Adding a fourth string, looking for options.

DakotaJim · September 2019

I have 3 identical solar panels on the roof of a 5th wheel RV (340W, 41.2 Vmp, 8.26 Imp) in parallel (running on individual 10G wires to a combiner box that has 10A fuzes, additionally each positive leg has a MC4 15A fuze at the panel connections themselves), I want to add an additional solar string on an extension at the ground (max 50 ft, current strings are 15, 20, and 30 ft long), for extra power if needed and in case of shading of the roof. I am using a Victron 150/100 MPPT Tr, so it should be able to handle the 4th string. I want to use 2 or 3 panels in series (so they are smaller and easier to store when not used) to equal the others, then connected in parallel with them. I know that they need to add up to (within 10% of) the same voltage as the current panels, but everything will be producing at the lowest values (V in parallel or I in series) when connected.

Are there any specific things that are more important to pay attention to (or anything else I'm missing) in my plan? Is there a problem if the 4th string is poly panels instead of the mono I already have? Any recommendations for cheap panels to do this? I'm in AZ so local is preferred and I originally purchased my panels from the forums sponsor.

Photowhit · September 2019

Just consider you are trying to build another panel like those you have already, So you have 3 panels that are;

DakotaJim said:

(340W, 41.2 Vmp, 8.26 Imp) in parallel (running on individual 10G wires to a combiner box that has 10A fuzes, additionally each positive leg has a MC4 15A fuze at the panel connections themselves),

Sounds like they are 72 cell panels (I'd dump the redundant fusing/breaker'ing will make trouble shooting a pain)

Try to stick with the same type of cells poly or Mono, but don't have a fit over it.

So You are looking for 170 watt + panels, with a VMP of 20.6 (IMP should take care of its self)

It doesn't look like NAWS has anything available under $1.50 a watt. I would look around locally, but larger 12 volt nominal panels are a pretty current thing.

So you could look at eBay or Amazon. Bottom dwelling this looks like it might work(they sell the panel without the mounting brackets for $4 less);

https://www.ebay.com/itm/200-Watts-Solar-Panel-12-Volt-Mono-crystalline-a-set-of-Z-Bracket-Charger-Kit/143347473655

or this

https://www.ebay.com/itm/1-200-Watt-12-Volt-Battery-Charger-Solar-Panel-Off-Grid-RV-Boat-200-watt-total/283431535074

DakotaJim · September 2019

Thanks for the comment, learning more everyday, I actually moved the 15A MC4 fuses to the negative lines, as the 10Amp fuses are on the positive lines; I know t-shooting may be a pain, but I'm paying attention to the solar panel fires that seem to be popping up on the news cycles now, do to subpar installs, and the fact I'm thinking of using mismatched equipment.

I looked around locally for panels and someone mentioned that I should be paying attention to the 'diodes' being overloaded or burned out, if one panel gets shaded. They stated that's how panels get 'cooked,' especially if they are in a mismatched system. These are actually 80 cell panels and its going to be hard to find smaller panels that can handle the same current loads. But I'm wondering if that is just one persons opinion. I searched for a few comments on diode shading here, but I'm still confused.

By the way the total specs for the Kyocera's are 80 cells, Pmax 340W, Vmp 41.2, Imp 8.26, Voc 50.8, Isc 8.86, P tolerance of +5/-0 and a system fuse rating of 15 A (which I assume are the diodes in the junction box) maximum system DC voltage is 1KW.

So I'm thinking 2 mono panels that are 25 Voc and around 8-9 Isc in series, then in parallel with the other 3 Kyocera. They should adjust with the Kyoceras to the current solar luminescence (Voc to Vmp), and the Victron MPPT should minimize the voltage differences, to pump the max current into the batteries (based on bulk/absorb/float needs). But if one panel was shaded, could it get burned out, or could I potentially fry the Victron charger since all will be connected in parallel? Since this 4th parallel ground mount string will probably only be used when one or more of the other 3 panels on the roof of the RV are shaded (I could shut off each of them individually in the combiner box if needed), I'm trying to make sure this whole idea is worth the trouble to do.

Did a little more reading, looks like if I do attempt this, I should either put in blocking diodes on all 4 strings (or only the 4th string if the two panels in series is less V and I than the other 3 Kyocerias) Unless all the panels already have blocking diodes, and instead I need to pay attention to the total voltages that are internal to each section of each panel, and make sure none of them could be large enough to break down any of their internal bypass diodes.

I also need to pay attention to the cables to handle the current from the Victron into the batteries and the batteries into the inverter. I haven't decided if I'm going to go 12v or 24V route for the inverter or batteries. I'm currently running 12v on 2 older AGM's in parallel, but will be upgrading to lithium soon. I know that 24V will not require as thick of cables, but not sure of the actual amperages that could be on those cables.

mike95490 · September 2019

Panel bypass diodes don't burn on the first hour of shade. They generally are not well heat-sinked and slowly get cooked to death by day after day of shade. They heat the J-Box, and if more diodes in that J-box are active, there is more heat. Eventually a diode burns out and dies. Often, nothing else happens. Sometimes the box gets hot and melts a bit. If a diode burns open, then the shaded cell becomes the high resistance and it start heating up, sometimes cooking the back sheet.

Don't put panels where they will be shaded is the simple answer

You don't need to add "extra" diodes anywhere.

Don't connect or disconnect any fuses in daylight, most fuse holders are not rated to disconnect and will arc. Use a breaker to shut the system down before opening fuse holders

DakotaJim · September 2019

Thanks, that helps.

So I should shut down any of the the panels on the roof (if they're going to be shaded) when I add the ground mounted panels, otherwise just periodically checking the diodes and the j boxes for heat damage.

mike95490 · September 2019

Simply don't mount panels in the shade.

Adding a fourth string, looking for options.

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