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 > Break-even point between cable length and voltage drop?

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profdant139

Southern California

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Posted: 04/05/22 07:20pm Link  |  Quote  |  Print  |  Notify Moderator

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!


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BFL13

Victoria, BC

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Posted: 04/05/22 07:36pm Link  |  Quote  |  Print  |  Notify Moderator

Voltage drop is related to amps. As the battery gets near full, amps are low so little voltage drop. Your system gets the battery near full before dark so no problem


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LittleBill

Scranton, PA USA

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Posted: 04/05/22 07:40pm Link  |  Quote  |  Print  |  Notify Moderator

your best bet, is to wire the panels in series, and put the controller as close to the battery as possible. that's the best your going to do.





profdant139

Southern California

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Posted: 04/05/22 07:47pm Link  |  Quote  |  Print  |  Notify Moderator

BFL, that is valuable info! The battery was full when I did this test, so that may explain why the voltage drop was so slight.

I should deplete the battery to a 50 percent state of charge (around 12.1 volts) and then re-try my side by side comparison of the two cables. I suspect that the drop will be more than two percent.

And LittleBill, that's a good idea, but the controller is built in to the portable panel and can't be removed.

LittleBill

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Posted: 04/05/22 08:26pm Link  |  Quote  |  Print  |  Notify Moderator

profdant139 wrote:

BFL, that is valuable info! The battery was full when I did this test, so that may explain why the voltage drop was so slight.

I should deplete the battery to a 50 percent state of charge (around 12.1 volts) and then re-try my side by side comparison of the two cables. I suspect that the drop will be more than two percent.

And LittleBill, that's a good idea, but the controller is built in to the portable panel and can't be removed.


at the end of the day it doesn't matter, even if the 75 foot cable had 15% loss, if it gets 50% more sun then the 40 foot cable, your still going to be ahead.

BFL13

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Posted: 04/05/22 08:26pm Link  |  Quote  |  Print  |  Notify Moderator

120W is at low amps already so batt SOC does not matter. Time in the sun is needed in both cases, and will be about the same

Gdetrailer

PA

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Posted: 04/05/22 08:45pm Link  |  Quote  |  Print  |  Notify Moderator

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..

BFL13

Victoria, BC

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Posted: 04/05/22 09:34pm Link  |  Quote  |  Print  |  Notify Moderator

PWM has panel V at battery V so his 13.5 means batt is not at controller Vabs yet. Batt not full or if full sun too low end of day to keep voltage up

Important once home to get batt right full

ktmrfs

Portland, Oregon

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Posted: 04/05/22 09:55pm Link  |  Quote  |  Print  |  Notify Moderator

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

* This post was edited 04/05/22 10:02pm by ktmrfs *


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ktmrfs

Portland, Oregon

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Posted: 04/05/22 09:57pm Link  |  Quote  |  Print  |  Notify Moderator

LittleBill wrote:

your best bet, is to wire the panels in series, and put the controller as close to the battery as possible. that's the best your going to do.


only if you have a MPPT controller, with a PWM controller series panels are a NO NO NO all you will do is waste the power from the additional panels.

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