profdant139 wrote:
A lot of thought-provoking discussion!! To start with Steve's comment, our trailer is so small that there is very little room on the roof for permanent panels. That fact, plus the "very shady campsite" factor, means that for now we will stay with a portable panel.
The big take-away from the earlier posts is that I have some more homework to do. I will have to test the cables with a semi-depleted battery, instead of a fully charged battery. And I will have to test not only the voltage but also the amperage. (I wrote myself a "non-expert's cheat-sheet" on how to use my multi-meter as an ammeter -- if anyone is interested in that cheat sheet, just say so and I will post it.)
I am surprised to hear that 13.5 volts won't even charge a semi-depleted battery -- we have found, after much real-life experience, that my 40 foot cable will do a fine job of charging a battery at around 13.9 volts.
I know this for a fact because we test our battery (at rest) with a voltmeter almost every evening, after the sun goes down. It's almost always at 12.7 volts. Even on a bad day (some clouds or partial shade), it's at 12.5. By contrast, in the morning, it tests out at about 12.3 or so.
The obvious solution to all of this is to bring both cables but then to deploy the 70 footer only if needed -- otherwise, the 40 footer will be preferable. The problem, though, is that the 70 footer is very bulky and heavy, and storage room is at a premium. So if it causes a significant voltage drop, and will not do a very good job of recharging the battery, then I'm better off without it.
get yourself some "anderson" connectors. Cut the 70 ft in half, or maybe a 50ft and 20 ft. put anderson connectors on the other cord and on the panel. Makes a quick and easy way to get reliable connections and use what you need for cable.
Take the PWM controller off the panel and rewire to have it either in the pass through to the batteries or in a portable configuration near the battery. THIS IS KEY. PWM CONTROLLER NEEDS TO BE CLOSE TO THE BATTERY. Voltage drop from the controller to the battery kills charging current. voltage drop from the panel to the PWM controller doesn't have much effect till you get around a 4V cable drop, not likely.
Now you have (a) a variety of cable lengths to choose from (b) a configuration that is pretty tolerant of voltage drops.
Since panel output voltage is around 18V or more under load, you need about a 4V drop, unlikely with the wiring and panel size before you wil see a drop in charging current when using a PWM controller. And the current is what you care about.
Let's you park in the shade and charge from the sun. We do it all the time and I have 480W of panels to a MPPT controller and #10 wire. yes I suffer some power loss, but the increase in sun more than makes up for it.
