Battery tender in absence of power outlet

Not sure if it'd work for your use case, but CTEK (same supplier as the OEM charger) has a new unit that might work called the CS-Free https://smartercharger.com/collections/jump-starter-for-car-alternative/products/cs-free I have no experience with this specific model, but have owned many CTEK over the years and been happy with them.

My understanding is it can trickle charge off of solar panel accessory (so no go) or by another 12v car battery source. It doesn't appear it's internal battery that's used for jump starting can be used without one of those sources as a tender. If you felt safe leaving a spare battery in your parking space, I guess you could use that and bring it up to recharge every so often. I am no EE so no idea how long one 12v trickle charging another 12v would last, unfortunately.

Another idea is a Antigravity LiFePO4 battery with "restart". For one, the chemistry difference apparently leads to less drain during charge, there are posts on this forum of owners reporting weeks to months of being unplugged with no issues. The second advantage is the Restart feature is branding for a built in battery management system that reserves a portion of the capacity by going to "sleep". When you discover the dead car, you hit a button on a remote fob and it activates the reserve which should give you power to get into the car and start it or at least be able to put it on the tender.

I do have experience with one of these Antigravity batteries installed in my recently acquired 458, and results are mixed. My first weekend, I came out to a dead car which I attribute to the Bovee ipod dongle keeping the car awake to drain it in one night. While the fob did work to get me in, it failed to start on two tries and then was well and fully dead. I was able to get it on the charger, but again results were slightly odd as the recommended "CTEK Lithium US" charger would only advance to 2 lights and stayed there for over a day. It did start, and after a long drive to get some recharge, I put it back on the charger and it now quickly went to 6 lights (again, stayed there for a day). So there seems to be something about the battery management system that's confusing the charger's progress reporting.

 

I’ve used the Antigravity batteries on both my 458 and my FF with excellent results!

What most folks don’t realize is using them removes all the little electrical buggies that happen over time with regular batteries. All gone!

SV

 

I’m in an apartment complex; the garage has no outlets. I purchased a power station that has an AC outlet. I connect it to my Ferrari charger (Portofino M). My 500 Wh power station lasts about 48h. Bigger stations of course last longer. Bringing it in and charging it is now a routine; since I’m in the garage a few times a day it’s not too inconvenient.

 

For your situation I would get the portable power source. I have this for when traveling and the car sits in valet for days with no power source. Excellent for your application.
https://www.goalzero.com/collections/portable-power-stations/products/goal-zero-yeti-1000x-portable-power-station