profdant139 wrote:
I have two cables for my portable solar panel – one is 40 feet long, and the other is 70 feet long. (We boondock, and we like to park the trailer in deep shade and put the panel out in the open sun.).
Obviously, the longer cable gives us more reach and flexibility. The trade-off, of course, is that the longer cable causes a slight voltage drop. How do I decide if the extra length is outweighed by the voltage drop? Is there a break-even point?
Here are some more facts: our "suitcase" solar panel is rated at 120 watts and supposedly puts out about 14 volts in bright sun. Both cables are ten gauge wire. When I hook up my 40 foot cable and measure the voltage at the battery terminals, it reads 13.8 volts. The 70 footer reads 13.5 volts. I think that's about a 2 percent drop (0.3 divided by 13.8).
Does that matter? Does a 2 percent drop mean, for example, that it will take the solar panel 2 percent longer to charge my battery? If so, that's probably trivial, and I will use the longer cable. For example, in a typical day, the solar panel is putting out juice for eight hours, or 480 minutes. Two percent of 480 is about ten extra minutes: no problem.
Or is this some sort of non-linear function, where a two percent voltage drop means that it takes a lot longer to charge the battery? In that case, I will use the shorter cable.
By the way, and in case it matters, we use a group 31 12V flooded lead acid 110 amp/hour NAPA battery, which is supposedly a true deep cycle marine battery. I always keep it on a BatteryMinder Plus when we are at home, and I never let it get below 12.1 volts when we are traveling. Our little solar panel has almost always fully recharged the battery, every day.
(And in case you're wondering, the 70 foot cable was a gift from a generous neighbor who was clearing out his garage. It weighs 25 pounds and is very bulky, but I could not pass up the chance to have a super-long cable.)
Thanks in advance for your wisdom and expertise!
OK, if you are using a PWM controller NEAR the batteries you need really long cables before you see much of a drop in charging current unless you have very little sun. Reason is that the panel voltage is well above battery voltage so you still get pretty much all the current the panel can provide. If the controller is AT THE PANEL, another story, then the effects are dramatic, even a few tenths drop can decrease battery current dramatically.
If you are using a MPPT controller, power drop is voltage drop x current in the cable. that is the watts you will loose. again with 10 gauge wire it will be some, but if the location gives you good sun your ok.
And as the batteries get close to full charge the effect drops off
I have over 150Ft of 30A RV power cable I use for my portable panels, cut the ends off, wire ground and neutral together to decrease resistance and use anderson connectors to splice the length I need. Then place them were sun is best. I use MPPT controller, yes I loose a few watts, maybe 0.2 A worst case charging current but the advantage of good sun outweighs the length. And once the batteries near full charge current drops enough that it has virtually no effect. Controller is mounted in the pass through
