the car), the traction battery definitely discharges relatively quickly. I can monitor the range but the Tesla Visible app. I forget the range reduction and time frame but it was obvious a one year absence would be a problem, maybe even in Sleep Mode. Tesla service center advised to just leave plugged in. Sorry to make references to Tesla, but that is my only experience with a BEV. FWIW, although lacking some features, I expect to be happier with my Niro.