Lifepo4 batteries & Charging a 12 battery bank with a 24v array?

9+ Months--That is pretty much full time off grid.

Not freezing weather, then Li Ion batteries are a good fit (and/or keep them in a heated space in the RV--They do not like being sub freezing--Vs Lead Acid which can work at well below 0F most of the time--details matter).

Refrigeration Compressors--Whether for a refrigerator/freezer or an A/C system. Those are very difficult to run on solar, especially smaller solar power systems. You already have a fridge (propane?). And the A/C system?

For the A/C system, need a few more details, but unless it is an "inverter" based compressor (internal VFD--Variable Frequency Drive--Basically gives you soft start and the ability to throttle back the compressor RPM to a fraction of power vs the On/Off cycle of a standard compressor). Need to know the peak starting VA (Watts--sort of), and the average running Watts, and how many hours per day.

If you already have the A/C--Now would be a good time to put a Kill-a-Watt or similar Watt*Hour meter on the system and gather some data (along with your other AC loads). Without the information, it is difficult to make any realistic guesses about energy usage.

https://www.amazon.com/s?k=kill+a+watt+meter (examples of various brands/models of WH meters).

There are DC Amp*Hour/Watt*Hour meters too--If you have significant DC loads (more than LED lighting, range hood fan, cell phone 
And here we are. The ideal methodology is to pick and measure your loads (most energy efficient possible--Such as inverter powered Mini-Split A/C or LG inverter window unit--or similar inverter RV AC--Not my field of knowledge what is out there).

And since we can only guess at that for the moment. Other methods of design... Use our "rules of thumbs" to size components. For example, the size of the solar array (Watts, maximum roof area, or amount you want to install--Must be free of shade/exhaust stacks/AC unit shading, etc.). Array can be flat, or tiltable (the farther away from the equator and the deeper into winter, the more tilting can be helpful). Some folks will even do "solar awnings" for more power (fold against side of RV when moving).

Or the size of the battery bank (if you can only fit 6x 6 volt @ 200 AH golf cart batteries, that is all you can fit/carry). Or you have a heavy load (like A/C) that needs a larger AC inverter--And battery bank + solar array + genset to keep up with its usage....

I am a huge believer in designing a "balanced" system. A huge AC inverter on a smaller battery bank+solar array--You might be able to get ~1,800 Watts for 30 minutes--Then the battery goes dead. For most people that is a "fail", but for some specialized energy needs, perhaps that is OK (like a Mac Truck with a 1 gallon fuel tank--It does what it does).

And be careful of reviews... Use something like this website. For example, the Westinghouse iGen2500 Amazon reviews are "questionable":

https://www.fakespot.com/product/westinghouse-igen2500-portable-inverter-generator-2200-rated-watts-2500-peak-watts-gas-powered

For example, 4x 200 Watt Solar array for your RV (and yes, using an MPPT charge controller, you can take the high voltage array and efficiently downconvert to 12 or 24 VDC).

I will start a new post--Just to keep things "contained" to useful chunks.

And as an aside--I high suggest doing (probably a few different) paper designs first... There are "hidden gotchas" in solar/electrical system designs--And it is very easy to get good deals on hardware only to find that they do not "play well" together.

-Bill

Just to get started, choosing the battery bank. I will stay flooded cell lead acid batteries as the "base design" and use "conservative" design rules (as if this was for a full time off grid home). AGM, Li Ion batteries, etc. will "work better", but the basic math for a "general off grid system" is not much difference in design rules (at least for first pass back of the envelope).

For a battery bank, I highly suggest the following guidelines (based on many reasons):

• 12 volt battery bank: 1,200 to 1,800 Watts maximum
• 24 volt battery bank: 2,400 to 3,600 Watts maximum
• 48 volt battery bank: anything >2,400-3,600 Watts

Note that Power = Voltage * Current ... So you can have a 12 volt @ 600 AH battery bank (example 2x 6 volt @ 200 AH golf cart batteries in series * 2 parallel strings for a 12 volt @ 400 AH battery bank)

That same bank could be 4x golf cart batteries in series for a 24 volt @ 200 AH battery bank--Both banks are the same size/weight/number of batteries, and store the same amount of energy:

• 12 volts * 400 AH = 4,800 Watt*Hours of storage
• 24 volts * 200 AH = 4,800 Watt*Hours of storage

Of course, your voltage needs may define your bank (HAM radios, 12 VDC appliances, etc.).

Starting with an 800 Watt solar array, 5% rate of charge for part time (weekend/sunny weather/genset supported) battery bank. 10%-13%-20% rate of charge for full time off grid (minimize genset usage). Using the 20 Hour battery capacity number (i.e., 200 AH @ 20 hour rate = 10 amps * 20 hours discharging = 200 AH to "dead").

• 800 Watt array * 0.77 panel+controller deratings * 1/0.20 * 1/12 volt battery bank = 257 AH @ 12 volts minimum bank size
• 800 Watt array * 0.77 panel+controller deratings * 1/0.13 * 1/12 volt battery bank = 395 AH @ 12 volts "minimum genset" usage optimum bank
• 800 Watt array * 0.77 panel+controller deratings * 1/0.10 * 1/12 volt battery bank = 513 AH @ 12 volts nominal/optimal bank capacity
• 800 Watt array * 0.77 panel+controller deratings * 1/0.05 * 1/12 volt battery bank = 1,027 AH @ 12 volts maximum (weekend) bank

Note the above calculations--Within 10% of the above numbers are within the design/meters accuracy. I am showing 3-4 digits so you can follow my math rather than using "variables" everywhere--I try to be a "nice guy".

Lets say you pick 395 AH @ 12 volts for your battery bank... That array is 13% rate of charge and will charge your battery bank about as quick as it can, and minimize genset use (get up into Canada during winter, you will use genset or shore power--Solar will not harvest much in winter).

Lets use 2x 6 volt #@ 200 AH Golf cart batteries in series (12 volts) times 2 parallel strings (2x 200 AH) for a 12 volt @ 400 AH battery bank.

For a full time off grid home, I would be suggesting 2 days of storage (bad weather) and 50% maximum (for longer battery life)... For an RV, you may choose 1x day of storage--But have to use shore power/genset every other day or so to fully recharge bank (RV trade offs). Sticking with off grid home/conservative design, your typical suggested battery usage would be:

• 400 AH * 12 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 1,020 Watt*Hours per day (overnight, cloudy day, etc.).
  

Now, lets assume that you run your AC inverter 5 hours per day (evening), the energy usage would be:

• 1,020 WH per day / 5 hours per night usage = 204 Watt average load

So, I would be suggesting an ~300 Watt AC inverter like this one (really a neat inverter with remote on/off and "search mode", no fans, quiet):

https://www.solar-electric.com/morningstar-si-300-115v-ul-inverter.html

And for a 12 volt @ 400 AH battery flooded cell lead acid (golf cart battery) bank, the supported loads (continuous discharge) would be along the lines of:

• 400 AH * 12 volt * 0.85 AC inverter eff * 1/20 hour discharge = 204 Watt @ 120 VAC load (5 hours per night, 2x nights, 50% max discharge)
• 400 AH * 12 volt * 0.85 AC inverter eff * 1/8 hour discharge = 510 Watt AC load (for less than 4 hours to 50% discharge)
• 400 AH * 12 volt * 0.85 AC inverter eff * 1/5 hour discharge = 816 Watt AC load maximum continuous (for minutes to an hour or so)
  
• 400 AH * 12 volt * 0.85 AC inverter eff * 1/2.5 hour discharge =1,632 Watt AC load max surge (for a few seconds)
  

And then we figure out how many hours per day of sun your solar array will see. Assume mount flat to roof, no shade, and Aurora Colorado as an example of the process/math:
http://solarelectricityhandbook.com/solar-irradiance.html

Aurora
Average Solar Insolation figures

Measured in kWh/m2/day onto a horizontal surface:

Jan	Feb	Mar	Apr	May	Jun
2.41	3.27	4.49	5.42	6.28	6.70
Jul	Aug	Sep	Oct	Nov	Dec
6.35	5.68	5.03	3.90	2.67	2.18

Assume that 9 months of the year, are "non-winter" months and flat mounted solar panels, gives us February with 3.27 hours of sun per average day as our "break even month" (may or may not need the generator):

• 1,020 WH per day * 1/0.52 off grid AC system eff * 1/3.27 hour of sun (Feb) = 600 Watt "break even" Array

And the predicted generation for an 800 Watt array:

• 800 Watt array * 0.52 off grid system eff * 3.27 hours of sun (Feb long term average) = 1,360 Watt*Hours per "average" February day

And to give you an idea of energy usage for a "conservation minded" home:

• 500 WH per day = Cabin with LED lighting, cell phone charger
• 1,000 WH per day = Above cabin, add RV water pump, small fan, computer/TV, charging battery devices
• 3,300 WH per day = Above cabin + full size energy star rated refrigerator/freezer, clothes washer on sunny day, solar friendly well pump, Laptop for editing pictures/website/"job"/printer...
• 10,000 WH per day (or 300 kWH per month) = Average very conservation minded North American home with natural gas heating/cooking/hot water/etc.

Genset--Typically 10% to 20% rate of charge. Derate Genset to 80% of rating (charging large battery banks is hard on gensets and NEC wiring). Assume 67% efficient charge (both losses and Power Factor issues--another time):

• 10% of 400 AH battery bank is 20 Amp charging:
• 400 Amps * 14.75 volts charging * 0.10 rate of charge * 1/0.8 genset derating * 1/0.67 AC charger deratings = 1,101 VA (Watt) minimum rated genset suggested
• 20% of 400 AH battery bank is 40 Amp charging:
• 400 Amps * 14.75 volts charging * 0.20 rate of charge * 1/0.8 genset derating * 1/0.67 AC charger deratings = 2,202 VA (Watt) minimum rated genset suggested

As I said before, details matter. The above Genset (and rest of the stuff) is just a simple rule of thumb design using nominal off grid solar hardware (nothing special/exotic/that expensive--not to say solar is cheap).

And this why it is difficult to design a system without a load... Here we designed a system based on the solar array and "conservative" usage in a cabin/small home.

Or design a system to support your loads--Or design a system that "fits your space" and figure out what it can output. Which what this post is... Adjust the above based on your needs and bank account.

I am not right, or wrong. It is just a stick in the sand for continuing the conversation. All the above has not made any assumptions about any of the hardware just yet (golf cart batteries are cheap and easy to find--Other batteries may be better for your needs).

-Bill

You are very welcome. We aim to please.

As you can see, even a "small" A/C system that draws ~350 Watts ("inverter type", low power/speed), inside of 3 hours, the system would use up 1 day's worth of solar harvest. Tilting the array will help (depending on where you are at). A larger AC inverter (probably 1,200 Watts or larger) would be required... But large AC inverters also "waste" more energy.

There is enough math and links to sources that you could, for example, do your own AC daytime calculation for 3,000 WH, Aurora Co, with a tilted array in summer, and see the expected power generation:

Aurora
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 65° angle from vertical:
(Optimal summer settings)

Jan	Feb	Mar	Apr	May	Jun
3.72	4.43	5.39	5.78	6.24	6.47
Jul	Aug	Sep	Oct	Nov	Dec
6.21	5.87	5.79	5.17	3.98	3.53

Summer, >6.0 hours per day (array tilted to 65 degrees), want 3,000 WH per day for A/C:

• 3,000 WH per day * 1/0.52 off grid system eff * 1/6.0 hours per day = 962 Watt array "break even"

So, in this case, you can get 3,000 WH (average summer day) with a 962 Watt tilted array (25 degrees from horizontal). At the cost of a tilting array--But notice that tilting the array in summer is not much better that a flat array for harvesting summer sun (previous post). During summer, in Aurora Co, you can still average >6 hours per day sun.

Is 3,000 WH (3kWH) enough for your needs... I don't know. Getting measurements will help nail down the requirements.

Have fun,
-Bill