Question about charging the battery

Well my problem is this.
I ride the bike on weekends only, and the bike got an active tracking alarming on it.

So basically it means that I can easily have no battery every week when I do want to ride it.
Riding 4-5 hours in the weekend aren't enough to fully charge the battery it seems (not a "relaxed" ride).

My main problem is that I do not have an electric outlet anywhere near the bike, so I can not get for example the ktm charger and plug the bike to the wall and forget about it until next week.

I am trying to find a solution, like connecting a 12V car battery to the bike to keep the alarm system from eating the bike's battery, or trying to find a charger which is being feed from a car battery instead of a normal electric outlet.

So anyone has an idea about what are my options except disconnecting the bike's battery and charge it at home (which I don't really want to do every week)?

Thank you.

Is there a window in your garage? If so how about connecting a solar cell to the battery via a plug and socket arrangement?

I used to do this for an old car I kept in France which got used once a month and it kept the battery well charged up. As you are lucky enough to live down there in Israel you won't have any problems with lack of sunshine.

Problem is that I don't have a garage.

The bike is in an underground parking in a shared building.