Once it has been flattened, your battery is damaged, and the more times it goes flat, the more damage is caused.
You will almost certainly need a new battery.
Don't leave your car unused for more than a couple of weeks without plugging in a battery maintainer.
These cars use a little power even when switched off, and the battery will also self-discharge over a few months, so you need to keep the battery charged if not using it even semi-regularly or only doing short runs. I use a cheap £20 7Amp (90W) maintainer that is plugged in in the garage. I just extended the 12v cables to reach where I park the car and fitted a good quality plug and socket to make it easy to plug in & unplug.
If you cannot do that, then you can get solar powered chargers that will do the job. Anything over 10W will do the job but 20W is better.
Ideally put the solar cell inside the window that gets direct sunlight (south facing). They even work on cloudy days, just less efficiently.
I wired one into a friends car with a simple plug so when he parks the car he just puts the panel in the windscreen and plugs it in.