August 31, 20169 yr Author Just now, superdiy said: 20 A seems a bit on the high side. According to your panels's datasheet, what is the short circuit current per panel? 8.55A per panel so in parallel gives me 17,1A. 20amp should be fine then or am I missing something. I followed the advice of my local supplier so please let me know if there is something wrong.
August 31, 20169 yr 4 minutes ago, superdiy said: I think you are still misunderstanding this. The total length of cable per string needs to be the same. If the total length of cable per string is equal, then you are good to go. 47 minutes ago, HeinTheTerrible said: The battery cable (16mm copper) and charge controller cable (8mm KBE solar) are all the same length (I'm a bit OCD).
August 31, 20169 yr Author 3 minutes ago, Chris Hobson said: With this provisio you could wire it up like this (excuse the farmer modification to your image). This negates the need for busbars which I think is an over kill on only 2 strings. batteries.pdf Its almost spot on in the image you modified. I cant take a picture of the batteries because its too much effort to take out the shelve
August 31, 20169 yr HTT, the guys are just super excited and want to make 100% sure that all is perfect for we have had too many sad stories, yours a ray of sunshine confussing us all. Just bear with them / us and all the questions that we can make sure all is spot on, that there is another successful solar installation giving you and your parents years of delight not using Eskom.
August 31, 20169 yr 8 minutes ago, Chris Hobson said: the need for busbars ... are VERY convenient and comfortable ... soon @Energy will have mine in stock.
August 31, 20169 yr 4 minutes ago, HeinTheTerrible said: Maybe I am I tested each battery after a week and the voltage was spot on over all 4 batteries so I'm 99% certain its all up to scratch You won't necessarily pick up voltage differences on new batteries. The problem is that in the long run the string with shorter cables (less resistance) will work harder than the string with longer cables (higher resistance) and therefor the batteries will become unbalanced and the batteries in the one string might fail sooner than the batteries in the other string.
August 31, 20169 yr Author 5 minutes ago, The Terrible Triplett said: HTT, the guys are just super excited and want to make 100% sure that all is perfect for we have had too many sad stories, yours a ray of sunshine confussing us all. Just bear with them / us and all the questions that we can make sure all is spot on, that there is another successful solar installation giving you and your parents years of delight not using Eskom. No worries! I like getting input for a outside party. I took all the info/advice I got from all the suppliers and designed the system myself. This isn't the 1st system I've built but it is the largest by far!
August 31, 20169 yr 16 minutes ago, Chris Hobson said: With this provisio you could wire it up like this (excuse the farmer modification to your image). This negates the need for busbars which I think is an over kill on only 2 strings. batteries.pdf So each string would have a short positive to inverter and one long negative to the inverter (these are common). Then one shortish link to connect two batteries in series (These two links need to be the same length) and two longer links (also the same length as each other) one on the positive side for one bank and one on the negative side for the other bank. Spot on
August 31, 20169 yr 48 minutes ago, superdiy said: 20 A seems a bit on the high side. According to your panels's datasheet, what is the short circuit current per panel? It's a 250W panel. If it's a 72 cell module, then Vmp is around 36V, so Imax should be around 7 amps. So 20A is a bit high, 10A would be better. Edit: Didn't see your reply. The fuses should be in line with each string (each panel in your case) before they are paralleled, so you should have 4 x 10A fuses. The point of the fuse is that if something goes wrong with one string, the current from the other string doesn't take a shortcut through there, heating things up and setting the roof on fire :-) Confession time: I also have two strings in parallel without a fuse. I bought the fuses... just need to find time to do the work :-)
August 31, 20169 yr 45 minutes ago, HeinTheTerrible said: 8.55A per panel so in parallel gives me 17,1A. 20amp should be fine then or am I missing something. I followed the advice of my local supplier so please let me know if there is something wrong. Yep, they are too high. To prevent nuisance blowing you have to overspec it by about 25% and you have to have 1 fuse per string (in your case per panel since you have one panel per string) before the strings (panels) are combined and connected to the charge controllers. So you'll have to use 4 x 12A fuses, one per string (panel).
August 31, 20169 yr 1 minute ago, plonkster said: It's a 250W panel. If it's a 72 cell module, then Vmp is around 36V, so Imax should be around 7 amps. So 20A is a bit high, 10A would be better. I think he only fused the combined parallel strings not each string (in this case 1 panel) on it's own
August 31, 20169 yr 1 minute ago, viper_za said: I think he only fused the combined parallel strings not each string (in this case 1 panel) on it's own That defeats the purpose.
August 31, 20169 yr There are also times when you can get away with not fusing between parallel strings. For high voltage setups it is essential, whenever one array can make enough voltage to potentially overcome the reverse breakdown voltage of the other that's wired in parallel. If you have two 12V panels in parallel, for example, chances of such a thing happening is so small that you can get away with it. With 24V strings (or 72-cell strings) you're sort of on the fence, and for strings longer than that you should fuse.
August 31, 20169 yr 2 minutes ago, superdiy said: That defeats the purpose. agree for his panels a 12A fuses should be used per string (even if a string is only one panel)
August 31, 20169 yr 18 minutes ago, HeinTheTerrible said: Quickly made a drawing on SketchUp Yes, that will do.
August 31, 20169 yr 3 minutes ago, plonkster said: There are also times when you can get away with not fusing between parallel strings. For high voltage setups it is essential, whenever one array can make enough voltage to potentially overcome the reverse breakdown voltage of the other that's wired in parallel. If you have two 12V panels in parallel, for example, chances of such a thing happening is so small that you can get away with it. With 24V strings (or 72-cell strings) you're sort of on the fence, and for strings longer than that you should fuse. Yes but will you be able to get away with what the new regulations that might someday come into play requires @SilverNodashi and @Mike should be able to tell us
August 31, 20169 yr Let me get into that a bit deeper. So if you haven't done so yet, read up a bit on how bypass diodes work in a solar panel. Basically, when a cell is shaded, its impedance rises and no current is going through there, causing a reverse voltage to rise across it. If the voltage rises high enough, the solar cell will be destroyed. So panel makers install diodes that conduct away this potential difference to protect the cells. Instead of placing such a diode across every cell, it is usually placed over a string of 2 or more. What this means is that a partially shaded string may bypass enough cells that the two strings become unbalanced (one is now a 72-cell string while the other is, for argument's sake, only a 36-cell string). Now... if the one string makes enough voltage to overcome the reverse breakdown of the disadvantaged string (or to push all the protection diodes to conduct)... things get hot and potentially broken. But, for very short strings (36-cell strings), even if the entire thing is shaded and reverse biased, there is never enough voltage to do that, and if it's just your 36-cell 12V camping rig... just leave the fuse already, it's never going to do anything :-)
August 31, 20169 yr More info, which I just went and researched. Typical breakdown for a Polycrystalline cell is between 12V and 20V. Forward it only makes about 0.5V. So one could say that the reverse breakdown is easily an order higher than the forward voltage (let's say a factor of 20). So the danger arrives when you have 20 times more cells in your array than the smallest bypass area. Since the bypass area is usually 4 cells, it means the trouble arrives at 80 cells. Now 24V panels are 72-cell modules... so do you see why it's sort of a fence-sitter there... better to do it, but if it is your camping rig and you don't have a fuse ready, there is probably no reason to worry.
August 31, 20169 yr But now say you have three 72-cell panels in series and one panel gets shaded. Also then how would one tell if the diodes couldn't prevent damage. Can you by looking at the cells see physical damage? Then will the non shaded ones show the damage or the shaded ones? Sorry @plonkster using you as an encyclopedia
August 31, 20169 yr Author I know about the feedback you can get between two panels but I never really put any thought into it. The local supplier of the charge controllers said I shouldn't worry about fuses I went and bought the two 20A fuses to atleast add some protection. Seems like my thinking was flawed. So you guys recon a 16A DC circuit breaker is too big for a single panel too? Can anyone send me a link to a online shop that sells the correct size fuse or circuit breakers? Thanks
August 31, 20169 yr Just replace the fuses, add some more if you want ... DC breakers / disconnects are nice to have if you work on the system daytime and want to "switch off" the panels to the controllers.
August 31, 20169 yr Author Just now, The Terrible Triplett said: Just replace the fuses, add some more if you want ... DC breakers / disconnects are nice to have if you work on the system daytime and want to "switch off" the panels to the controllers. Currently there are only two fuses for the 4 panels and each individual panel is supposed to be fused. I have two bussmann fuse holders at the moment.
August 31, 20169 yr 12 minutes ago, HeinTheTerrible said: Can anyone send me a link to a online shop that sells the correct size fuse or circuit breakers? https://solar.rubiconsa.com/ You should be able to get what you need from them
August 31, 20169 yr Author 4 minutes ago, viper_za said: https://solar.rubiconsa.com/ You should be able to get what you need from them Everything is on Pre-Order
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