Need help with solar!!

KEDWARD · 07-14-2022, 01:19 PM

Hello, new to forum and could use some help!! I purchased a new to me 2004 Winnebago journey 32T and would like to add solar. I plan on putting 4x 175w Renogy solar panels on the roof to feed a house battery bank of 3x 170ah Renogy lithium batteries.. I know I will have to address the issue of not frying my alternator but right now I’m trying to gather all necessary components, and could use some help sizing. I would like to go with a Victron DC/DC charger and Victron MPPT, but I’m not sure of which ones to get!! Any help would be greatly appreciated. I have many more questions but don’t want this post to drag on too long... thanks in advance for any help you can give me ... Jim C.

BarryLS · 07-14-2022, 02:49 PM

All the math is quite simple. I'd suggest getting Will Prowse's book on Motor home solar. "Mobile Solar Power made Easy" available from Amazon.

You need to know the voltage and current of the cells you will purchase and then decide how you plan to wire them: series, parallel, or both. Series will result in the smallest wire as the current doesn't increase but the voltage does. This is important for the charge controller to make sure you do not exceed the incoming voltage maximum (and be above the minimum). The drawback is operating in shade. Any panel in shade will kill the whole system. For this reason it is suggested wiring in parallel or a pair in series and the the two pairs in parallel.

Look at combination devices, charger controller and inverter built into one unit with shore power incoming connection as well. Very simple to wire up. And eliminates an array of big wires.

Also: https://www.youtube.com/results?sear...ry=will+prowse

Marine359 · 07-14-2022, 07:13 PM

When panels are wired in series, the voltage is additive and amperage is unchanged just like batteries. When wired in parallel, voltage is unchanged and amperage is additive. The advantage of series wired panels is that the increase of voltage makes a mppt controller more efficient, and allows the controller to produce more charge current in low light conditions than otherwise. This is because a charge controller needs to see a delta between pv voltage and battery voltage. If the delta is less than 5v, many controllers won’t turn on. Therefore, when the pv array is at its weakest voltage (like morning and evening), series will produce charge amperage when parallel can’t (due to the pv array lower voltage). However parallel wired panels allow each panel to produce its max voltage independent of the other panel. Which is why parallel is favored when you might expect one panel to be shaded. In series, when one panel is shaded, both are shaded (electrically). This also leads you to determination of the gauge of wire you need for your install. Parallel needs thicker wire than series. Since series does not increase amperage, 10awg wire is usually sufficient for two panels. The controller just needs to be large enough to handle the incoming voltage, and maximum amperage. If you’re planning 2x 170w panels, a 100/30 controller is suitable for either series or parallel because two of those panels will not exceed 100v in series, or 30amps in parallel. If you have additional solar inputs, like a portable panel, it is preferable to have it on a separate controller, as wiring becomes more difficult, and matching the pv inputs is very difficult. If you’re thinking Victron, get the smart version, as Victron smart will interface seamlessly with Victron Connect, the mobile Bluetooth app. LFP batteries are normally 13.6v at 100% SOC. You’re likely to see 13v at 60% SOC. So, your pv array must produce at least 18v in order for the charge controller to produce charge current.

KEDWARD · 07-15-2022, 01:55 PM

Quote:

Originally Posted by BarryLS

All the math is quite simple. I'd suggest getting Will Prowse's book on Motor home solar. "Mobile Solar Power made Easy" available from Amazon.

You need to know the voltage and current of the cells you will purchase and then decide how you plan to wire them: series, parallel, or both. Series will result in the smallest wire as the current doesn't increase but the voltage does. This is important for the charge controller to make sure you do not exceed the incoming voltage maximum (and be above the minimum). The drawback is operating in shade. Any panel in shade will kill the whole system. For this reason it is suggested wiring in parallel or a pair in series and the the two pairs in parallel.

Look at combination devices, charger controller and inverter built into one unit with shore power incoming connection as well. Very simple to wire up. And eliminates an array of big wires.

Also: https://www.youtube.com/results?sear...ry=will+prowse

BarryLS,
Thank you for your response. I will order Will Prowse’s book!! by knowing the voltage and current of the cells were you referring to the solar panels?? If so the panels are Renogy RNG-175D which have an optimum operating voltage of 17.95 V and an optimum operating current of 9.75 A. Are these the correct values that I should be looking at?? If those are the correct values and I were to wire the system in 2S2P would that be 35.9v and 19.5 A for the 4x175w panels??
I will look at the combined devices but the motorhome does already have an inverter. Thanks again for your help..

BobC · 07-15-2022, 05:59 PM

Quote:

Originally Posted by BarryLS

Look at combination devices, charger controller and inverter built into one unit with shore power incoming connection as well. Very simple to wire up. And eliminates an array of big wires.[/url]

I'm aware of Renogy's DC to DC Charger with MPPT controller and similar units by others but I'm not aware of anything that combines a DC to DC charger, an MPPT controller, an inverter and incoming shore power connector, at least for RV use. In any case, such would seem to be putting too many eggs in one basket, not to mention the inadvisability of bypassing your MH's Automatic Transfer Switch (ATF). Here's a link to Renogy's unit and one to Will Prowse's review:

https://renogy.com/dcc50s-12v-50a-dc...ger-with-mppt/

https://www.youtube.com/watch?v=JtAUrS4zNSA

BarryLS · 07-15-2022, 06:13 PM

"If those are the correct values and I were to wire the system in 2S2P would that be 35.9v and 19.5 A for the 4x175w panels??"

Yes, that is correct.

"I will look at the combined devices but the motorhome does already have an inverter. Thanks again for your help.."

Since you have the inverter, then no need for a combination controller.

marine359 took the time to write it up quite nicely. I'm way too lazy to do that

BarryLS · 07-15-2022, 06:30 PM

Quote:

"...but I'm not aware of anything that combines a DC to DC charger, an MPPT controller, an inverter and incoming shore power connector, at least for RV use."

Well, no on the DC to DC charger as far as I know. That would be a separate item should you desire one. Any of these can be considered for "RV use". No magic there. Perhaps you are considering the battery voltage and of the opinion that it needs to be 12vdc and not a 24 or 48. I prefer to use a higher voltage for the battery and plan to use 48, maybe 24. A 48 to 12 converter can then be used for the 12vdc devices, mostly very low current in my MH. A charge controller with inverter and incoming shore power connection and programmable transfer is the norm for combination controllers. They even have an output to start the generator.

I plan to use this one depending on how I can organize the cells on my roof:
https://signaturesolar.com/eg4-3kw-o...er-3000ehv-48/

The advantage is the quite simple installation and space required. And cost - that's a 5K PV input, 3Kw pure sine wave inverter, with shore power/generator connection/switching for $750. At that price you can purchase a spare. Also, they have a 5KW battery at $1500 that is designed to work with this charge controller. The battery actually directs the charging, another advantage.
https://signaturesolar.com/eg4-lifep...ery-48v-100ah/

The problem is the PV input minimum. If I can only get 4 of the used cells I plan to use on the roof then I have to wire them in series and suffer if I am in partial shade. I'd prefer to wire 2S2P but that would require a different charge controller. I'm going to make some measurements this weekend to see what I can do and that will determine what I use.

Victron also has a nice combination controller. I'm just not of the opinion the price can be justified.

moabdds · 07-20-2022, 08:20 PM

Jim,

I wonder if you are not going to be seriously 'under-paneled' with your proposed set-up. I have 3 100AH Battle Born batteries and they recommended at least 250w of solar panel per 100AH battery. Since I installed 4 x 250w panels, I'm actually over-paneled by one panel, which means I charge more quickly now and have the option to add another battery later if I need to. I also broke my 4 panels into two banks with a 40 amp charge controller for each. There were two advantages to that. First, 2 40 amp controllers were cheaper than one 80 amp controller. Secondly, if one of my panels it getting shaded during the day, it only knocks out 2 panels instead of all 4. if only one of the cells on one of your panels is shaded, it knocks you output down to about 10% or thereabouts!

KEDWARD · 07-22-2022, 04:02 PM

First of all let me thank everyone for their input there’s a lot of amazing help on this forum and I’ve learned quite a bit from it... I did get Will Prowse’s book and I am trying to educate myself on solar. I like the idea of the DC/DC-CC but if I wire the 4×175 W panels in 2S2P would the voltage be too high for that controller??? And if wired in parallel would the amps be too high for the MC4 branch connectors??? please correct me if I’m wrong.. maybe only two 170ah batteries would make more sense if I only use four panels?? Possibly expanding the system later if needed... once again thanks everybody for your help I greatly appreciate it....

Marine359 · 07-22-2022, 05:34 PM

Assuming your panels ar 18v and 6amps.
2 panels in serial yields about 36v and 10 amps per pair ( or whatever the output amperage of your panels is)
So when you wire the series pair in parallel, the yield going into the mc4 gland box will be:
36v and 20 amps ( or 2x your panel amperage.)

From the gland to the controller is normally 10awg wire., which can carry 30amps.
So your wire should be ok.
And you will not exceed the voltage of your controller.
100/30 controller or larger should be used for your install.

BarryLS · 07-22-2022, 08:11 PM

Wired in series the voltage will double for two panels, the current remains the same. Wire two pairs in series and parallel the two pairs together and you get double the voltage and double the current (of a single panel).

I don't have all your specs, perhaps you might want to list them. Every charge controller has a minimum and maximum voltage input. Stay between those and you should be good. The same is true of the current, there is a max, stay under that and you are good. I prefer to try and keep the voltage as high as possible above the battery voltage for the best charging performance. OTOH, as previously mentioned the possibility of partial shading should be assessed.

Someone else mentioned to me that the Prowse book was simple. My answer was that it is pretty simple once you spend a bit of time reading. That's not to belittle any questions, only to tell you that a reasonable person is more than capable of doing this. Figure out what you want from solar, then do the rough design. Then run that by the collective here and you should get some assurance or redirection.