Newer car battery dies below starting level while parked for a month at airport. Have 1+ amp solar panel glued to trunk attached to battery. (Same panel model keeps my boat battery charged, but not car.) Computer draw is less than 1/4 amp. How do I return to my car without needing a jump?
A quarter of an amp is way too much draw when off. 50 milliamperes, (0.05 amps). yours is drawing 0.25 amps.
Most vehicles draw some battery current when the key is off , thanks to the clock and the internal memory of engine computers, body-control modules, and radio presets. Altogether, they draw a very small amount of current . Fifty milliamps would be a safe upper limit for this, though many vehicles will draw less.
Your solar panel only produces power during periods of sunlight. I think it can not keep up with the excess power drain.
Indeed, the 250 milliamp draw is excessive so you are going to have to track down the source of the problem. Once that problem is eliminated I think your solar panel will do fine to keep a charge up on the battery. Here is a link you can refer to that will help you find the problem.
By chance, you don’t have anything plugged into your cigarette lighter, do you?
I’m asking because your cigarette lighter is always hot, so anything plugged in could drain the battery
Even a charger with nothing attached . . . those little LEDs draw current
Do you have an aftermarket alarm or sound system?