Solar Planning - Tell Me I'm Stupid

[RamRod4]

Need to run this by our expert solar gurus as I'm hung up on charge controller sizing/quantity. Here's my planned specs:

( 8 ) 200W Rich Solar panels with 4 strings of 2 series panels - 1600w total
(4) 200ah LiFePo4 batteries - 800ah total
(1) Victron Multiplus-II 12/3000/120-50 2x120V
(2) Victron SmartSolar 150/70

Let me know your opinions on the solar array (4 strings of 2) and the size/amount of charge controllers. Also, I'm open to hearing about going with a 24v since I think Victron makes the same inverter in 24v.

 

[Albatross]

I would run more series and less in parallel, you'll get more efficiency out of it that way.

If you check the data sheet on the charge controller and compare with the datasheet on the solar panels, you'll see that first, the controller is capable of handling the higher voltages per line, and two, that the self-consumption decreases when you feed it higher voltages.

There is an efficiency loss every time you convert from one voltage to another, but that's actually the whole point of the charge controller in the first place--to convert the voltages you feed the batteries to the exact voltage they like based upon their current charge. But in order for this controller to actually work, it wants +5V above whatever the batteries are already at. In the early mornings and late evenings, when the solar is at a minimum, with more in series versus more in parallel, you could wind up with an appropriate voltage with a minimal current compared to more current at a voltage too low that the controller shuts things off--which would result in all the energy going to waste as heat, and more wear/tear on the device.

 

[rossvtaylor]

I concur with Albatross...well said. The only real advantage that parallel strings have over series is that parallel strings are more resistant to shading of one or more panels. But in my mind, all the advantages of the higher voltage (and longer charge opportunity) from a series string outweighs the shading issue in most cases. In larger arrays, where one controller won't handle the entire array, you can do series strings into separate charge controllers...which are connected to the batteries in parallel. If one of those arrays is shaded, the other will still charge.

As to 12V vs 24V, I've written a post specifically about this. Some (whom I really respect) disagree with me...but my general design philosophy is to use 12V banks in a bus for systems up to about 5000W in inverter size. Using a 12V house battery bank makes lots of things easier and cheaper, in addition to making it easier to find compatible parts or accessories while you're on the road.

Having said that, I'm working on a system design with a 24V battery bank because of the need to have 240V AC power for a dual-zone mini split. That system exceeds my general 12V guidelines. And for all the off-grid and village systems I did, those were all 48V...so I know there are advantages in some cases...but in a bus, with a native 12V system and 12V appliances/bulbs/chargers I don't see a reason to stray from that in smaller systems.

 

[Albatross]

I largely agree with Ross, but I would also add a caveat with longer buses and any kind of need for significant sources of power at the other end of the bus.

If you need to run more than 10-20 feet for a high-power device, I would consider it generally worthwhile to go to a higher battery voltage and use a step-down transformer at the other end.

Personally, I would rather keep voltage higher and currents lower as much as possible.

But it makes sense to stick with a 12V system as much as you can when you are already having a 12V system built-in.

There is actually another option I don't see too often (for a fair reason, TBPH), and that's to use a 24V (or higher) system and essentially try to load-balance across the different legs of the battery bank like 'circuits', but you also would lose the chassis as ground for the other half and have to run return lines for everything. It's a bad hack that would cause problems for most people and most situations, but in a select few cases (where you're expecting to have higher throughput because of active loads) it is one way I could see doing some things. Oh, and you'd really need some kind of fused link for safety's sake that could cause problems later on.

 

[Dbacks2k4]

Broken record here, but yeah, 24v or 48v. I got "skooled" on that pretty early in my build planning

I'm going 24v, really would like to go 48v but it's not as common so less options for equipment. Probably a little safer anyway, self-proclaimed "professional amatuer" electrican am I. 110v AC isn't a fun zap. 48v DC electrical shock is deadly.

 

[RamRod4]

Awesome feedback. Thank you guys.

I’m leaning towards sticking with the 12v/3000w split phase inverter and running 2 charge controllers. We don’t plan to full time in this bus or boondock for more than 2-3 days at a time so the solar setup may be a little overkill but I’m trying to cover my bases so I can use the bus wherever I please (I’m the guy that packs enough clothes a weeks worth of clothes for a 2 day weekend trip). Most boondocking will occur in the winter when we’re skiing and my heat is diesel hydronic as well as hot water so my power consumption should be minimal when off grid.

 

[rossvtaylor]

I definitely don't want to argue about bank voltages...but I'm hard pressed to think of any high draw 12V appliances which might require the larger wire people mention. Definitely, from the bank to the inverter...but that ought to be a 3-foot max run. Even a 12V fridge doesn't draw enough current to cause me any concern about wire sizing. And from the inverter (again...placed close to the batteries) everything is 120V. In a 24V system, to use DC items, you need a converter...transformers don't work on DC, they just become an electromagnet. By the time you get 24V>12V converters, I'll bet you've spent more than the larger wire would cost.

Anyway, these are good questions and a good discussion. I just raise these points as things to consider. Because, like all parts of a good system design, each decision is a tradeoff and impacts other parts of the system.

 

[HamSkoolie]

We're going with Victron gear and 10.24kWh (about 800ah at 12VDC)



We are going 24 volt batteries and a 24 volt "backbone" with three separate buck converters (front, mid, rear) feeding three separate 12VDC fuse blocks. This is to reduce the number of wires coming from any single fuse block, reduce the gauge wire required from each block to the circuit device. Thus cutting wire bulk. Construction cost is likely to be about the same due to smaller/cheaper terminal circuitry offsetting the cost of the large gauge backbone and additional buck converters but operating efficiency should be increased due to higher voltage on the backbone and larger wire with less resistance.
We're doing the Victron multiplus but the 24VDC version. Be sure to check the specs, including output at various temps, to ensure it will cover your needs.
Panels will be two banks in series and parallel config for each bank feeding TWO Victron controllers. This is to provide shade resistance and redundancy. Each positive PV lead to the controller will have a T class fuse and after the controllers will be circuit breakers.
The multiplus will also supply the AC distribution box with a 50amp main (6000W at 120V which is the peak for the multiplus).


Other than going 24 volts and splitting the PV's into at least two banks feeding two higher voltage controllers, your description sounds fine.

 

[RamRod4]

Much appreciated guys. This is one of the reasons why I appreciate this forum so much. So much knowledge.

Per the advice, I’m thinking I’ll run 2 arrays each with 4 panels wired in series. Each array will produce 80v/10a

Then to 2 SmartSolar 100/50’s outputting 12v. Any reason I should go bigger on these?

To 800ah if LiFePo4 battery bank

With a Multiplus-II inverter.

I understand the benefit of 24v systems but I’d like to keep some complexity out of the system so I can run my 12v lights, sound system, camera system, etc without the addition of converters and whatnot.

 

[HamSkoolie]

Much appreciated guys. This is one of the reasons why I appreciate this forum so much. So much knowledge.

Per the advice, I’m thinking I’ll run 2 arrays each with 4 panels wired in series. Each array will produce 80v/10a

Then to 2 SmartSolar 100/50’s outputting 12v. Any reason I should go bigger on these?

To 800ah if LiFePo4 battery bank

With a Multiplus-II inverter.

I understand the benefit of 24v systems but I’d like to keep some complexity out of the system so I can run my 12v lights, sound system, camera system, etc without the addition of converters and whatnot.

It works. 12 or 24 volt is a choice that, so long as you understand the pros and cons, can go either way.