What do you experts think of this solar+battery+inverter system for my small RV?

[MegaMosquito]

Some background (why I chose this specific design):

I have a small (24 foot long) motorhome with a 30A shore service. I have designed an off-grid 1kW solar array (the most I can fit on the roof) and a 10kWh battery bank with a 3kW inverter so we can sometimes "boondock" when we travel (i.e., park in places where we cannot plug in to the grid). I wish I could add more solar for charging this big battery bank, but 1kW is the most I can fit (and that is very tight). We want to be able to boondock as much as possible but we realize that we may sometimes need to stay at a campsite with hookups to top up the batteries (we need that to dump waste water from time-to-time anyway). I designed the solar array with ten 12V 100W panels in series because I understand that serial setups are going to enable charging earlier in the morning, and later in the afternoon, than parallel setups. As a result, the solar array has an open current rating of 195V. I only have enough space for 10kW of battery in the form of two rack-mount 48V (nominal) 51.2V 100Ah batteries. I'm planning a 3kW inverter (i.e., 25A at 120V) so I can power almost everything that I can power from the 30A "shore" connection. Another option might be an inverter of 3.6kW (30A at 120VAC) or more, but the inverter is for boondocking when we will hopefully be conserving power. So I think we can live with 3kW.

The best controller, inverter, monitor, etc. system I have found so far for this setup is an all- Victron Energy system. I considered less expensive all-in-one systems (like EG4) but the Victron system gives me complete flexibility for each component and I also really like the CerboGX monitoring system.

I have bought nothing yet.

I'm new to all of this so I am really hoping you can criticize my design before I buy all this expensive stuff and begin this project.

Here's my design:



So what do you think? What would you change if this was your project?

Thanks in advance!

 

[time2roll]

228 volts (Voc) into a 150/100 controller will let the smoke out. Or is this 5s2p?
Any reason not to use a victron 48/12 converter? I would put the fuse in the distributor if possible as it does about nothing close to the converter.
Only going to get 25 amps out of the MPPT so I would reduce that 100a fuse to 50/60.
DC load to the inverter is probably a bit less with 48 volt system. Smaller wire and fuses are probably appropriate. Inverter manual will have the details.

 

[PotusGobbleBottom]

Are you really looking to draw 400 amps or is that a type-o??
I dont know much about multiplus's, is this one only 3000 watts?

 

[sunshine_eggo]

I'm just not a fan of the Lynx Shunt. Prefer BMV-712, 702 or Smartshunt with temp sensor.

2/0 @ 95'? WTF?

400A? WTF?

I would go with SOK batteries.

195V will destroy your MPPT. Limited to 150Voc in the coldest weather you would experience. I would go with 5S2P.

150/100 is spendy. You only need 1000W/48V = 20.83A.

I would consider a 100/30 and do 3S3P to save some $ and better shade tolerance. Invest in 3 or 6 flexible 100W panels you could deploy as a lightweight ground array in parallel with your roof array (either 3S or 3S2P)

Converter must be sized to handle any heavy 12V surge loads you might have.

I personally prefer to leave the converter and 12V in place and feed the converter with the inverter. Yes, you're going to lose some 12V efficiency, but you'll have backup 12V power and tolerance of 12V surge loads.

 

[MegaMosquito]

228 volts (Voc) into a 150/100 controller will let the smoke out. Or is this 5s2p?
Any reason not to use a victron 48/12 converter? I would put the fuse in the distributor if possible as it does about nothing close to the converter.
Only going to get 25 amps out of the MPPT so I would reduce that 100a fuse to 50/60.
DC load to the inverter is probably a bit less with 48 volt system. Smaller wire and fuses are probably appropriate. Inverter manual will have the details.

Thanks @time2roll ! And ouch! I changed up the solar array from my previous design but forgot to recalculate the controller. Will fix. THANKS!

I did not know Victron made 48V-to-12V converters. I'll check that out.

Good point on the 100A fuse too. I sized based on the max charge rate of the MPPT, but these panels won't deliver that.

Thank you so much for responding!

 

[MegaMosquito]

Are you really looking to draw 400 amps or is that a type-o??
I dont know much about multiplus's, is this one only 3000 watts?

I wrote 400A for the wire because that is the battery peak discharge amperage (15 sec) but I fused it in the distributor to cap it at 200A because that's the max continuous discharge rate of the battery. And yeah the "Multiplus II 48/3000..." is 48V 3000W.

 

[PotusGobbleBottom]

I wrote 400A for the wire because that is the battery peak discharge amperage (15 sec) but I fused it in the distributor to cap it at 200A because that's the max continuous discharge rate of the battery. And yeah the "Multiplus II 48/3000..." is 48V 3000W.

Personally, I would use the 2/0 wire and fuse it at 100A. That way if anything happened, that wire would be perfectly fine except for where it shorted. 100 amps would cover your inverter draw. Fuses are not very fast blowing and even a 200 amp surge for .2 seconds would probably not blow the 100amp fuse.

 

[MegaMosquito]

I'm just not a fan of the Lynx Shunt. Prefer BMV-712, 702 or Smartshunt with temp sensor.

2/0 @ 95'? WTF?

400A? WTF?

I would go with SOK batteries.

195V will destroy your MPPT. Limited to 150Voc in the coldest weather you would experience. I would go with 5S2P.

150/100 is spendy. You only need 1000W/48V = 20.83A.

I would consider a 100/30 and do 3S3P to same some $ and better shade tolerance. Invest in 3 or 6 flexible 100W panels you could deploy as a lightweight ground array in parallel with your roof array (either 3S or 3S2P)

Converter must be sized to handle any heavy 12V surge loads you might have.

I personally prefer to leave the converter and 12V in place and feed the converter with the inverter. Yes, you're going to lose some 12V efficiency, but you'll have backup 12V power and tolerance of 12V surge loads.

Thanks @sunshine_eggo! I'll check out that shunt.

Lol. Sorry. I'm not buying 95ft of 2/0. I'm just noting the maximum length of that gauge wire at that voltage and amperage. My little RV is only 24ft long so I would have a hard time using 95ft of wire. Where I am locating things the inverter will be within 2 feet of the battery bank.

And 400A is just the maximum peak discharge rate of the batteries. I figured I needed to cable for that, but fuse to cut that to 200 (the max continuous discharge rate).

I clearly need to re-think my choice of MPPT. Thanks for highlighting this. "time2roll" also nailed me on that one. Oops.

I have measured the 12V loads in my RV and they top out (everything on at once) and just under 17A. I thought that sizing the converter to 40A would handle any surges. Are you suggesting instead of 48V to 12V DC/DC converter, that I use a 120V transformer (connected to the inverter) to provide 12V instead?

 

[MegaMosquito]

Personally, I would use the 2/0 wire and fuse it at 100A. That way if anything happened, that wire would be perfectly fine except for where it shorted. 100 amps would cover your inverter draw. Fuses are not very fast blowing and even a 200 amp surge for .2 seconds would probably not blow the 100amp fuse.

Thanks @ScrotusGobbleBottom! I think you (and others) are highlighting my fuse strategy is incorrect in a few places. I will rethink all of the fuses.

 

[Brucey]

. I designed the solar array with ten 12V 100W panels in series because I understand that serial setups are going to enable charging earlier in the morning, and later in the afternoon, than parallel setups. As a result, the solar array has an open current rating of 195V.

There could be a small theoretical difference but you may have bigger problems with shadows from rooftop obstructions reducing your long series string. I believe many have multiple controllers to allow for shading on one or more panels.

 

[MegaMosquito]

I would go with SOK batteries.

I forgot to respond to your SOK battery comment. I did look at SOK, but I can't fit a comparable SOK battery bank in the small compartment where I want to put my batteries. The EPOCH batteries are similar in build, but 48V (which tend to be smaller per Wh) and internally heated (I travel to very cold places about once a year, like -40 outside), and they fit in my compartment.

 

[Brucey]

For example, on my 150/35s I can series 3 300W 37V panels. Charging can start at 5V above battery voltage, so once the array gets to 60V it's starting to charge. With my 450/100, I can series ten panels. But the minimum charge start voltage is 120V. In either case they both get up to a voltage beyond 60 or 120V very quickly in the morning once there's enough light.

 

[MegaMosquito]

There could be a small theoretical difference but you may have bigger problems with shadows from rooftop obstructions reducing your long series string. I believe many have multiple controllers to allow for shading on one or more panels.

Cool! So I think you are saying I could use multiple MPPTs (in parallel) in this system (connecting a subset of the panels to each. Is that right? I guess then I would have to have multiple wires going through the roof. And I would essentially have multiple completely independent solar arrays. That's very interesting!

 

[MegaMosquito]

For example, on my 150/35s I can series 3 300W 37V panels. Charging can start at 5V above battery voltage, so once the array gets to 60V it's starting to charge. With my 450/100, I can series ten panels. But the minimum charge start voltage is 120V. In either case they both get up to a voltage beyond 60 or 120V very quickly in the morning once there's enough light.

Yeah, that's fantastic. I hadn't considered the possibility of doing something like this. Thank you so much!

 

[Brucey]

Yeah, that's fantastic. I hadn't considered the possibility of doing something like this. Thank you so much!

The problem is if you are running a 48V system you'll need to get to at least 60V or so per string to get started with charging. It would be easier if you went with a 24V design, then say a pair of 20V panels per string would be sufficient to start charging.

 

[PotusGobbleBottom]

I hadn't considered the possibility of doing something like this.

This is certainly an application for a 24v setup. An Rv with a typical max draw of 1500 watts. For sure. Not too scalable for running 5000 watts so the biggest issue would be buying 24v batteries and regretting later because you moved them into a house.

 

[sunshine_eggo]

Thanks @sunshine_eggo! I'll check out that shunt.

Lol. Sorry. I'm not buying 95ft of 2/0. I'm just noting the maximum length of that gauge wire at that voltage and amperage. My little RV is only 24ft long so I would have a hard time using 95ft of wire. Where I am locating things the inverter will be within 2 feet of the battery bank.

And 400A is just the maximum peak discharge rate of the batteries. I figured I needed to cable for that, but fuse to cut that to 200 (the max continuous discharge rate).

Manual lists 125A recommended fuse and 2/0awg for 5-10m run

I clearly need to re-think my choice of MPPT. Thanks for highlighting this. "time2roll" also nailed me on that one. Oops.

In retrospect, my 3S3P recommendation would be very marginal in hot temps with only 58.5Vmp.

5S2P on a 150/XX would be better. That would also require 5S as the separate deployable ground array (suggested this for maximizing your boondock longevity).

I have measured the 12V loads in my RV and they top out (everything on at once) and just under 17A.

Does anything have an electric motor? If so, its surge may be 5X its run current. Converters have negligible surge capability.

I thought that sizing the converter to 40A would handle any surges. Are you suggesting instead of 48V to 12V DC/DC converter, that I use a 120V transformer (connected to the inverter) to provide 12V instead?

No. I'm proposing you leave the existing AC-DC converter/12V system in place and power the converter with the inverter.

 

[MegaMosquito]

The problem is if you are running a 48V system you'll need to get to at least 60V or so per string to get started with charging. It would be easier if you went with a 24V design, then say a pair of 20V panels per string would be sufficient to start charging.

I agree it would be easier but my unfortunate space constraints are holding me back from that. To get the same (in kWh) storage capacity with 24V batteries takes too much room for my tiny "wiring closet" (which originally used to be an actual closet, lol). However, your excellent suggestion to use multiple MPPTs would allow me to keep 48V, I think. I can use 2 MPPTs, each with a string of 5 panels in serial attached (making 97.5 Vmp each).

 

[sunshine_eggo]

I forgot to respond to your SOK battery comment. I did look at SOK, but I can't fit a comparable SOK battery bank in the small compartment where I want to put my batteries. The EPOCH batteries are similar in build, but 48V (which tend to be smaller per Wh) and internally heated (I travel to very cold places about once a year, like -40 outside), and they fit in my compartment.

I was recommending the SOK 48/100ah batteries compatible with Victron BMS-CAN.

I call BS on this:



I'll poo myself if you can actually charge down to -25°C.

 

[Brucey]

I agree it would be easier but my unfortunate space constraints are holding me back from that. To get the same (in kWh) storage capacity with 24V batteries takes too much room for my tiny "wiring closet" (which originally used to be an actual closet, lol). However, your excellent suggestion to use multiple MPPTs would allow me to keep 48V, I think. I can use 2 MPPTs, each with a string of 5 panels in serial attached (making 97.5 Vmp each).

Can get rack mount 24Vs

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