Anyway, this is my $15 setup that I'm testing out now (without dashcam load). Just monitoring to see if it's enough to sustain the battery voltage on a small car parked, and not being driven for 2-3 weeks at a time. Controller is set to float ast 13.7v, to turn off the load if voltage falls below 11.6v (that's the highest it can go), and to turn the load back on when battery is at 12.6v. At this point the load settings are irrelevant because there is nothing connected to its load terminals. The car battery fuse tap is connected to battery terminal of the controller. Technically when the car is not running, all the controller sees is the battery. Desulphication voltage is set to 14.6v, but I don't know what that really does because the manual does not describe any sort of routine for this purpose.
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Fuse tap and ground are connected to fusebox as if powering a dashcam with parking mode. The quick connector is tucked away into the glovebox for easy storage and access.
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So when the car is parked, I take the panel + controller glued behind it out and put it against the windscreen with the sunshade holding them up. The quick connector snaps to the other end from the glovebox, and away we go. I've left the car like this for 5 days so far, and I read the battery voltage via whatever being reported by the insurance company's telemetry device which gets updated about twice a day when the car isn't moving.
The car's battery is a paltry 34B19L, and the panel is rated at 30w (no-load voltage under direct sunlight at the hottest time of the day is 20.7v). On the first day, the voltage started out at 12.5v. 3 days later it's down to 12.2v (it had been raining every day from about 2~3 pm onwards, so there's only really hot sun from about 9:30am to 1:30pm). Let's see how long more the voltage can hang on at 12.2v.
At the rate it's going, I don't think this can sustain any sort of motion detection parking mode, or time lapse / low bitrate parking mode. Perhaps the sleep --> G sensor --> wake up --> record --> sleep sort of parking mode is possible to keep going for weeks/months on end. Perhaps a 50w panel would be able to do better, but the controller didn't regulate current and I didn't want the panel to be dumping a large current onto the battery for
residential solar installation, therefore I figured 2.1A maximum was pretty safe. Maybe 3.6A x ~40-50% efficiency might have been able to keep the battery's voltage higher.
By the time we get to 100w panels, it's taking up too much space already, and clumsy to move in and out. Will continue to keep watch and update if anything interesting happens.