Brunton Solarflat 2w 12v 12x5x.075” Amorhous 3 Solar Panels - $39.95

I’m not a "Physics Buff", but a retired Navy submarine electrician... That said, some of the parameters involved, such as energy vs. power, and current flow vs. voltage are taught in a typical HS Physics class, and I have a significant amount of education in Physics independent of my knowledge of and expertise with lead-acid batteries.

This thing is intended to simply maintain the fully charged condition of a battery that is not frequently used. It will take a long while for any device that generates only 2 watts (power) to fully charge a car battery.

The parameter that is really important for how long it takes to charge a battery is the peak rate of current flow in amps (or in this case, milliamps (140 ma)), at which you can start a charge. That's 0.14 amps. Even though the charge rate drops off later in the charge, if you can start a charge at the max rate that the battery will handle, the time to fully charge the battery is significantly less.

Consider that my garage battery charger has two settings: 2 amps and 6 amps. At the 6 Amp setting, I can charge a typical car battery in a matter of hours. But that is nearly 43 times the max rate available from this panel. You can’t calculate total charge time based on that, because as the battery charges, the individual cell voltages rise and total battery voltage rises across all six cells, and the charge rate decreases exponentially and tapers off. So the charge rate after six hours on the six amp setting may very well have dropped to .14 amps.

If you want to quantify how long it will take to charge a particular battery with this thing, you can get a reasonable estimate by finding out the Amp-Hours capacity (AH) (stored energy) of the battery in question. Because of the diversity of vehicles, those capacities vary widely, but let’s assume a mid-range capacity of 50 AH.

Divide 50 AH by the 140 ma charge rate (0.14 amps), and you get about 357 hours, or roughly two weeks. We’re also assuming 100% efficiency here, which is not realistic. What I mean by that is that you might have to charge in 70 or 80 AH to get 50 out of it. That’s going to vary from manufacturer to manufacturer, and the condition of the battery. All batteries lose capacity as they age, and many car batteries are not well maintained. Even sealed “No Maintenance” batteries have terminals and ground connections that must be cleaned from time to time, and the outer surface of the battery should be cleaned to minimize power drain to ground.

Every submarine electrician is intimately familiar with cleaning in the battery well to improve DC bus ground readings (though the main concern with submarine DC bus voltage is electrical safety, rather than maximizing battery capacity and efficiency).

Did I make all of that understandable for anyone who’s not a “Physics Buff”?

really more of a comment i guess, because it would take quite a while to charge a car battery if at all. I think there are a lot of Smart people here, the dumb ones pay full retail.