profdant139 wrote:
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.
13.8V is barely charging and 13.5V is "maintaining", that is the difference.
A two percent difference can and does make a difference when you are dealing with a tenth of a volt when it comes to 12V batteries.
What matters is how much difference in amperage your battery actually is charging. Along with your voltage measurements take current readings.. Bet you will see a larger current reading with the shorter cable and that means your maximizing your charge current.
Batteries act in a non linear way making it difficult to predict, think of a battery like a variable resistance. When the battery is partially discharged the resistance is different from when it is fully charged..
Your solar panel voltage rating is pretty low for charging a 12V battery. Typical old school solar panels designed to charge 12V batteries will typically have a no load output voltage of 17V or a bit better.
More modern panels now days have voltages over 19V open circuit..
Example, Renogy 100W 12V panels are rated 24.3V open circuit, optimum operating voltage is 20.4V
The higher panel voltage maximizes the hrs of energy harvest..
