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

Rv_Adventure_Couple

So this is for our RV. My first question is can I buy the lifepo4 batteries as I can afford them and add them to the system along the way?

My second question is can I set my panels up in a 24v array and charge a 12v system?

List of components I currently have.

Currently I have 2 - 200watt panels

1 -  epever 150v 40amp mppt charge controller.  Here is a link to the CC

https://www.amazon.com/Controller-Negative-Grounded-Regulator-Flooded/dp/B077HQPV1Y/ref=sr_1_3?keywords=epever+150v+40a+mppt+solar+charge+controller&qid=1568843505&sr=8-3

1 - 3000 watt 12v dc  - 120v ac gowise pure sine wave inverter. Here is a link to the inverter

https://www.amazon.com/gp/product/B0131PZ9J2/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

Need to buy

charger / converter for lifepo4 batteries

lifepo4 batteries as I can afford them

shunts / wire / terminals

1 - 30 amp automatic xfer switch

2 - 200 watt panels same as above

BB.

Welcome to the forum RV folks.

How much time offering? Weekends, 9+months a year?

Where? Southwest? Canada? Subfreezing weather?

Genset use?

How much energy will you be using per day?

Just as a rough idea, if you have a 400 Watt array, a 400 Watt inverter + 400 ah @ 12 volt battery bank.

A 3000 Watt inverter would really "like" a 1,200 AH @ 12 volt lead acid battery back.

So.... Back to loads. Lots of conservation and very efficient loads.

The question about lithium batteries. They are very nice, light, more efficient at high charging and discharging currents. Don't like operating below freezing.

But they are not cheap.

Your thoughts and needs?

Bill

Rv_Adventure_Couple

How much time offering? Weekends, 9+months a year?  I would say about 9 months a year not quite full time yet.

Where? Southwest? Canada? Subfreezing weather? We try and stay out of freezing weather.

Genset use? We are going to get an inverter generator just havent done it yet, I want the i2500 westinghouse

How much energy will you be using per day? We don't use a ton, I want the ability to run our 12,500 btu ac set at like 80 degrees F for our animals while we go out and hike. So ideally it wouldn't kick on much, but we don't want them to get too hot.\

Your thoughts and needs? So in the spirit of not buying things twice, can I put the 4 200 watt panels in a 24v array to use the current charge controller at 800 watts and charge a 12v battery bank?

Also would I be able to add the lifepo4 batteries as I can afford them. Obviously we wouldn't be able to run the ac on 1 100ah battery but eventually as we add them we could.

BB.

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

Estragon

The short answer is yes, you can charge a 12v nominal bank with 24v nominal panels using an mppt controller.  And yes, you can probably add batteries (assuming you can afford them soon).

The "gotchas" are in the details though.  Can the controller be programmed to match the charging needs of the lifepo batteries under consideration?  Are there limits in the lifepo bms limiting parallel connections?

800w rated panels might produce 600w in many locations on a properly tilted rack for the full sun equivalent of 4-5hrs.  That gives you ~3000 watt-hours on a sunny day.  Enough to run a fridge and some lights etc, but likely not air conditioning for long.

BB.

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

Rv_Adventure_Couple

Wow, thank you for taking the time to type all that. I currently have the 2 panels up on the roof charging our house battery which is a walmart rv/ marine 114 ah deep cycle 12v battery. It has been sufficient enough in charging our laptops/ phones/ the small electrical draw of the propane fridge. Running the tv from time to time. We leave a small fan running when we leave currently for the animals and have planned around being in decent weather so its not too hot. I wouldnt expect the ac unit to run constantly but you are right with out the numbers we are just throwing around thoughts. Thanks for the food for thought.

BB.

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

mcgivor

Rv_Adventure_Couple said:

How much time offering? Weekends, 9+months a year?  I would say about 9 months a year not quite full time yet.

Where? Southwest? Canada? Subfreezing weather? We try and stay out of freezing weather.

Genset use? We are going to get an inverter generator just havent done it yet, I want the i2500 westinghouse

How much energy will you be using per day? We don't use a ton, I want the ability to run our 12,500 btu ac set at like 80 degrees F for our animals while we go out and hike. So ideally it wouldn't kick on much, but we don't want them to get too hot.\

Your thoughts and needs? So in the spirit of not buying things twice, can I put the 4 200 watt panels in a 24v array to use the current charge controller at 800 watts and charge a 12v battery bank?

Also would I be able to add the lifepo4 batteries as I can afford them. Obviously we wouldn't be able to run the ac on 1 100ah battery but eventually as we add them we could.

What voltage is the A/C unit? The12500 btu may be 240V and not compatible with the inverter linked.

The advantage to LFP batteries is energy density, LFP are~ 3.5 times that of lead acid, meaning a significant weight and space savings, especially helpful in a mobile application.

Lifespan also favors LFP, to approach their cycle expectancy with lead acid a significantly larger bank would be required to keep the discharge at 25% of capacity, meaning an addition in both mass and volume. Keeping in mind all additional mass will affect vehicle fuel consumption.

State of charge, lead acid needs to be kept at or close to full charge on a regular basis to avoid damaging effects of sulfation, LFP has no such issues, they can also accept higher charging current until fully charged, making generator charging way more efficient without an inefficient absorption cycle.

Initial costs may be higher but when broken down over time LFP is competitive, this however depends on what the initial costs are, some are way overpriced in my opinion, it pays to shop around.

Downside is low temperature tollerance, if temperatures can be kept above 0°C, 32°F this shouldn't be a problem, the optimal temperature is 30°C or 86°F, if in warm climates this could be another advantage as LA prefer 25°C or 77°F.

Having said all this, it is prudent to do load calculations first as real estate is limited in a mobile application, not only for batteries but also for PV, I would seriously consider a higher nominal voltage, 12V is so limiting, but that's just an opinion.