Posted by Sherman in Idaho [108.162.246.23] on Friday, May 27, 2016 at 14:46:19 :
In Reply to: AC Powered Trickle Charger? posted by Dan (NH) [108.162.219.144] on Friday, May 27, 2016 at 10:55:42 :
And in theory you need to charge float-charge a lead-acid battery with a voltage that is adjusted correctly according to the temperature of the electrolyte, and in theory the only measure of "full charge" is the specific gravity of the electrolyte, not the open-circuit voltage, which in any case will drop significantly in the first day or so after the charger is disconnected.
But in reality, you can put 13.8 volts across the battery forever, under normal conditions, or you can put several amps through it continually, without doing any significant harm. Likewise, the charge cycling due to the sun coming up and going down has no discernible effect on battery life. A battery that isn't used and is kept under a constant voltage will still go bad eventually.
The self-discharge rate of a good battery varies with all sorts of things, but a fair guess is that it's in the tens of milliamps range. Pretty much any solar panel that works at all will suffice to make up for that small self-discharge current. If you have anything in the vehicle that draws current even when "off" (modern radio, car alarm, etc) then you need a bigger solar panel. But in terms of charging cycles, a small solar panel, even without any kind of charge controller is very gentle on batteries. Batteries don't like to be charged quickly. That diminishes their overall capacity. Slow and steady is the way to go, and that's what you get with a small panel.
The problem with cheap panels is that even if the wires don't break and the seals don't leak, both of which are likely, the material they make them out of wears out. I used to use Harbor Freight panels on my cabin and I could figure that within 2 or 3 years the output would be down to half of what it was when they were new. So, no matter what you use, always check with a digital voltmeter to make sure that the voltage at the battery actually goes up when you attach the panel. It doesn't have to go up much, but at least make sure it goes up.