Things got a bit crazy a few years ago when I had an E92. I had an all out security package by Viper: glass break sensor, tilt sensor, proximity sensor, back up siren and, flashing logo. I also had a tracker and a dash cam. In the trunk was a JL Slash Amp that had a large current draw when in use (their RIPS technology). On top of that I had comfort access.
When left not in use for 3 days the voltage would drop to 11.8v-11.7v. A week without driving and I was down to 11.6v. Three weeks down to 11.3v. We moved office and for six months my drive was 6 miles / 15 minutes. The engine rarely got up to temperature.
I invested in two things. First I bought a smaller and cheaper variant of this.
https://www.westfalia.net/shops/elec...le-sockets.htm
There are many versions out there with different output, charge and, features.
I placed it on the rear parcel shelf at an angle. It worked. When I tested the battery with a multimeter I was holding around 12.1v-12.2v. It came with overcharging protection so there were no concerns.
Secondly to add to my current draw I bought this to monitor it all but only hooked it up when I needed it:
https://www.smarterchargers.co.uk/in...product_id=103
Of course there was the usual backlash on the forum of why would you waste your money on that Bluetooth monitor? Hey it worked for me.
If you do go down the solar panel route then either use some velcro to dampen back shelf rattle or what I did was just remove it when ready to drive. Never drive with it connected. Other than risking voltage spikes it can turn into a missile when you brake. Mine once fell off the shelf while driving but it was disconnected. I didn't want to damage it. Some are designed for outdoor use so if you are in a safe area you could leave it on the roof or trunk lid with something beneath to avoid paint scratches. You probably won't need as powerful one as I had but there is no harm because the output is effected by the weather. If the weather is like a fat dead pigeon (cold and grey outside) then there will be reduced output.
If you have access to a power source then all of this be unnecessary by using a battery tender such as CTEK. I had no power at the office car park. I would use a battery tender when home on the weekends though. Solar still worked though for me as I parked at the airport a few weeks at a time. If you travel regularly or don't have easy access to power where you park then solar is the best way in my humble opinion.
For our F30, I bought a weaker version as I parked at the airport for a month. The car had no aftermarket extras. When I got back there were no issues with comfort access. I parked the rear of the car towards the direction of sunrise to get the most from it.
In case of emergency:
https://www.amazon.co.uk/Roypow-J12-.../dp/B00VA0XBKK