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diy solar

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Ked

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Dec 29, 2021
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I have an RV I would like to add solar to and could use some input. I plan to install 3x renogy 170ah lithium ion batteries with a renogy dc/dc mppt charge controller. On the roof I plan to use 3x renogy 175W Monocrystalline panels (I could add another panel if needed?? ) and of course the appropriate size wiring and circuit breakers. I may eventually add a pure sine-wave inverter.

I am curious to know if this system is viable and any input would be greatly appreciated, as I am new to this and could use all the help I can get. Thanks.
 
That will definitely work, but you could stand to add about as many panels as you could fit on the roof with a total of 510ah (at I assume 12 volts). That is 6500 watts of power stored in your battery. Your solar panels will produce no more than 500 watts an hour, probably closer to 400 per hour when it's sunny. A full day, at 5 hours of good sun on the RV roof (being generous here), will make about 2000 watts per (good) day.

Of course I imagine you'll also be charging with a combination of a generator, shore power, and alternator, so what you lack in solar could easily be made up for.
 
I am no longer a fan of Renogy products. You can, and I have, found better quality panels off Craigslist for a fraction of what Renogy costs. I think you're better off buying Epever than Renogy as your charge controller. I don't have any experience though with their Lithium batteries.
 
renogy dc/dc mppt charge controller.
What is the max charge current of that controller? 40A? If so, with a 12V system that supports a max PV of about 500W. You can over panel a bit so maybe 600W. 3 175W panels is fine but 4 is possibly too much. It largely depends on the orientation of the panels, shading, and other factors and how much wattage you really get out of the panels at any one time.

you could stand to add about as many panels as you could fit on the roof with a total of 510ah (at I assume 12 volts). That is 6500 watts of power stored in your battery. Your solar panels will produce no more than 500 watts an hour, probably closer to 400 per hour when it's sunny. A full day, at 5 hours of good sun on the RV roof (being generous here), will make about 2000 watts per (good) day.
FYI - Here's a few corrections using the correct units. 3 12.8V 170Ah batteries is 510Ah and 6,528Wh (watt hours, not watts). 3 175W panels is 525 watts (not watts per hour). Same for the 400W figure. In 5 hours per day at 400W the panels provide 2000Wh/day (watt hours per day, not watts per day).
 
FYI - Here's a few corrections using the correct units. 3 12.8V 170Ah batteries is 510Ah and 6,528Wh (watt hours, not watts). 3 175W panels is 525 watts (not watts per hour). Same for the 400W figure. In 5 hours per day at 400W the panels provide 2000Wh/day (watt hours per day, not watts per day).
OP would definitely need more (or larger) charge controllers to support more panels, I was just saying that his batteries could definitely support more.

I didn't abbreviate any units for a reason, I was trying to make it clear what I was talking about without having to worry about unit conversions on top of everything else.
 
I didn't abbreviate any units for a reason, I was trying to make it clear what I was talking about without having to worry about unit conversions on top of everything else.
The issue wasn't with using abbreviations or not, it was with using the wrong units. You used watts when it needed to be watt hours. You stated watts per hour when is needed to be just watts, etc. Correct units (abbreviated or not) is the best way to be clear. :)
 
The issue wasn't with using abbreviations or not, it was with using the wrong units. You used watts when it needed to be watt hours. You stated watts per hour when is needed to be just watts, etc. Correct units (abbreviated or not) is the best way to be clear. :)
Fair enough. I appreciate the correction.
 
I’m sorry I should’ve been more descriptive with my post. The mppt is the renogy 50a dc/dc mppt charge controller. I believe I can fit 4x 175w panels for a total of 700 watts that would be wired with two panels in parallel, then the two pairs in series...( is this correct?? ). would this be to much for the 50a controller?? Thank you all for your input as I can use all the help I can get....
 
50A x 12V = 720W. So 700W of panels is just fine.

If you want the panels in 2S2P you would wire 2 panels in series (connect the positive of one to the negative of the other). You would wire the other 2 panels in series. You would then connect those two strings in parallel using a single pair of MC4 Y connectors.

Just make sure the combined temperature adjusted Voc of the panels is less than the max PV input voltage of the charge controller. If you are not sure then post the Voc of one of your panels, the coldest possible temperature your panels will ever be exposed to, and post the max PV input voltage of the charge controller and we can do the math.
 
Here is the cut sheet for 175 W solar panels and the installation instructions for the 50A DC/DC charge controller...as I live in San Diego, California I don’t think the lowest temperature would be below freezing
 

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Ugh. That charge controller only allows 25V PV input voltage. Your panels have a Voc of 21.6V. Your panels must be wired in 4P. You can't put any in series with that controller.

21.6V is 86.4% of 25V. That allows for an increase of 13.6%. The panels have a temperature coefficient of -0.31%/ºC. 13.6% / 0.31%/ºC is 44ºC. So the panel would need to get to 25ºC - 44xºC = -19ºC or -2ºF to get over 25V. So you should be fine there, especially in San Diego.
 
Max solar input voltage is 25V. Egads, why would anybody bother making a SCC with limitations like that?

I plan to install 3x renogy 170ah lithium ion batteries with a renogy dc/dc mppt charge controller.
If you have not purchased this or can return it, you'll be far better off with a quality SCC.
 
Max solar input voltage is 25V. Egads, why would anybody bother making a SCC with limitations like that?


If you have not purchased this or can return it, you'll be far better off with a quality SCC.
Seconded on both points. At LEAST let it take closer to 50v. F Renogy.

That particular charge controller seems to be designed for DC to DC alternator charging, and solar capability was added as an afterthought. In my opinion, it sort of defeats the purpose of an MPPT controller if it won't take higher voltages.

The one "top-teir" piece of equipment that I have is a Victron MPPT solar charge controller. I went through a handful of cheap ones before I realized that you get what you pay for, ESPECIALLY with SCC's. Everything else in my system is able to be bypassed, EXCEPT for the SCC. That makes it, IMHO, the most "mission critical" part of a solar setup, and I don't want to risk failure in an emergency. (My system is designed for backup power in grid-down situations, and my trailer / "camper").

You can most likely get by with a cheaper SCC, but I can guarantee that you won't regret buying a nice one. Plus, I'm a sucker for data export. ?

I would definitely return that "SCC" if it's still able to be. There's other options that cost about the same, that are much better then Renogy. If you can't return it, it's not the end of the world, but you'll definitely be limited in regards to how you can configure your panels.

A side note;

OP could probably rig up a Reverse DPST switch on the input of that controller, and switch between solar and alternator charging. That IMHO, may redeem that controller, unless OP already has alternator charging capability. Just thinking out loud here.
 
The reason I was going to go with the Renogy 50a dc/dc SCC was I wanted something that would charge both the chassis battery and the house batteries of the motorhome and not fry my alternator in the process.. I had read lithium batteries will take a lot and can overheat the alternator?? I watched Will‘s video on that MPPT and he seem to rate it quite highly. Well I guess I need to go back to the drawing board and find an alternative way to do this....
thank you for all your input..
 
You should look into a good dedicated DC-DC charger and a separate good solar charge controller.
 
Well it sounds like I need to get two separate SCC’s. a DC/DC for the chassis batteries and a good SCC for the lithium coach batteries. I guess my issue now would be how to separate the two systems in the motorhome as they are both wired together with an auxiliary start solenoid... that way if the chassis battery is dead you can use the Coach battery to start the motorhome. Separating the two systems may be a good thing as I have read that lithium batteries should not be used for a vehicle starting battery. Once again thanks for all your help looks like I need to do some more reading and research...
 
I need to get two separate SCC’s.
Do you have 2 different arrays or why would you need 2 SCC's? (solar charge controller)

how to separate the two systems in the motorhome as they are both wired together with an auxiliary start solenoid...
Why would either of these be connected to an auxiliary start solenoid?
The DC to DC would be connected to alternator on one side and the house lithium (presumably lithium iron phosphate, LiFePO4) battery(s).

The auxiliary solenoid is a tricky bit that i have not needed or experimented with. I have a 120A BMS so suspect the Sprinter started would draw too much for the BMS to not cut out. My plan, should i need engine battery help, is to connect LiFePO4 house battery for several minutes with auxiliary switch before attempting to start the engine. I dunno, maybe 50/50 chance for success?
 
Now I’m really confused.. it sounded like rmaddy suggested a DC to DC charger and a good solar charge controller... I take it the DC to DC is not an SCC??
 
Now I’m really confused.. it sounded like rmaddy suggested a DC to DC charger and a good solar charge controller... I take it the DC to DC is not an SCC??
correct.
Although sometimes the functions are combined in one device.
this one for example.
I don't recommend that one though.
 
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