The bottom line is that a properly functioning LEAF will have no issues. Reality states that all really depends on the condition of your 12 volt battery. I have never had issues with storage including 15 days in July-August of this year. So what you MUST do;Now after posting this, I felt that some elaboration on how I came to these opinions was needed. Nissan's 12 volt battery management system was failing enough that it became a concern to me when it first started cropping up because the LEAF was my only source of transportation at the time and being stranded was simply not an option for me so I started collecting data on both my 2011 and 2013. Now, both cars pretty much acted the same so it begins.
DO NOT leave your vehicle plugged in. This prevents the car from topping off your 12 volt battery.
Charge your vehicle to roughly 40% SOC give or take. (LEAFs are shipped with about 30 % SOC) In my 15 days of storage, I lost 9 GIDs (116 to 109) but more importantly, my 12 volt battery voltage was stable at 12.42 volts. I was unable to determine voltage drop because the car had not gone anywhere on the day I departed but had been boosted as it read anywhere from 12.7 to 12.8 volts (the norm is 12.30-12.40ish)
Soooo, how do we determine our 12 volt battery health? Well good question and since the LEAF randomly shuffles the deck so to speak, its really anyone's guess but things to consider such as what is bad for battery.
Extended periods of inactivity; now how long is anyone's guess and ***IMHO***; the factor is cumulative. So more than say 24 hours probably contributes. This opinion of mine came about by observing 12 volt battery voltage both as rest, during startup and at random times during the same drive cycle. Things like timers, being plugged in, CARWINGS, etc all use your 12 volt battery which makes idle time MORE critical.
Radical changes in temperature; aka "My LEAF does not live in the garage" Both cold and heat weakens your battery. If your LEAF sits at the window with sad eyes looking in at night, expect your battery to be weaker than mine who lives with me (in the garage)
Accessory mode. Be like me. Pretend you don't have one. its not like you are going to smoke out any nearby pedestrians while "idling" away... and finally THE most important thing to remember
The LEAF especially ones equipped with CARWINGS has different modes when its turned off. A type of high readiness mode happens when the car is plugged in. This can simply be the car listening for the timer to start a charge or whatever. Either way; this is a 12 volt function and unless the car is in ready mode, its a 12 volt "battery" function. Either way, these are things that can cause your LEAF to remain at a higher state (which includes using more battery power) longer.
Now I did post some observations a few years back on my 2011 SL with CARWINGS and as mentioned, I repeated the same measurements with my 2013 S (without CARWINGS) and found little if any difference which I think is caused by the fact that both my 12 volt batteries were in good shape.
In the blog above, I noticed that my battery at rest generally sat in the same range of 12.3-12.4 volts if measured a few hours after being driven. Checking it right after parking, it was in the 12.6-12.8 range. The voltage here would slowly drop to the 12.4 volt range. This tells me that when in ready mode, there is some charging going on but is it all the time the car is in ready mode? Well no.
The battery was at 12.18 volts this morning (The lower voltage due to the fact that it was cold last night. The garage which is normally in the mid 50's was at 45º this morning. During Summer when the garage varies from the mid 60's to the mid 80's, the voltage is between 12.4 and 12.5. The temp outside was 24º this morning when I got up. that is about 15º lower than normal) About 2 hours later, I checked it again and it was up to 12.36 volts and the garage temp was 50º. I started the car and the voltage went up to 13.80 volts but dropped to 13.00 volts within one minute. This tells me the LEAF checked my 12 volt battery and determined everything was ok, IOW, no boos needed. 13.00 volts is generally where the 12 volt system is at all the time when the car is in ready mode. There have been a few times I checked it when it was higher (over 14 volts once in my 2011!) and my assumption was that it was a scheduled boost charge of the 12 volt battery. I think the 12 volt DC converter is active I do not believe the 12 volt battery is actively providing a charge when the car is running which probably accounts for the steady voltage I see most of the time. Some think the battery might be acting as a buffer for the 12 volt system and that maybe true to some extent but the Car's ability to seemingly overwhelm the battery makes me think the 12 volt battery's role while in ready mode is limited at best.
In an attempt to vary the voltage in the system, I tried both A/C and heat. Neither seemed to change anything at least during the short period of time I observed it.
Either way, I am confident that Nissan has heard about the issues in the earlier models and has tweaked the software to insure weaker 12 volt batteries were properly maintained.
Now Lead Acid simply does not store well. If you plan to be away for an extended period of time like over 3 months, its probably best to put it on a battery tender. Lead Acid batteries can lose up to 10% SOC per month. Sealed Lead Acid batteries do much better losing half as much in the same period of time but that is still not good. For periods lasting under 3 months, your LEAF should be ok. FYI; your traction battery should lose roughly 1 percent per month so no worries there.
Finally; temperature is probably the biggest factor. Best case for lead acid is 59º F. Now, heating the garage for several months while you are away is probably not doable but if you are in a very cold climate, something as little as a 40 watt incandescent light bulb (Yes, they are still around) will do the trick. If you are in a moderate climate, a timer that runs the light during the coldest part of the day will be fine. For most of us in the Puget Sound region, I think simply parking in the garage will be fine.
The chart below (provided as an FYI only) is for hot areas so not really a concern here but notice the difference in recoverable charge for Li for a 40% SOC storage charge verses storage at full SOC? Chart from http://batteryuniversity.com/. A great site for battery tips!
One thing this chart does illustrate is the great long term storage for Li. I wish I could find an updated chart with the new chemistry that Nissan is coming out with in 2016 but guessing someone will figure something out soon enough!