I looked into this.
In my plane, there is no way to get to the battery via the cigar lighter socket unless the Master is on, and then there is about 2A current draw due to the gyros etc. The CB on the cigar lighter socket is 5A, so the charger would have to be current limited to about 2A, and voltage limited to something which fairly accurately charges the battery at the same time. One could trickle charge in this way but anything faster you can forget, and the charger would have a hard time switching to a trickle charge mode due to the other avionics current draw. Also there is the pointless wear on the KG-102 gyro. One could pull the gyro CB but those CBs are not great for longevity
An "unofficial" connector via the clock or the courtesy light would work for trickle charging.
I have a
solar panel too, which delivers about 1-2A at full sunlight into a 24V battery. It rolls up into a tube and weighs about 3kg. I have carried it on long trips but it never got used.
Many years ago, in the days of my ex wife and her horses, I completely successfully used a 2ft x 2ft solar panel to charge a car battery used for an electric fence. That works very well.