diy solar

diy solar

Use the same negative cable for multiple seperate panels and controllers

svfalkor

New Member
Joined
Apr 20, 2020
Messages
43
Location
Panama
Hello crowd,

I would like to pick your brain :)

I have multiple panels that are mounted close to each other but in different angles towards the sun and with different shading (they are on a sailboat).
Therefor (and for redundancy reasons) I want to connect each panel via a mppt controller to the battery bank:
1599700497594.png

I am now wondering if I can simplify the cables and use only 1 negative cable between the charge controllers and the panels (given that I size the negative cable to handle the load from all panels):
1599700619782.png

Do you see negative effects? Does the negative and positive cable between the controller and the panel need to be of the same length and strength? If so why?
Thanks a lot!
 
Why one panel per MPPT? I have 9 on mine and will eventually have 18.

Shading can be addressed by paralleling panels on the same charge controller, i.e., 3 panels in parallel to each other connected to 1 charge controller will perform the same as 3 panels on 3 different charge controllers.
 
It breaks the concept of redundancy somewhat too. There would be a single point of failure in the common - wire.
 
It breaks the concept of redundancy somewhat too. There would be a single point of failure in the common - wire.
There you are right. I would think that not so much can go wrong with the cable itself but there is a chance...
 
Why one panel per MPPT? I have 9 on mine and will eventually have 18.

Shading can be addressed by paralleling panels on the same charge controller, i.e., 3 panels in parallel to each other connected to 1 charge controller will perform the same as 3 panels on 3 different charge controllers.
hmm, when should you use a second controller? Whenever the panels are different?
 
If all your panels are in parallel and all the same model, and the voltage from the panels is > than battery voltage, and the total current does not exceed the rating of the controller, there isn't any need to use more than one controller; other than for redundancy purposes.

There's a lot of parameters that while not 100% necessary to keep within they should still be noted to get the best out of a given system. If we put different voltage panels* in parallel we won't get the paper watts out of them. If we put different amp panels in series we won't get the paper watts out of them. That's just scratching the surface.

*can be a single panel, can be a string of panels
 
@gnubie got it, thank you!
What I called "panel" is are actually 2 panels in series...
So I guess the idea is:
one charge controller per group of the same panels, wired in parallel, a (group of) different panel(s) gets their own controller
one plus and minus cable (no common minus cable) per group.

1599708860882.png
 
Looks good, less $ spent and you get to keep your redundancy. If you like post the specs of your two types of panels (a total of 6 panels), the model charge controller and your battery voltage / amp hour rating and I'm sure someone will check the numbers to make sure it's all good.
 
Depending on the voltage of panel 3, it may be possible to parallel it with the other panels.
 
Looks good, less $ spent and you get to keep your redundancy. If you like post the specs of your two types of panels (a total of 6 panels), the model charge controller and your battery voltage / amp hour rating and I'm sure someone will check the numbers to make sure it's all good.

that would be great! :)
I have one panel of 365 watts bifacial. I regularly get up to 400 Watt peak power. I can't find it's spec right now but the maximum voltage seems to be around 43 volts. This one is connected to a Victron MPPT 75/15.
I think I would leave that one as is to have the redundancy.

The new panels are HQST 100 watt poly:
1599798841461.png
I believe these are the ones that Will recommended.

So far I have 2 panels but I plan on ordering another 4, so 6 in total. I plan on series wiring them into pairs of two since I have a 24 Volt system. Each pair will be in a different location so that only one set can get full sun at a given time. My take away from this thread is that I can wire the 3 pairs together in parallel (2s3p) and run them through one solar controller, right?
Do I need to make sure that the cables from the pairs to the connection point are the same length or doesn't it matter?
Thanks so much for your help!
 
(1) My take away from this thread is that I can wire the 3 pairs together in parallel (2s3p) and run them through one solar controller, right?
(2) Do I need to make sure that the cables from the pairs to the connection point are the same length or doesn't it matter?
Thanks so much for your help!

(1) Right. Each of the two series pairs will operate independently of the other pairs. Shading on 1 or 2 pairs will not affect the 3rd, etc.
(2) No.
 
Why one panel per MPPT? I have 9 on mine and will eventually have 18.

Shading can be addressed by paralleling panels on the same charge controller, i.e., 3 panels in parallel to each other connected to 1 charge controller will perform the same as 3 panels on 3 different charge controllers.

From what I understand multiple smaller MPPTs is a common practice (best practice?) in the marine world, since partial shade is common, unpredictable, and not consistent with all the sails and rigging, and often when one side is partially shaded the other is sunny. Multiple MPPT's gets you:
(1) redundancy
(2) the same shade tolerance as parallel panels on a sngle MPPT
(3) the additional shade / angle / etc optimization of multiple maximum points points (MPPs), at the panel level or at least sub array level. Same concept as microinverters or DC optimizers.
 
you would need a thicker cable for the one that is shared since you pass more current in it.
it is a common mistake to think that only the "live" wire thickness is important.
 
Last edited:
From what I understand multiple smaller MPPTs is a common practice (best practice?) in the marine world, since partial shade is common, unpredictable, and not consistent with all the sails and rigging, and often when one side is partially shaded the other is sunny. Multiple MPPT's gets you:
(1) redundancy
(2) the same shade tolerance as parallel panels on a sngle MPPT
(3) the additional shade / angle / etc optimization of multiple maximum points points (MPPs), at the panel level or at least sub array level. Same concept as microinverters or DC optimizers.
yes the concept seems to be common on sailboats. Everything on a boat will break eventually :cool:
about (3): do you know what that means in real world numbers? Does that mean that also the performance of the non shaded panel will be effected by the shaded ones since the MPPT controller can't optimize for the the sunny one?
 
(3): do you know what that means in real world numbers? Does that mean that also the performance of the non shaded panel will be effected by the shaded ones since the MPPT controller can't optimize for the the sunny one?

I believe this is the logic yes, and no, I have no idea what real world numbers would look like, I think they would vary a lot depending on the scenario. One of the benefits of MPPT over older technologies is it optimizes for the conditions, more localized MPP tracking in theory allows for the controller to optimize more precisely for the conditions of the panel or panels wired to it. Other than shade, one situation where multiple MPPT's are sometimes utilized is east-west facing arrays. I suspect (and this is pure speculation, the real world improvement would be less than 20% take this with a grain of salt its just a shot in the dark)
 
yes the concept seems to be common on sailboats. Everything on a boat will break eventually :cool:
about (3): do you know what that means in real world numbers? Does that mean that also the performance of the non shaded panel will be effected by the shaded ones since the MPPT controller can't optimize for the the sunny one?
Shading is an interesting thing. If you have two identical panels (or series strings of), one sloped east the other west, either will float up to Voc under no load across the entire day. Now parallel wire them. As the sun moves across the sky the current both panels can provide at Vmp changes but they will still provide as much current as they can since they are both sitting at Vmp. If one side is shaded the current it can provide falls, but that's it. You'll still get as much power out of the array as you possibly can.

There's more to it than just light though. There is also temperature to consider.

Vmp wanders around a bit with temperature. The panel with direct sunlight / no shading will be hotter than the other panel, so it's Vmp will be a bit lower ( ranges across 0.3% to 0.5% per degree C drop in Vmp ). The hotter panel will pull the cooler one down to it's voltage lowering its output a bit more than the shading alone would suggest.

If you are trying to wring every last drop out of things separate controllers will get a bit more power but not a lot unless the parallel panels / strings are mismatched voltage in the first place.
 
  • Like
Reactions: Dzl
@gnubie, thank you for the explanation.
what I have is somewhat similar to what you describe. I have 3 sets of panels. 2 sets are mounted to each side of the boat, one above the cockpit roof.
You can think of it having as having the panels on each side of a van and one on the roof.
=> At a given time only one set of panels will get full sun light or 2 sets will get sun light but at a non ideal angle.

So I guess (based on what you guys said) it is safe to assume that I will get with parallel wiring and one mppt controller almost as much out of the setup as if I would use 3 controllers and separate wiring: almost same gain for roughly a 3rd of the costs :)
That makes sense, you saved me good dollars, thank you!
 
Not to hijack but this brings up a question I have. So let's say you have two (or three) controllers as mentioned. When it comes to distribution through load or to the battery, how is that handled?

Edit - I assume you just combine the outputs and fuse accordingly?
 
Last edited:
So I guess (based on what you guys said) it is safe to assume that I will get with parallel wiring and one mppt controller almost as much out of the setup as if I would use 3 controllers and separate wiring: almost same gain for roughly a 3rd of the costs :)

I'm not sure this is the case (the cost savings), it would depend on your specific situation (panels, controllers, voltages, etc) but as an example, here are two options of roughly the same output capacity from Victron:

1 x 100V 50A Victron Smartsolar = $325
3 x 75V 15A Victron Smartsolars = $355

The single controller is a bit cheaper with a bit more capacity (in both cases about 10%), but nowhere near '1/3 the price', it may be different with other companies, but Victron's SCC pricing from what I've observed is often somewhat linear with a small premium paid for multiple smaller controllers.
 
Back
Top