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Old 04-07-2020, 08:24   #1
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Limit to the number of 12V solar panels in series?

Looking at how many 75watt 18V mp panels can be put into series for a string? Is there a voltage limitation from the internal bypass diodes? I would like to go 4 in series each string into a mppt, but can the internal bypass diodes of each 75watt panel handle that? I also have not been able to remove the juction box to see what is inside, too hard to get the thing off, feels like the panel will break. The alloy panels are thin & lite with croc skin coating, heard a rumor they are Sunpower grade A mono, but not sure.

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Old 04-07-2020, 11:17   #2
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Re: Limit to the number of 12V solar panels in series?

The controller will have a max voltage input.

Ie the victron 75/15 is max 75v.

The panels won’t care the current is the same

Also anything over 48v dc is considered deadly. And becomes harder to find breakers and switches rated.

On a boat I would not use series panels unless you have no shading. (Unliky)
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Old 04-07-2020, 11:24   #3
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Re: Limit to the number of 12V solar panels in series?

Agree with Smac. Panels are so frequently shaded even by shrouds that parallel is the way. Diodes each panel. If the MPPT needs higher volts then make two panel series sets and connect those sets parallel.
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Old 04-07-2020, 15:15   #4
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Re: Limit to the number of 12V solar panels in series?

Not just the controller but also the panels have max system Voltage in the specs. Look up the spec sheet.


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Old 05-07-2020, 06:38   #5
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Re: Limit to the number of 12V solar panels in series?

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Originally Posted by smac999 View Post
The controller will have a max voltage input.

Ie the victron 75/15 is max 75v.

The panels wonít care the current is the same

Also anything over 48v dc is considered deadly. And becomes harder to find breakers and switches rated.

On a boat I would not use series panels unless you have no shading. (Unliky)
Yep, but this is a fold up panel I only set out when I can. Cheers.
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Old 05-07-2020, 07:05   #6
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Re: Limit to the number of 12V solar panels in series?

In addition to the controllers the wire and connectors each have a max voltage rating as well as fuses/breakers. But the charge controller and maybe the breakers will likely be the lower limiting factor here.

Wire insulation is probably 600v or thereabouts. I don't know offhand what the the voltage maximum is for the standard PV connector. The PV panels themselves also have a maximum voltage rating too. At some point the internal insulation will be overwhelmed by the voltage potential.
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Old 05-07-2020, 08:59   #7
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Re: Limit to the number of 12V solar panels in series?

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Not just the controller but also the panels have max system Voltage in the specs. Look up the spec sheet.


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Yes, but this is for legal reasons, not electrical ones. For installations over 50V, electrical codes require panels to be UL or IEC certified. Panels that don't have the certifications have the max system voltage "limitation".
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Old 05-07-2020, 10:05   #8
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Re: Limit to the number of 12V solar panels in series?

Be VERY careful with DC voltages above 30v. Get across it and it can/will stop your heart. I have 10 solar panels and have them all wired in parallel to minimize the effect due to shading and also for safety reasons.
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Old 05-07-2020, 15:19   #9
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Re: Limit to the number of 12V solar panels in series?

When you hook the panels in series, the maximum voltage across each panel canít exceed the panelís open-circuit voltage, normally about 20 volts. The maximum total voltage across ALL of the panels is the sum of their individual OC voltages. So, if you hooked up four 12-volt panels in series, the voltages at the connection points would be 12, 24, 36 and 48 volts respectively. Most larger MPPT controls will allow an INPUT voltage of more than 50 volts. My Outback controller specs around 140 volts maximum.

Once the output voltage across the entire array exceeds 50 volts, you need to follow the rules for high-voltage connections.

Boat cable is usually rated for 600 volts. The usual A-frame switch breakers are good to 60 volts DC. Finding fuses or breakers that are rated at more than 60 volts DC takes some looking around, but they are available, but most of the commonly available fuses and breakers with DC ratings donít go above 60 volts DC. When you look at the voltage rating of circuit protection, be careful. Your old-fashioned glass 3AG type fuse is rated for 120-250 volts of AC, but only about 60 volts DC, for example.
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Old 05-07-2020, 15:47   #10
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Re: Limit to the number of 12V solar panels in series?

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Originally Posted by Bycrick View Post
When you hook the panels in series, the maximum voltage across each panel canít exceed the panelís open-circuit voltage, normally about 20 volts. The maximum total voltage across ALL of the panels is the sum of their individual OC voltages. So, if you hooked up four 12-volt panels in series, the voltages at the connection points would be 12, 24, 36 and 48 volts respectively. Most larger MPPT controls will allow an INPUT voltage of more than 50 volts. My Outback controller specs around 140 volts maximum.
A 12 volt nominal panel has an open circuit voltage of about 20 volts. So your 4 panels in series would measure 20, 40, 60, and 80 volts. This is what the controller sees. At 80 volts you need a controller capable of 100 volt input to be safe. In some conditions panels can output higher than rated voltages.

As far as the max voltage across the panels not exceeding their open circuit voltage that doesn't make sense as with more than 2 panels as you are exceeding open circuit voltage by panel 3.
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Old 06-07-2020, 10:10   #11
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Re: Limit to the number of 12V solar panels in series?

The "open circuit" voltage from a panel is determined by the panel's construction, itís orientation to the light source, and the characteristics and brightness of the illumination. Thereís a standard set of measurement conditions. So a "12 volt rated" panel will put out "about" 18-22 volts open-circuit under "standard" conditions. Under certain lighting conditions, it might be slightly more, but thatís ONLY when there is zero load on the panel, ie OPEN CIRCUIT. Even small loads can limit the output voltage. So, for discussion's sake, about 20 volts is a reasonable number.

Any rational attempt at system design will not operate components at their maximum rating. So even if one were to assume that the rated open-circuit voltage was the absolute maximum that the panel(s) could output, no one would assume that the controller only needed to tolerate that voltage. Personally, for four 12-volt rated panels, Iíd want at least a 100-volt rating on the controller.

But the example of 12,24,36,48 was only an example to show that whatever the total, open-circuit voltage of multiple "identical" panels connected in series, under "identical" condition is going to be evenly divided across the panels. Even if you contrive conditions where the panel voltages are not equal, youíre not going to have ONE panel with a voltage greater than itís open-circuit voltage. The connection points between the panels will, of course, be at different voltages relative to "ground." The total output of the array will be the sum of the panel voltages.
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Old 06-07-2020, 10:31   #12
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Re: Limit to the number of 12V solar panels in series?

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Originally Posted by Bycrick View Post
The "open circuit" voltage from a panel is determined by the panel's construction, itís orientation to the light source, and the characteristics and brightness of the illumination. Thereís a standard set of measurement conditions. So a "12 volt rated" panel will put out "about" 18-22 volts open-circuit under "standard" conditions. Under certain lighting conditions, it might be slightly more, but thatís ONLY when there is zero load on the panel, ie OPEN CIRCUIT. Even small loads can limit the output voltage. So, for discussion's sake, about 20 volts is a reasonable number.
The open circuit voltage is pretty close to what the controller sees - whether it is charging the batteries or not.
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Old 06-07-2020, 10:44   #13
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Re: Limit to the number of 12V solar panels in series?

Here guys and girls you like to mix things up quite a bit.


Where I mentioned max panels voltage I meant


Maximum System Voltage


One of the markings on any CE panel.


I checked it on ours and it says 1000 V. So likely not an issue in an average boat (but I am not sure as some boats are truly big these days).



Cheers,
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Old 06-07-2020, 12:21   #14
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Re: Limit to the number of 12V solar panels in series?

Quote:
Originally Posted by barnakiel View Post
Here guys and girls you like to mix things up quite a bit.


Where I mentioned max panels voltage I meant


Maximum System Voltage


One of the markings on any CE panel.


I checked it on ours and it says 1000 V. So likely not an issue in an average boat (but I am not sure as some boats are truly big these days).



Cheers,
b.
Most regular single insulated wire is rated for 600v max

I just looked this up at Renology and they state their MC4 quick-disconnect PV connectors are rated for 1000v @ 30A.

0.0

I've spent many hours working in live 480v AC commercial and industrial panels and motor control centers. I have some experience building and modifying medium-voltage primary gear, transformers, generators, and associated primary cableing running at 13.8KV. No way in hell would I ever entertain the idea of even touching or disturbing one of those flimsy PV quick-connectors energized at 1000v DC. Just the thought makes my palms sweat and the hair on the back of my neck stand up. Hell no!

https://www.renogy.com/mc4-connector-cable-for-solar-panels-pair-male-female/
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Old 07-07-2020, 05:51   #15
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Re: Limit to the number of 12V solar panels in series?

Apparently, high voltage PV systems are on the way. I received a newsletter from DigiKey, the parts supplier, talking about high-voltage semiconductor components for use in PV systems above 1000 volts and greater. Not something Iíd put on my boat though.
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