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Break-even point between cable length and voltage drop?

profdant139
Explorer II
Explorer II
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!
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)
Our trips -- pix and text
About our trailer
"A journey of a thousand miles begins with a single list."
44 REPLIES 44

BFL13
Explorer II
Explorer II
Once in the sun all day worth rotating tilted array three times.
Sunset to SE
Mid morning to S
Mid afternoon to SW

Also tilt lower mid day with sun higher

Eg 130W in May at 49N daily AH haul

Flat 56
South tilted all day 70
Tilted rotate three times 90

OP 110AH batt 50% needs 55 plus 5 for heat so
60 is in range after using some to run things during daylight too and 120W other times of year and latitudes
1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.

SteveAE
Explorer
Explorer
Short of modifying your existing system (or better yet, building your own system that will better work for you....panels and a controller are super cheap these days), I like the idea of having two different length cables that you can mix and match as needed. Just keep in mind that every time you have a connection point there is an increase in resistance.....and a corresponding voltage drop.

But, keeping with my theme of looking for alternative solutions (I guess they call it thinking outside the box) here are some more (dumb???) ideas:
- Get a second portable system that you put in parallel with your existing system and double the fun of chasing the sun around.
- Have a second battery that you put right by your panel, then swap batteries as needed. If you can't bring Mohammad to the mountain, then move the mountain to Mohammad...or some such nonsense.
- Put another cable in parallel with your existing cable.

profdant139
Explorer II
Explorer II
The only thing worse than humble pie is when it comes with a side order of crow. Don't ask how I learned this.
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)
Our trips -- pix and text
About our trailer
"A journey of a thousand miles begins with a single list."

pianotuna
Nomad II
Nomad II
Munches on humble pie on sometimes a daily basis!
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.

profdant139
Explorer II
Explorer II
Very clever -- just bypass the controller and use another controller near the battery! I'm not sure I will do that -- depends on the cost and my lack of skill -- but that is definitely thinking outside the box.

It's interesting (in a 30,000 foot way) to see myself as a captive of my own assumptions, for example: "Well, the controller is permanently attached, so I guess I am stuck with it."

It is very hard to force myself to think in a non-linear way: what if we ignore the existing controller?? Once someone else points out the obvious alternative (usually DW), I feel humbled. It is not a pleasant sensation.
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)
Our trips -- pix and text
About our trailer
"A journey of a thousand miles begins with a single list."

time2roll
Nomad
Nomad
Avoiding shade is the most important. Why choose length? Carry a 25' and 50' and use the shortest combination of 25, 50, or 75' to stay in the sun.

Always better to have the controller close to the battery.

ktmrfs
Explorer
Explorer
profdant139 wrote:
OP here. Very interesting discussions!!

I will take another hard look at my controller -- I am pretty sure it is molded onto the back of the panels with plastic -- not designed to be removed. It would take a lot of courage for me to cut it out and move it to the battery. What if I mess it up? These suitcase panels are not cheap.

I think I understand, though, why the manufacturer designed a less-efficient "all in one" system. The goal is a consumer-friendly "plug and play" portable solar panel -- nothing to do but just hook up the cable. The more complex the system, the greater the sales resistance.

I know that my 12.7 daily reading is not a full 12.8, but the proof of the pudding is in the eating, as they say. We routinely boondock for a week at a time, using the solar panel to top up the battery to 12.7 every day. We have never lacked for power. (Admittedly, we are very frugal.) So the panel is providing us with more than mere maintenance -- the power system does just what we need it to do.

If it were doing nothing more than maintaining the battery at its then-current charge, we'd gradually run out of power during the week. We don't.

I like the idea of chopping up my cable into pieces and attaching as many segments as needed. But I'd still have to transport all of the pieces.

If the controller is molded on, a decent PWM controller for that size panel is likely pretty inexpensive, you may need to do some rewiring depending on how the controller is connected to the panel.

In any event, having the controller on the panel is a big drawback and is going to be a definite negative to getting decent charging current.

I've 'modified" over a dozen portable panels for friends and myself, I've managed to bypass the "attached" controller pretty easily. In some cases they use mc4 connectors betwen the panel and controller, so bypassing the controller is "easy peasy, others I needed to open the controller and disconnect and add MC4 connectors. In all cases the typical use was 2 or 3 panels, so in the pass through either went a MPPT controller or a 30A PWM controller from midnite solar (BRAT)
2011 Keystone Outback 295RE
2004 14' bikehauler with full living quarters
2015.5 Denali 4x4 CC/SB Duramax/Allison
2004.5 Silverado 4x4 CC/SB Duramax/Allison passed on to our Son!

Gdetrailer
Explorer III
Explorer III
MNRon wrote:
Gdetrailer et al - check my math here, good catch on return path:

Simple answer, solar panel placement affects charging more than the difference between 40ft and 70ft of 10ga.

More complex answer:
Based on OP's first post the controller puts out 14v and he measured 13.8v at battery and 13.5v at battery on 40/70ft runs. This would imply with 40f he is getting 2.5A charging current (200mV drop across 80ft (round trip) at 1mOhm/ft); and at 70ft he's getting 3.6A charging (500mV across 140ft...). The reality is that his measurements etc aren't that accurate and I'm sure that he's getting roughly the same current charging with either run, the limiting factor in his charging is *not* the IR drop in the wire, but the internal IR drop inside the battery (if you will, actually more complicated). The truth is that with ~13.5v across the terminals the battery when nearly fully charged is the current limiter and isn't accepting much current.

If he did the same experiment with 50% discharged batteries (12.1v) he will be putting a larger voltage differential onto the batteries and will drive more current. In this case the wire IR resistance might come in to play but I'm guessing second order compared with sun exposure on solar panels. Assuming full 7A charging from his panels he'd be putting ~13v across the battery with 70ft (including round trip) and 13.4v with 40ft; I suspect a 50% depleted battery will draw 7A regardless of 13v or 13.4v across it, and as it charges up the battery internal resistance will be a larger current limiter than the IR drop (as evidenced by his measurements in the first place).

Thoughts?


Pretty much.

On of the biggest catch 22's is the battery internal resistance changes as it charges in a non linear and non predictable way.

The wire resistance will stay linear but as the battery draws more current during the charging phase the voltage at the battery drops. That drop will limit the maximum voltage the battery will see which in turn limits the maximum charging current.

Toss on top of that is the fact that the solar panels also have internal resistance which is not 100% linear, that internal resistance is what limits the peak current the panel can deliver.

Resistance is the enemy, some resistance in the circuit can't be removed and what you can control is the distance and wire ga you use.

Basically, in a nutshell the line resistance does have a limiting effect so a 100W panel on the end of a long run of wire will deliver substantially less than 100W.. In reality even if one were able to harvest all 100W that is roughly 7A of charging for at best 10hrs per day. That nets you a absolute max of 70Ahr at best, but with the OPs setup, might get 30Ahr due to the controller being remote.. If that is acceptable, then roll with it.

Personally out of curiosity, adding in a ammeter between the controller and the battery would give the OP a much better read on the situation.. Voltage alone doesn't give the full story of what really happens.

profdant139
Explorer II
Explorer II
OP here. Very interesting discussions!!

I will take another hard look at my controller -- I am pretty sure it is molded onto the back of the panels with plastic -- not designed to be removed. It would take a lot of courage for me to cut it out and move it to the battery. What if I mess it up? These suitcase panels are not cheap.

I think I understand, though, why the manufacturer designed a less-efficient "all in one" system. The goal is a consumer-friendly "plug and play" portable solar panel -- nothing to do but just hook up the cable. The more complex the system, the greater the sales resistance.

I know that my 12.7 daily reading is not a full 12.8, but the proof of the pudding is in the eating, as they say. We routinely boondock for a week at a time, using the solar panel to top up the battery to 12.7 every day. We have never lacked for power. (Admittedly, we are very frugal.) So the panel is providing us with more than mere maintenance -- the power system does just what we need it to do.

If it were doing nothing more than maintaining the battery at its then-current charge, we'd gradually run out of power during the week. We don't.

I like the idea of chopping up my cable into pieces and attaching as many segments as needed. But I'd still have to transport all of the pieces.
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)
Our trips -- pix and text
About our trailer
"A journey of a thousand miles begins with a single list."

ktmrfs
Explorer
Explorer
MNRon wrote:
Gdetrailer et al - check my math here, good catch on return path:

Simple answer, solar panel placement affects charging more than the difference between 40ft and 70ft of 10ga.

More complex answer:
Based on OP's first post the controller puts out 14v and he measured 13.8v at battery and 13.5v at battery on 40/70ft runs. This would imply with 40f he is getting 2.5A charging current (200mV drop across 80ft (round trip) at 1mOhm/ft); and at 70ft he's getting 3.6A charging (500mV across 140ft...). The reality is that his measurements etc aren't that accurate and I'm sure that he's getting roughly the same current charging with either run, the limiting factor in his charging is *not* the IR drop in the wire, but the internal IR drop inside the battery (if you will, actually more complicated). The truth is that with ~13.5v across the terminals the battery when nearly fully charged is the current limiter and isn't accepting much current.

If he did the same experiment with 50% discharged batteries (12.1v) he will be putting a larger voltage differential onto the batteries and will drive more current. In this case the wire IR resistance might come in to play but I'm guessing second order compared with sun exposure on solar panels. Assuming full 7A charging from his panels he'd be putting ~13v across the battery with 70ft (including round trip) and 13.4v with 40ft; I suspect a 50% depleted battery will draw 7A regardless of 13v or 13.4v across it, and as it charges up the battery internal resistance will be a larger current limiter than the IR drop (as evidenced by his measurements in the first place).

Thoughts?


Yup, Once the OP corrects the poor (wrong) placement of the charge controller and gets it close to the batteries, He will find, like I have found, that long runs may loose a fraction of an amp in peak charging current with a PWM controller, but gain with best sun placement. Even a slight cloud cover or shade will have a much bigger detrimental effect on charge current than the long run. And once he does that he will find that peak charging current with a reasonably discharged battery will be close to Imax for the panel. The power loss in the cable is a linear function of current, while the the panel conversion efficiency is a highly nonlinear function of sunlight, slight drop in sun or panel shade= large drop in output.

And he won't notice the power loss in the cable unless the voltage drop is enough to drop the voltage at the controller down to around 15 volts. Given that loaded output voltage of most panels is in the 18+ volts you need either REALLY skinny wires or really LONG run to get that much drop with the panel size he has.

Recently we have had days with high thin clouds. My roof solar panel will go between 2A and 9.5A charging current with the sun goin "behind" the thin clouds, a difference in shading that one hardly notices when standing outside. Same with shading part of the panel.
2011 Keystone Outback 295RE
2004 14' bikehauler with full living quarters
2015.5 Denali 4x4 CC/SB Duramax/Allison
2004.5 Silverado 4x4 CC/SB Duramax/Allison passed on to our Son!

Lwiddis
Explorer II
Explorer II
13.5 and even 13.8 charging volts at the battery isn’t going to cut it with a Lithium battery.
Winnebago 2101DS TT & 2022 Chevy Silverado 1500 LTZ Z71, WindyNation 300 watt solar-Lossigy 200 AH Lithium battery. Prefer boondocking, USFS, COE, BLM, NPS, TVA, state camps. Bicyclist. 14 yr. Army -11B40 then 11A - (MOS 1542 & 1560) IOBC & IOAC grad

MNRon
Explorer
Explorer
Gdetrailer et al - check my math here, good catch on return path:

Simple answer, solar panel placement affects charging more than the difference between 40ft and 70ft of 10ga.

More complex answer:
Based on OP's first post the controller puts out 14v and he measured 13.8v at battery and 13.5v at battery on 40/70ft runs. This would imply with 40f he is getting 2.5A charging current (200mV drop across 80ft (round trip) at 1mOhm/ft); and at 70ft he's getting 3.6A charging (500mV across 140ft...). The reality is that his measurements etc aren't that accurate and I'm sure that he's getting roughly the same current charging with either run, the limiting factor in his charging is *not* the IR drop in the wire, but the internal IR drop inside the battery (if you will, actually more complicated). The truth is that with ~13.5v across the terminals the battery when nearly fully charged is the current limiter and isn't accepting much current.

If he did the same experiment with 50% discharged batteries (12.1v) he will be putting a larger voltage differential onto the batteries and will drive more current. In this case the wire IR resistance might come in to play but I'm guessing second order compared with sun exposure on solar panels. Assuming full 7A charging from his panels he'd be putting ~13v across the battery with 70ft (including round trip) and 13.4v with 40ft; I suspect a 50% depleted battery will draw 7A regardless of 13v or 13.4v across it, and as it charges up the battery internal resistance will be a larger current limiter than the IR drop (as evidenced by his measurements in the first place).

Thoughts?
Ron & Pat
2022 F350 Lariat CCSB SRW Diesel
2019 VanLeigh Vilano 320 GK

BFL13
Explorer II
Explorer II
BFL13 wrote:
PWM controller passes batt V to the panel so that is the V seen at the panel,less Vdrop not the controller's Vabs voltage of say 14.4

The IV curve for the panel uses batt V
1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.

BFL13
Explorer II
Explorer II
Deleted
1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.