Community Forums

okmunky · 01-26-2020, 07:55 AM

I think the key factor is keeping everything as close together as possible. Plan your layout so wires are as short as possible. I'm putting my tiny house batteries and charge controller under my bed. That will require isolating and venting the batteries to the outside but that way the wire run is less than 3’ and I can use 4 gauge wire.

Oventek · 01-26-2020, 12:20 PM

Quote:

Originally Posted by okmunky

I'm putting my tiny house batteries and charge controller under my bed. That will require isolating and venting the batteries to the outside but that way the wire run is less than 3’ and I can use 4 gauge wire.

If you relocate the batteries to under the bed, what gauge wire are you using to extend the wire to the alternator/generator etc? I would think this would also be a consideration.

astrocamper · 01-26-2020, 02:07 PM

I used 2/0 wire from each of four 100 amp-hour Battle Born LiFePO4 batteries to a pair of 650 amp bus bars near the batteries between the pass through storage area and the fold down couch. I used 4/0 wire from the bus bars to another pair of bus bars in the pass through strorage area near my 2200 watt inverter which can surge to 6600 watts.

I brought the cables that ran to the batteries on the lounge and routed them to the bus bars in the pass through storage area. All my solar controllers are in the pass through storage area.

okmunky · 01-26-2020, 07:52 PM

Quote:

Originally Posted by Oventek

If you relocate the batteries to under the bed, what gauge wire are you using to extend the wire to the alternator/generator etc? I would think this would also be a consideration.

This is a separate set of 4 six volt golf cart batteries just for the house. I'm running 10 gauge about 10' from the roof to the MPPT charge controller. In southern Arizona I won’t need a separate generator for the house. I'm planning to park the RV next to the house and run an additional remote start switch from the house to the RV. There will be a connection from the generator to 2 plugs on the wall that separates the kitchen from the storage room. Primarily it will be used to run the microwave.

The tiny house is 32’ of a 40’ high cube cargo container. The remaining 8’ will be storage/workshop on the doors end of the container.

Rocketman3 · 01-26-2020, 09:32 PM

Remember panels are “cheap” these days. If needed add an extra set if you have room. (And a big enough controller).

My setup is four 200w panels wired in a 2S2P with 10ga wires to a combined box - one series is about 25 feet from the box - the other is 5’. From the combiner box I use 6ga (3 feet) to the Victron 100/50 mppt controller. Then I use 6ga for a 15’ to 20’ run to the batteries (four 6v AGM’s). Would I like that run shorter - Yep - but it’s not practical. The 2series panels gives me about 40 volts into the controller. Plenty of juice going in.

I am also using the Victron BMV-712. One really useful feature is the battery monitor will send the temperature and battery voltage to the mppt through Bluetooth. That way any voltage losses from the mppt to the batteries will be adjusted out. (If I have .2 volt drop - the mppt will boost the voltage by .2 volts so the batteries are getting exactly what they should.

The 800 watts of panels keeps my MotorHome batteries full. (As long as I had sun the batteries were full even in Alaska).

Just buy the pre-made MC4 cables - that way your first wiring joint is at the combiner box. Then you don’t need the MC4 crimper.

Good luck

Brianna · 01-27-2020, 06:05 AM

I'm a newbie, so be clear that I am asking, not telling.

While it seems logical that heavy 4ga cables between solar panels on a roof would reduce current loss, Is it sgnificant enough to even bother?

In a typical solar system on a 36--40' motorhome let's say there is 20 feet of cabling between the panels and controller. Are we talking, what..... 3% of loss using 10ga vs 4ga?

Do i have this math figured correctly?

A hypothetical 680W system (4x170) sending maybe 80% of that at (series) 90 volts open circuit would produce 6.04 amps. Right?

So 3% of that is only .2 amps.

Is that right?

Park Super · 01-27-2020, 07:03 AM

According to what I have read from folks that claim to be Electrical Engineers and have rv solar installation experience the difference of wire size is a system that can charge the batteries to 100% and a system that cannot. You should do your research. Am Solar has been installing systems for 25 years. Garret at AM Solar is an electrical engineer, will answer your questions via e-mail. Also they have wonderful videos. Handy Bob solar has lots of great info based on his education and experience but he is not big at answering questions. Your math may be correct but it may also be an oversimplification of the issue. Charging the batteries is the goal and batteries need specific charging and current parameters depending on many factors. Think of wire as a water hose. Electric current as the water. Bigger the hose the more water flows and more easily. Solar only has a limited capacity based on the Solar Day. The Solar Day varies on latitude, time of year. Atmosphere conditions also play a role: clouds, smog, and the like. You are going to have a large solar array but it will still be limited to the solar day and atmosphere conditions. To get your batts charged make the most of those few hours your system is working....get that energy to your batteries with 4ga wire.

jcussen · 01-27-2020, 10:13 AM

Quote:

Originally Posted by Brianna

I'm a newbie, so be clear that I am asking, not telling.

While it seems logical that heavy 4ga cables between solar panels on a roof would reduce current loss, Is it sgnificant enough to even bother?

In a typical solar system on a 36--40' motorhome let's say there is 20 feet of cabling between the panels and controller. Are we talking, what..... 3% of loss using 10ga vs 4ga?

Do i have this math figured correctly?

A hypothetical 680W system (4x170) sending maybe 80% of that at (series) 90 volts open circuit would produce 6.04 amps. Right?

So 3% of that is only .2 amps.

Is that right?

Plenty of charts out there telling you voltage drop at certain voltages, wire size and lengths. You are right, at higher voltages, amperage goes down and wire size goes down. I am running 2500 watts of solar at 240 volts series/parallel, so unless a long distance,#8 or #10 wire is fine. Don't forget, your normal extension cord that carries 120 volts at 15 amps is probably only 12 or 14 gauge.
Lower voltages like from your controller to your battery banks will of course have to be much heavier because of the increased amperage.

Rocketman3 · 01-27-2020, 11:57 AM

You will have voltage drop not amperage drop. But is it in a place that matters. Your solar panels are putting out about 20 volts each (look at the panel to get the exact number).

You add all panels in series (7 of them = 140 volts - can your solar controller handle 140 volts? Or do you need to go to a serial-parallel layout?). This voltage will go through the wires and will have a voltage drop. Because this voltage is high, a drop here doesn’t matter much.

Your MPPT controller will change this to something like 14.4 volts at 41 amps. From here to your batteries voltage drop WILL matter a lot. This is where you want bigger cables and short runs.

If it will fit go with 8 panels in a 4S2P arraignment (4 in series & 2 in parallel).

Good luck

Brianna · 02-05-2020, 02:47 PM

When the tundra melts I need to get up on the rv roof and do some serious measuring.
I was originally thinking I would run PV cables down the refrigerator vent but now I am thinking of adding a new combiner box on the roof directly above the battery compartment. So maybe a maximum of 8 or ten vertical feet of heavy gauge cable straight down to the batteries. Then just use 10ga PV cable between panels on the roof.
Probably time to get one of those fancy new stud finders that show you an image. I'm not afraid to drill a hole in the roof, but I am afraid of hitting existing wires that might be hiding in the 8" of roof styrofoam.

astrocamper · 02-05-2020, 05:35 PM

I put a AM Solar combiner box in the front of my roof and ran a 4 x 6 AWG cable down to the controllers in the front pass through storage. I have a 265 watt 24 volt panel on the original prewire run. The combiner box is supporting a pair of 300 watt 24 volt and 3 180 watt 12 volt panels going to two different controllers.

okmunky · 02-07-2020, 09:20 AM

Brianna,

You probably know this, but I thought I would remind you to avoid all shadows from obstacles on the top of your roof. Good luck with planning and implementing your solar project.

jaearhart · 02-07-2020, 03:41 PM

People buy a $70,000 diesel truck for torque and MPG to tow a $60,000 TT that has a 6 kw gas generator that consumes 1.5 gph for boondocking. And buys Solar panels and batteries to be energy conservative for four hrs per recharge on sunny days. I just lost my " Let's go Camping " appeal..

ShelbyM · 02-08-2020, 04:05 PM

Quote:

Originally Posted by jaearhart

People buy a $70,000 diesel truck for torque and MPG to tow a $60,000 TT that has a 6 kw gas generator that consumes 1.5 gph for boondocking. And buys Solar panels and batteries to be energy conservative for four hrs per recharge on sunny days. I just lost my " Let's go Camping " appeal..

??? Not sure what the cost of the truck and trailer have to do with anything, but if adding solar reduces the use of a noisy generator, it's a good thing, right?