Off grid box truck RV conversion

[95Z28A4]

Hi everyone. I'm new to solar power. I have attempted to educate myself over the last couple months, but have hit a roadblock.

In approximately 5 weeks I will be traveling from Louisiana to New Mexico to convert an International 4700 24ft box truck to an off-grid RV. The conversion will take place off-grid in a New Mexico desert. I will pack everything except lumber in a 18ft cargo trailer (including tools, generator, table saw, air compressor, etc., etc.). I am doing as much work as possible prior to leaving home. The water system is built on 3/4" plywood and will be screwed to an interior wall of the RV. The breaker panels, switches and solar monitoring devices are pre-wired and built into a box constructed of 3/4" plywood and will be screwed to the wall. The solar/charging system will be built in a ventilated box made of 3/4" plywood and screwed to the wall. I can cite several more examples, but I think you get the idea.

The solar/charging system consists of a Victron MPPT 250/100, a Progressive Dynamics PD9180ALV converter, an AIMS Power 3000 watt pure sine wave industrial inverter, a Champion 3500 watt open frame inverter generator, 6 Trina 260 watt solar panels and two Renogy 200 ah Lithium batteries with 200 amp BMS each.

Here's where I'm having the issue. My plan is to parallel 2 sets of 3 panels in series. I don't understand the diode arrangement. I think I should have a total of 6 diodes - 2 blocking diodes (1 for each set of seriesed panels) and 4 bypass diodes (2 diodes per seriesed set).

Is my thinking correct? For the blocking diodes, I plan to use Schottky diodes. Do I use Schottky diodes for the bypass diodes? Or is a different type of diode better suited for this service? What size diodes should I use? I've also read that large diodes produce a lot of heat. I have seen large diodes mounted to a heat sink. Any experience with this?

The specs for the 6 panels are:
Watts - 260
VOC - 38.2 volts
ISC - 9.0 amps

I know this is a lot for my first post, but I have to start somewhere. I thought this would be the right place to increase my knowledge. Thanks.

 

[timselectric]

1. Bypass diodes are already included in the panels junction box.
2. Blocking diodes are not needed. The open circuit voltage will take care of power flow.

 

[95Z28A4]

I believe the panels have diodes, but could not visually confirm. They appear to be potted/concealed in a rubbery-like material. I attached a photo.

Can you explain "The open circuit voltage will take care of power flow"?

 

[timselectric]

Can you explain "The open circuit voltage will take care of power flow"?

The old theory for using blocking diodes was to keep power from flowing from good producing strings to shaded strings. Instead of flowing to the charge controller. But even a shaded string produces enough voltage to keep it from happening.
A few of us have tested it with and without blocking diodes. And the results were the same.
We decided that if the blocking diodes helped at all. It was a small enough amount. That it was lost in the inefficiency of the diodes and extra connections.

 

[95Z28A4]

OK. That simplifies the installation. I plan to protect each panel with a 15 amp ATC fuse. Do you see any issues with this?

 

[timselectric]

OK. That simplifies the installation. I plan to protect each panel with a 15 amp ATC fuse. Do you see any issues with this?

Check the voltage ratings for your fuses.
FYI, no fuses are needed for 2 strings in parallel.
But I do recommend a disconnect (I use a 2 pole DC breaker), near the charge controller for maintenance and emergencies.

 

[pvgirl]

OK. That simplifies the installation. I plan to protect each panel with a 15 amp ATC fuse. Do you see any issues with this?

ATC fuses are only rated to 32volts. With 3 panels in series you string voltage will be as high as 138.5 volts in cold weather, these fuses will not be safe. Any fuses, circuit breakers, or disconnects need to be 150volt DC rated.

 

[95Z28A4]

Thanks for the information about the ATC fuse voltage limit. I thought they were rated for 48 volts. I figured if I fused each panel individually, I'd be OK. I now understand that logic is flawed because 3 panels are in series. I started planning this project about 2 months ago with 6 panels that I was going to wire in parallel. After learning more about solar I realized two parallel strings of 3 would be a better solution. Obviously, I was still stuck on needing to fuse each individual panels. Thanks again.

I have a question(s) concerning the 2-pole DC breaker. Considering the Victron 250/100 specs, should I use a double pole 50 amp DC breaker? One pole for each string? I will be using 2 AWG cable. I attached an Amazon link to a breaker as an example. This one is obviously made in China. I try to avoid inexpensive import components because of safety and reliability concerns. Can you suggest a breaker?

https://www.amazon.com/LANGIR-Photovoltaic-Polarized-Circuit-Certificates/dp/B012MWNSUW/ref=sr_1_1_sspa?crid=20WTHPDRDZKQC&keywords=250%2Bvolt%2Bdc%2Bbreaker%2B50%2Bamp%2Bdouble%2Bpole&qid=1694194346&s=hi&sprefix=250%2Bvolt%2Bdc%2Bbreaker%2B50%2Bamp%2Bdouble%2Bpole%2Ctools%2C109&sr=1-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1

 

[pvgirl]

2 AWG wire is massive over kill for the pv side of the charge controller, max current there will be no more then 28 amps when both strings are paralleled(14 amps per string), 10 awg should be fine for the PV side of the charge controller. Midnite Solar's MNEPV breakers, are my recommendation, combine them their BABYBOX, and you have a nice setup for PV disconnect. 1 breaker in the positive will work, you can use 1 30 amp breaker if you combined both stings before the breaker, or 2 individual 15a breakers for each string.

On the output of the charge controller 2 awg is ok for short runs, should be fused per the manual between 120-140 amps.

 

[timselectric]

This is what I use for all of my solar disconnects.

DC Miniature Circuit Breaker, 2 Pole 1000V 32 Amp Isolator for Solar PV System, Thermal Magnetic Trip, DIN Rail Mount, Chtaixi DC Disconnect Switch C32 https://a.co/d/9UxsKM2

 

[95Z28A4]

Thanks for the help. I ordered two 15 amp Midnite solar MNEPV breakers, a big baby box, 100ft of 10 gauge PV wire and MC4 connectors and a few Heyco strain reliefs for NAZ.