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

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Gdetrailer

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Posted: 04/06/22 01:50pm Link  |  Quote  |  Print  |  Notify Moderator

MNRon wrote:

10ga wire is approximately 1 ohm/1000ft. So if the panels try to push 7A, that would be a ~0.5v drop from panel to battery (70ft x 0.001ohm/ft x 7A). The 40ft cable will be ~0.3v drop at 7A. As others have said, sun position will be more important than the difference between these.


This would be all well and good IF the OPs charge controller was at the battery end.

But, it is not the case for the OPs setup.

The OPs charge controller is integrated into the portable solar panel. The output voltage of the controller is fixed 13.5V.

Drop .3V and the battery sees only 13.2V which is nothing more than a trickle charge.

However, your assumption of .5V drop is not correct as it is only taking into account for 1 wire, to complete the circuit there is two wires involved.. So the actual length of wire is 140ft for the 70ft run and 80ft for the 40ft run..

Now that turns out to be 1V drop for 70ft (140ft total length of pos and negative) run at 7A and .6V drop for 40ft (80ft total length of pos and negative)run at 7A..

Now things would be better if the OP could separate the controller from the panel and mount the controller at the battery..

This would mean the input of the controller gets full panel voltage (17V or more)- the voltage lost from the wire and the battery now gets the max voltage output of the controller (13.5V) instead of 13.5V - the voltage lost from the wire run = 13.1V..

You can't make "amps" if you don't have the "volts"..

BFL13

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Posted: 04/06/22 02:14pm Link  |  Quote  |  Print  |  Notify Moderator

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BFL13

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Posted: 04/06/22 02:17pm Link  |  Quote  |  Print  |  Notify Moderator

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


MNRon

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Posted: 04/06/22 03:15pm Link  |  Quote  |  Print  |  Notify Moderator

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?


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Lwiddis

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Posted: 04/06/22 03:20pm Link  |  Quote  |  Print  |  Notify Moderator

13.5 and even 13.8 charging volts at the battery isn’t going to cut it with a Lithium battery.


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ktmrfs

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Posted: 04/06/22 05:28pm Link  |  Quote  |  Print  |  Notify Moderator

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.


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profdant139

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Posted: 04/06/22 08:11pm Link  |  Quote  |  Print  |  Notify Moderator

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.


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Gdetrailer

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Posted: 04/06/22 08:42pm Link  |  Quote  |  Print  |  Notify Moderator

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.

ktmrfs

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Posted: 04/06/22 09:12pm Link  |  Quote  |  Print  |  Notify Moderator

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)

time2roll

Southern California

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Posted: 04/06/22 09:43pm Link  |  Quote  |  Print  |  Notify Moderator

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.


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