I've hit a wall whist building my first system. Having a hard time sizing my system.

Hi. I'm trying to build my very first RV system and I have run into a bit of a wall. I plan to be quite comfortable with a ~1200-1400w array and ~2400wh of battery storage. My original plan was to buy 6 used 230w SanTan Solar Panels for a total of 1380w to use on my 12v system. These panels are rated at a Maximum Power Voltage of 29.8v and a Maximum Power Current of 7.78a. Here's where I run into a problem. 1380w/12v=115a. This is far bigger than any SCC I can afford (or have found for that matter). So is my only option at this point to increase the voltage of my system to 24v and buy a 60a charge controller? (1380wh/24v=57.5a would 60a be cutting it too close?) If this is the case, it kind of throws a monkey wrench in my plans. I was looking to build my own battery out of loose cells. However, all of the cells that Will recommended are 3.2v and usually come in packs of 4. This means that I would need to either find some 6v cells elsewhere or buy twice as much battery as I need. I feel like there's something I'm missing here. If anyone has any helpful advice on how to achieve my goal here I'd love to hear it. Thanks for your time!!

Solar panel voltage does not need to match battery pack voltage thanks to MPPT solar charge controllers

If battery is 14V, current will be lower than at 12V
It's OK to over-panel, clip a bit when sun is directly overhead. It will be off-angle most hours and season.
If you can tilt panels in two directions, peak wattage will be reduced 30% (90 degree angle) or 50% (60 degree angle between panels.)

Two charge controllers in parallel gives redundancy and doubles power handling.

1200W of PV, 2400W of battery. 0.5C, Typical peak charging for some lithium batteries. Only do that near 25 degrees C, current should be lower if battery is hot or cold.

1400 watt solar and a 2400 watt/hour battery is where you are out of balance. Need more battery or less solar.
I am thinking 1400 watt solar is needing close to 7000 watt/hour battery.
Or 2400 watt/hour battery needs 480 watts of solar.
Or something in the middle.

This is why you cannot find a SCC that seems practical for your set up.

1400 watts solar is a lot of power for most RVs unless you expect significant run time of the air conditioner or have some other large draw.

Spoonathan said:

Hi. I'm trying to build my very first RV system and I have run into a bit of a wall. I plan to be quite comfortable with a ~1200-1400w array and ~2400wh of battery storage. My original plan was to buy 6 used 230w SanTan Solar Panels for a total of 1380w to use on my 12v system. These panels are rated at a Maximum Power Voltage of 29.8v and a Maximum Power Current of 7.78a. Here's where I run into a problem. 1380w/12v=115a. This is far bigger than any SCC I can afford (or have found for that matter).

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Those panels will probably only put out 80-90% rated and has been stated, panels flat on a roof are not going to make rated power unless the sun is directly overhead. Additionally, RV roof protrusions can cause partial shading on panels, and partial shading can have a devastating effect on production in some cases.

Yes, high power into 12V means mucho current.

Spoonathan said:

So is my only option at this point to increase the voltage of my system to 24v and buy a 60a charge controller? (1380wh/24v=57.5a would 60a be cutting it too close?) If this is the case, it kind of throws a monkey wrench in my plans. I was looking to build my own battery out of loose cells. However, all of the cells that Will recommended are 3.2v and usually come in packs of 4. This means that I would need to either find some 6v cells elsewhere or buy twice as much battery as I need. I feel like there's something I'm missing here.

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Those same cells in groups of 4 are available in two groups of 4 or 8 individual cells - perfect for 24V. 280Ah cells + BMS would probably run you about $1200 for 7.2kWh of storage - much more appropriate for your proposed solar per @time2roll.

A simple 24V to 12V converter should power most of your RV's needs. For generator or shore power charging, you'll need a 24V AC-DC converter, or you get an inverter/charger.

You will need to watch for items like slide outs or other motors that might have high drain. If that's the case, you'll want to retain a modest 12V battery with a 24V-13.8V converter keeping it charged.

Spoonathan said:

If anyone has any helpful advice on how to achieve my goal here I'd love to hear it. Thanks for your time!!

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time2roll said:

1400 watt solar and a 2400 watt/hour battery is where you are out of balance. Need more battery or less solar.
I am thinking 1400 watt solar is needing close to 7000 watt/hour battery.
Or 2400 watt/hour battery needs 480 watts of solar.
Or something in the middle.

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Why?

I do think more battery is probably reasonable, weight of 200 Ah 12V LiFePO4 isn't that much.
Cost, though ... 1400W of PV is probably much less than the battery, so I could see skimping on battery.

It could be reasonably balanced, especially running A/C as you mention.

My biggest concern would be temperature limits of battery if charging at 0.5C

My own (grid-backup) system has a higher amount of PV in proportion to battery, and it is AGM. It regulates charging to 0.2C

Most of the SCC's I've seen limit your wattage based on the battery voltage, IIRC even mine max out at 780w max for a 12v system. I think with that much panel you're either going to need a higher battery voltage (24 minimum, more likely 48v) with some version of buck converter, or you're going to have to split your panels into multiple arrays on multiple controllers and parallel them together at the batteries.

time2roll said:

1400 watt solar and a 2400 watt/hour battery is where you are out of balance. Need more battery or less solar.
I am thinking 1400 watt solar is needing close to 7000 watt/hour battery.
Or 2400 watt/hour battery needs 480 watts of solar.
Or something in the middle.

This is why you cannot find a SCC that seems practical for your set up.

1400 watts solar is a lot of power for most RVs unless you expect significant run time of the air conditioner or have some other large draw.

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This is what I was planning on. I have a desktop computer and an air conditioner that I would like to run during the day. ~900w/h between the two of them.

sunshine_eggo said:

A simple 24V to 12V converter should power most of your RV's needs. For generator or shore power charging, you'll need a 24V AC-DC converter, or you get an inverter/charger.

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Thanks for your reply. Sorry for the noob question, but what's the difference between a converter and an inverter? Doesn't an inverter handle the conversion from 12vdc into 120vac? Would the converter be between the battery and the inverter to change the battery voltage into something the inverter can use?

If you put in a 24V battery and inverter, that will carry half the current for charging and feeding inverter. Could work better and be more economical.

Also need 12V DC, like for lights and a water pump, not much power? A 24V/12V DC/DC converter can supply that.

Rednecktek said:

Most of the SCC's I've seen limit your wattage based on the battery voltage, IIRC even mine max out at 780w max for a 12v system. I think with that much panel you're either going to need a higher battery voltage (24 minimum, more likely 48v) with some version of buck converter, or you're going to have to split your panels into multiple arrays on multiple controllers and parallel them together at the batteries.

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They really throttle it that much? Dang. I'm leaning towards maybe upping my battery power and going to 24v (I don't think there's any way I could pull off 48v and still have enough money for groceries haha). How difficult would it be to split my arrays between two charge controllers? Is it as as simple as just connecting the two SCCs to the battery terminals and the panels? Or is there some sort of balancing I would need to do? Thanks for the reply!

Spoonathan said:

Is it as as simple as just connecting the two SCCs to the battery terminals and the panels?

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Yup! Super simple.

Rednecktek said:

Yup! Super simple.

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Nice! That doesn't seem too bad. Thanks for your help. Earlier, you mentioned a Buck Converter. What is the purpose of this? From what I read on Google, it's supposed to basically just lower your voltage, right? So would I place this between my PV and my SCC and it would decrease the voltage going into my SCC? Wouldn't that decrease the wattage going into my system therefore causing my batteries to charge slower? I wonder if it would just be more beneficial to just buy two SCCs. My only concern there is that the cells I'm looking at have little toothpick terminals. I worry I wont be able to fit everything on them haha.

Spoonathan said:

This is what I was planning on. I have a desktop computer and an air conditioner that I would like to run during the day. ~900w/h between the two of them.

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Even for this usage the battery could be twice the size. Especially if you want to continue running the air into the evening as the sun sets and still have some battery to get you through the night. Seems like a small A/C and that is good for power usage. Keep in mind 1400 watts of panels mounted near flat on the roof will not create 1400 watts from the inverter five hours a day. Some days may come up a bit short and need battery support.

Spoonathan said:

They really throttle it that much?

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Their primary limiting spec is amps out.

Secondary one is watts dissipated inside them. That will go as amps out squared, as amps in squared, amps through transistor squared (but also times on/off duty ratio.) Better brands like Midnight Classic publish efficiency curves.

Spoonathan said:

Nice! That doesn't seem too bad. Thanks for your help. Earlier, you mentioned a Buck Converter. What is the purpose of this? From what I read on Google, it's supposed to basically just lower your voltage, right? So would I place this between my PV and my SCC and it would decrease the voltage going into my SCC? Wouldn't that decrease the wattage going into my system therefore causing my batteries to charge slower? I wonder if it would just be more beneficial to just buy two SCCs. My only concern there is that the cells I'm looking at have little toothpick terminals. I worry I wont be able to fit everything on them haha.

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The buck transformer is for getting a lower voltage DC out of a higher DC source. In your case, if you needed to go to a 24v or 48v battery setup, you couldn't feed your 12v lights and appliances with that or you'd end up with greasy orange smoke/fire/angry wife.

The buck transformer would kick that 24v down to the 12v that your 12v appliances need. That buck transformer would go between your battery bank and your 12v fuse block.

I had to do the same thing for my camp since I kicked the main battery bank up to 24v but I do have appliances that need 12v so I needed a couple of buck transformers. Here's my wiring diagram so you can see what I'm doing there.

time2roll said:

Even for this usage the battery could be twice the size. Especially if you want to continue running the air into the evening as the sun sets and still have some battery to get you through the night. Seems like a small A/C and that is good for power usage. Keep in mind 1400 watts of panels mounted near flat on the roof will not create 1400 watts from the inverter five hours a day. Some days may come up a bit short and need battery support.

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Yeah I've considered all of this. The only reason I'm not going with 5000wh of batteries (right now) is cost. I'm creating an identical build for my Dad; he saw some guy on youtube with 2400wh of battery and 1200w of panels and he though that seemed like a good idea so I've kind of just based my whole build off of that metric. I like to stay up late on my PC so 2400wh of battery is probably nowhere near enough for me. My dad on the other hand wont be using as much power as I will be. He wants to run two (energy efficient) AC units in the daytime but otherwise wont be using a whole lot of power. Hence the oversized panel array with the smaller battery bank. I'm thinking that for my own build I need to just save up and buy more battery. Maybe my dad just needs to accept a smaller array? Thanks for your reply!

Rednecktek said:

The buck transformer is for getting a lower voltage DC out of a higher DC source. In your case, if you needed to go to a 24v or 48v battery setup, you couldn't feed your 12v lights and appliances with that or you'd end up with greasy orange smoke/fire/angry wife.

The buck transformer would kick that 24v down to the 12v that your 12v appliances need. That buck transformer would go between your battery bank and your 12v fuse block.

I had to do the same thing for my camp since I kicked the main battery bank up to 24v but I do have appliances that need 12v so I needed a couple of buck transformers. Here's my wiring diagram so you can see what I'm doing there.

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Ahhh. I think I get it. So you would use the buck converter to control your dc-dc current and use your inverter to convert your DC-AC? I promise I tried to understand your wiring diagram but I still wonder how you connect your converter and your inverter to the same system. It looks like your converter is at the top left of the diagram, and it looks like you have your inverter's positive running straight to the converter and the negative running to the wire just below. Is that right? I really appreciate the help man. Sorry for all the questions.

Spoonathan said:

cost.

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Exactly. PV is cheaper than batteries.

Spoonathan said:

I like to stay up late on my PC so 2400wh of battery is probably nowhere near enough for me.

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Buy a different PC. PC is cheaper than batteries.

Use the power-hog bitcoin-mining CPU with nVidia V100 when the sun shines, ultralight laptop at night.

Spoonathan said:

Here's where I run into a problem. 1380w/12v=115a. This is far bigger than any SCC I can afford (or have found for that matter).

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Output amps is a soft limit, generally speaking, you can exceed it with the only downsides being that when your solar panels are operating at 100% of STC rating (which is rare, especially on an RV) you will not be getting 100%, and you will be running at closer to 100% of the charge controllers rating much of the time so ventilation/cooling may be more of a concern. 100A controllers are available.

However there are two other options. (1) up the battery bank voltage (2) use two or more controllers (this option has other advantages in a mobile context as well).

The most important limit you need to make sure you don't exceed is the controllers input voltage limit. The Voc of your panels after accounting for low temperature cannot exceed the controllers max input voltage. There is also a max input current for controllers, but from what I've heard this is much less critical (still should be followed).

Spoonathan said:

So is my only option at this point to increase the voltage of my system to 24v and buy a 60a charge controller?

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See discussion of options above.

Spoonathan said:

(1380wh/24v=57.5a would 60a be cutting it too close?) If this is the case, it kind of throws a monkey wrench in my plans.

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Thats actually pretty ideal (so long as the input limits of the controller are not exceeded).

Spoonathan said:

I was looking to build my own battery out of loose cells. However, all of the cells that Will recommended are 3.2v and usually come in packs of 4. This means that I would need to either find some 6v cells elsewhere or buy twice as much battery as I need. I feel like there's something I'm missing here. If anyone has any helpful advice on how to achieve my goal here I'd love to hear it. Thanks for your time!!

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All LFP cells are 3.2V, no such thing as 6V cells in the LFP world.
"12V" = 4 x 3.2V,
"24V" = 8 x 3.2V,
"48V" = 16 x 3.2V
As you can see from the math, the '12, 24, 48' values are nominal values, not actual values.

As to needing to buy "twice as much battery as you need" to go with 24V, I think you are misunderstanding something here, but I cant tell exactly what. If you want/need 2400Wh, 8 100Ah cells could accomplish that (8 x 3.2V x 100Ah = 2560Wh), and could be configured as either a 12V or 24V battery.

The relevant thing to pay attention to is Watt-Hours (or Kilowatt-Hours), especially when comparing between different voltages. New builders often focus in on Amp-Hours which is a useful unit in some contexts, and the most common unit in marketing materials and specs but also leads to a lot of new-builder confusion.