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

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pianotuna

Regina, SK, Canada

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Posted: 04/06/22 01:25am Link  |  Quote  |  Print  |  Notify Moderator

Hi profundant,

Solar panels are considered to be constant voltage devices. Bright sun ought not make much of a difference.

I'm a little surprized that the suitcase only produces 14 volts. I'd want at least 17 volts.

Have you measured the panel voltage in a no load situation?


Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

theoldwizard1

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Posted: 04/06/22 04:18am Link  |  Quote  |  Print  |  Notify Moderator

Gdetrailer wrote:


13.8V is barely charging and 13.5V is "maintaining", that is the difference.
.
.
.
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.

Concur !

I think your voltage is low because you are using smallish portable panels. Check their spec sheet for open circuit (not connected to anything) voltage. It might be time for an upgrade !!

StirCrazy

Kamloops, BC, Canada

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Posted: 04/06/22 06:45am Link  |  Quote  |  Print  |  Notify Moderator

if you want a 1% or less voltage drop the calculator says you need a 2/0 cable and you can get away with a 6 ga at a 5% voltage drop over 70 feet. 10ga is serioulsy undersized for a 120 watt set up with a 70 foot run. myself I would probably try go as large as I could but there are a couple things I look at, cost and storage. so is the cost worth the extra benifit and can I store that extra cable easy. if I was doing it I would probably replace it with a 2ga if you can wire that into your controler, but some controlers will only accept a max size if thats the case just go to that and call it a day, or leave it and get what you get.


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valhalla360

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Posted: 04/06/22 06:49am Link  |  Quote  |  Print  |  Notify Moderator

Simple solution: Next time you are out, spend 20 min and test it.

Find a reasonable location the 40ft cable will reach and hook it up with an amp meter in the circuit (preferably right at the battery).

Write down the voltage and amps.

Then leaving the panel unmoved, connect with the 70ft cable and check the voltage and amps.

Try to do it on a clear day and fairly quickly. Otherwise you risk variations in the solar input (like clouds casting a shadow) messing up your estimates.

But in general terms, if you can reach the preferred location with the 40ft cable, use the shorter cable. If you can eliminate shadows, it's almost always worth it because a bit of shade on a panel can pretty much kill all output from the panel.


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2oldman

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Posted: 04/06/22 07:38am 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.
X2

BFL13

Victoria, BC

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Posted: 04/06/22 07:59am Link  |  Quote  |  Print  |  Notify Moderator

It is PWM. All Vdrop does is make the controller hold the high set point at a lower V but by the time the batt gets to a high SOC the amps have tapered

Also amps are lower later in the day when the batt gets full with the 120W only doing about 7 amps at high noon

The 13.8 vs 13.5 is a difference not the Vdrop for each wire at pick a number amps

OP needs an hydrometer. His batt could be sulfated with those 13 .x voltages


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CA Traveler

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Posted: 04/06/22 08:03am Link  |  Quote  |  Print  |  Notify Moderator

Sunshine is King, use the shorter cable but if a longer cable is needed for sun then use it. You can even try both cables in series. Wiring loss will limit the charging.

FYI Unfortunately the controller at the panel results is the most inefficient charging due to it’s output voltage drop to the battery. As an example the typical panel voltage will be 18V+. Better to have the long cable wiring loss to the controller input with the controller output voltage at 14.8V for a discharged battery and short cables to the battery vs a lower charging voltage at the battery.


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Posted: 04/06/22 08:13am Link  |  Quote  |  Print  |  Notify Moderator

Just saw some of the prior posts. NOTE OP stated his controller is built into the panel and hence the wiring loss due to the cable is more significant and series panels/MPPT are not a option. But good considerations for a new setup.

BFL13

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Posted: 04/06/22 08:34am Link  |  Quote  |  Print  |  Notify Moderator

Who says the longer wire will have fewer amps to the batt with whatever the amps the 120W array is supplying at the time? Max is about 7 amps briefly at noon Will the longer wire choke even 7 amps by more than the shorter to notice?

* This post was edited 04/06/22 08:41am by BFL13 *

SteveAE

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Posted: 04/06/22 08:47am Link  |  Quote  |  Print  |  Notify Moderator

Hey Dan, hope you guys are well.

Looks like you are getting lots of advice, so I am going to take a different approach the problem.
I considered all this when I got panels and concluded that (for me) it was better to permanently mount panels on the trailer roof. No, you don't get as much of a charge when in the shade (surprisingly, I often do get enough, especially under conifers ..... but not so much under deciduous trees). However, IMO, the advantages of roof mounted panels outweigh the negatives with; shorter (and heavier i.e.; larger diameter) cable runs, no worry about theft, the batteries charge when on the road and parked at home, no set-up and monitoring of the panels, no storage space needed for the panels/wire/controller, and no fussing (more time for hiking, etc.). And I suspect if you made this change, you would wonder why you hadn't done it sooner.
Additionally, there is no reason (other than cost) that you couldn't use a combination of permanently mounted panels and your portable system if you wanted that occasional extra boost when in the deep shade.....though I suspect that after a few trips you will start leaving the portable system system at home.
Anyway, there you have it. A different approach to the problem.
Regards.

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