Whenever I know they're going to be sitting for an extended period I only charge them to 80% at most as it's easier on the batteries.I am assuming with the mild weather, they dont burn much conditioning or maintaining battery temp or the like? How long are they happy to sit before they want a top up?
The Volt uses pretty much nothing sitting still unless it's plugged in at which point it'll cool the batteries if they get too hot (really only happens after driving), or heat them if it's too cold out. When it's unplugged it does neither when sitting still, so no use.
The Ioniq does have a "battery maintainer" that automatically tops up the 12V accessory battery when it notices it's voltage has sagged. It seems, from experience (a notice pops up on the dash at the next stop when it's been triggered) that it only happens after 2-3 days. When we were away on our cruise the last week of February I charged the car to exactly 70% and then unplugged it. When we got home the notice about the battery maintenance popped up...but the main battery was still at 70%. So, it seems it could sit for months or years without any issues.