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Should I wire my RV with 48v or 12v batteries? Wonder about solar panel charge efficiency.

Dragoth

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Joined
Mar 28, 2023
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Utah
I'm currently designing my solar system for my 45 foot fifth wheel travel trailer. I am going to be living out of the trailer for at least 3-6 months and want to be able to use just about anything I want whenever I want. (I know A/C is a tricky one, but let's ignore that for now).

I currently own 6 200w 12v solar panels with the following specs:
  • Max Power Output(W): 200W
  • Voltage MPP Vmp(V): 18.60V
  • Current MPP Imp(A): 10.76A
  • Voltage Open Circuit Voc(V): 21.8V
  • Short Circuit Current Isc(A): 11.47A
I am considering wiring them in a series/parallel configuration with 3 panels in series, and 3 in parallel. (Not sure if this is the best option and I'm not opposed to adding more panels if needed).

I have decided to purchase at the minimum 2 server rack batteries and highly considering getting 2 more within the next 6 months. (Likely going with the Lifepower since the added features of the LL-ii don't seem justifiable for me).

I like the low amperage of the higher voltage systems, but it does create more problems with having to convert the DC voltage to 12v to run the chassis systems. I was ultimately going to stick with 12v for simplicity reasons and because of solar charge efficiency.

Now correct me if I'm wrong, but the way I figured it out was that a 48 volt battery needs 56 volts to charge meaning that if I were to wire my panels in series/parallel I would only be at 65.4 volts (Voc) at peak output so if my panels drop in efficiency to 88% my batteries wouldn't be able to charge, but in the same scenario with a 12 v battery requiring only 14v to charge my solar panel output could drop to 22% before it would not be able to charge my batteries.

Is this the correct way of looking at it? If so, then to go with 48v I would probably want to wire all the panels in series to bring up the voltage so that my batteries don't lose the ability to charge after dropping only 12% efficiency on my panels. I assume at this higher voltage I would need to use 2 charge controllers since using the Victron charge controller calculator it says they have nothing that will work with 6 of my panels wired in series.

It really doesn't matter to me that I have to use massive wires to ensure a safe system when using 12v, but If it makes sense to go 48v I want to do that out of the gate, I'm just getting a bit overwhelmed trying to make sure I don't screw up the system. Any advice is helpful and I thank you all in advance for your input.
 
I currently own 6 200w 12v solar panels
wiring them in a series/parallel configuration with 3 panels in series, and 3 in parallel.
3S3P is 9 panels.

I think you mean 3S2P:
3 panels in series, twice. These 2 strings in parallel. But...

For 3S, 18.60Vmp = 55.8V
Most SCCs require at least 5V headroom over charging voltage (~58V) so this will not work for a 48V battery.

With 6 panels, 6S would be the next best configuration but would require an SCC that accepts about 150Voc.

6 x 200W = 1200W
1200W / 58V charging = 20.7A

Something like this would work nicely and allow for expansion: (Victron 150/35 Smart)
 
3S3P is 9 panels.

I think you mean 3S2P:
3 panels in series, twice. These 2 strings in parallel. But...

For 3S, 18.60Vmp = 55.8V
Most SCCs require at least 5V headroom over charging voltage (~58V) so this will not work for a 48V battery.

With 6 panels, 6S would be the next best configuration but would require an SCC that accepts about 150Voc.

6 x 200W = 1200W
1200W / 58V charging = 20.7A

Something like this would work nicely and allow for expansion: (Victron 150/35 Smart)
You are correct. I wasn't understanding how the layout would be. I am looking at the 3S2P setup.

I have no problem with wiring them all in series, I'm only hesitant to do so due to my research showing that if a single panel is shaded it will bring down the entire system rather than affecting only the one, this is my first system though, so I'm not sure how that plays out in real world practice.
 
if a single panel is shaded it will bring down the entire system
That is correct. You could do a lot better for shade handling with different panels.

Or, you could add 2 more panels and run 4S2P. (2 shade zones)
Or, you could add 3 more panels and run 3S3P. (3 shade zones)
Or, you could add 4 more panels and run 5S2P. (2 shade zones)
 
Sell the six 200 watt panels and buy 300+ watt panels instead. For an RV that size, I think you're going to want more than 1200 watts anyhow. My dinky 22' trailer can consume 1200 watts easily with the air conditioning on.
 
Sell the six 200 watt panels and buy 300+ watt panels instead. For an RV that size, I think you're going to want more than 1200 watts anyhow. My dinky 22' trailer can consume 1200 watts easily with the air conditioning on.

Definitely something to consider....I was worried it wouldn't be enough for my needs, but I'm new to this and figured I could add on later if I find it's not enough.
 
You are also under-paneled for 10kWh of battery. You likely should have a minimum of 2kW.
 
With any reasonable sun I can go indefinitely on 600w solar and a large 12v battery. Although air conditioning would deplete the battery in 8 to 12 hours by my estimate. If I need air I am generally looking for utility power. Hot water and fridge run on propane.

1200w solar maybe go 24 volts. 2kW+ solar to run the air is best at 48 volts. I doubt 2kW would run the air 24/7 unless maybe if it was a smaller mini-split.

If you plan to have a generator to run the air.... I would stay at 12v for simplicity. (I did)

45' gives a lot of roof area for solar and also creates a larger cooling load.
 
You are also under-paneled for 10kWh of battery. You likely should have a minimum of 2kW.
Yeah, I understand that. Which is why I'm considering purchasing a few more panels. That being said, my eventual use for the batteries will be to transfer them into a much larger system for my home setup once I have that. So I'm okay for now being under powered at the panels if need be. Too much battery power is never a bad thing imo.
With any reasonable sun I can go indefinitely on 600w solar and a large 12v battery. Although air conditioning would deplete the battery in 8 to 12 hours by my estimate. If I need air I am generally looking for utility power. Hot water and fridge run on propane.

1200w solar maybe go 24 volts. 2kW+ solar to run the air is best at 48 volts. I doubt 2kW would run the air 24/7 unless maybe if it was a smaller mini-split.

If you plan to have a generator to run the air.... I would stay at 12v for simplicity. (I did)

45' gives a lot of roof area for solar and also creates a larger cooling load.
Yeah, I'm not sure how much A/C I'm going to be using. I know I want to be able to use it, and my trailer has an onboard 5kw Onan generator, but honestly, If I don't have to fire that thing up much, I'll be a happy camper. I guess I'll climb up there tomorrow with my template and see what I can get away with as far as panels go.

Anyone know if 48v converts more efficiently to 120 to run the A/C or does it really not matter that much other than Amps creating the need for much larger cables in 12v systems?
 
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