24 volt to 12 volt

I am a full-time RVer. I am planning a 24 volt solar system to minimize power loss. My question: Since my RV system is 12 volt, can I install a 24 to 12 60-amp step down converter between the battery bank and a 3000 watt inverter? Or are there any 24 volt inverters on the Market that have both 120 volt and 12 volt terminals on the output side? Thank you.
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Stepping down DC input voltage to run a large 12v inverter strikes me as unwise. Even running a 3000w inverter directly off a 12v bank isn't ideal IMO. At 12v nominal, 3kw could be ~250-300a (or more with surge loads), requiring a fairly large bank and heavy wiring.
If only ~700w (60a * 12v) is needed for AC loads, I'd consider a smaller inverter. Running a larger than needed inverter can waste a lot of power on self-consumption, particularly when used for small constant loads (eg phone chargers).
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
Most of the MS controllers are unique in their ability too be powered by a battery to charge a lower voltage battery
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A
solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,
For RVs, they generally do not support very large battery supported loads. For example, say you have 8x 6 volt @ 200 AH "golf cart" flooded cell lead acid batteries. That would be 4x series (24 volts) times 2x parallel strings for a 24 volt @ 400 AH battery bank... Pretty good size for an RV.
The average heavy load support would be C/8 (8 hour) discharge rate (C/5 would be the largest I would every suggest for a FLA battery bank).
- 24 volts * 400 AH * 1/8 hour discharge * 0.85 AC inverter eff = 1,020 Watt optimal "larger" AC inverter
And, such an inverter (and battery bank) would support about 2x rated capacity (~2,040 Watt surge load).And if you are going to use solar power to run dry camping/off grid... We suggest 5% to 13% rate of solar charging for the battery bank. And 10%+ for full time off grid operation:
- 400 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,506 Watt array "nominal"... That is a fairly big array for a smaller/standard size RV.
We really try for a "balanced system" design. Loads drive battery bank size. And loads+battery bank size drives your solar array (and other charging sources).For larger, continuous loads (such as an AC system or running a lot of heavy power tools), a genset is probably going to be more cost effective in the long term.
Run the solar/RV system overnight and non-work days. And for work days, run a genset during the day/when larger load support is needed.
-Bill
Also, for the 24 to 12 step-down, couldn't I have a separate cable from the bank for that with the output going to the 12 volt fuse panel and the 24 volt inverter hard-wired to the breaker box?
Whatever you do, DO NOT TAP 12V FROM A 24V BANK, the result will be rapid battery failure. Not sure if this was proposed, but it is a common mistake.that results in dissapointment.
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
With off grid solar power, you really need to conserve. It is sort of like having a 15 gallon gas tank, that you want to (for example) supply 2 days of fuel to 50% maximum discharge... For say 5 hours a night (7.5 gallons / 2 days = 3.75 gallons per day then divide by 5 hours per night, means 0.75 gallons per hour of average usage.
You can supply a 1,200 HP motor at 150 Gallons per hour for ~1/10th of an hour from a 15 gallon tank. It works, but usually not very useful unless that is your only load.
The microwave is possible--It is only ~1,000 Watts or so (if you get a smaller one). And if you only run it for 10 minutes a day--It is not a lot of energy.
Use a thermos to hold your coffee and boil the water on the stove, and you may get away with no electricity at all.
I would suggest starting with flooded cell batteries for your first bank or two--Most people "murder" their first bank of batteries on solar.
If the array is not going to be mounted on the vehicle--What about making a fixed solar system shed (or on a trailer, if living location is short term) and simply (for the time being) just use the RV power system when traveling (which you are not planning on doing a lot for now?). The size and weight of a "big&cheap" lead acid battery bank.
The details of your system design can be more flexible that way. Put the panel and shed close to each other and run your 120 VAC back to your RV spot.
Or, we need to choose an MPPT solar charge controller and array configuration that optimizes the Array to RV/charge controller/Battery Shed configuration. It is much easier to send higher voltage/lower current longer distances than low voltage/high current (from solar array) those 60+ feet.
Choice of 120 VAC (remote shed), or Vmp~100 Volts (150 VDC MPPT max input controller), or higher voltage MPPT controllers (the higher Vpanel input controller voltage, the much more expensive MPPT controller--There are models that will take Vmp-array~400 VDC as solar input--Great if you need to send the power longer distances from the array).
A 1,000 WH per day system is a "small". Good for LED lighting, RV water pump, running a Laptop computer, charging a cell phone, etc.
A 3,300 WH per day system is a "medium" size system that will give you a "near normal" electric lifestyle (using propane/etc. for cooking, hot water, heating). The above system plus a refrigerator, well pump, and clothes washer.
Say you don't want to use very much genset in the winter, a 3,300 WH per day system could look like this:
- 3,300 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge * 1/24 volts batt = 647 AH @ 24 volts
Basically, 4x 6 volt @ ~220 AH golf cart batteries in series times 3 parallel strings = 24 volts @ 660 AH bank (12 GC batteries total). If you use "golf cart batteries", plan on them lasting 3-5 years full time off grid use. Although, your first bank may only last a few months or a year if you are not "careful" (no over discharging, no deficit charging, getting >~90% state of charge at least once per week, keep batteries watered and monitor specific gravity once a month--more often SG monitoring as you first start).Then to charge the battery bank, 5% to 13% rate of charge (20 hour battery bank capacity). 10%+ highly recommended for full time off grid:
- 660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,243 Watt array minimum
- 660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,486 Watt array nominal
- 660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,231 Watt array "cost effective" maximum
And then sizing based on hours of sun per day (by season). A fixed array facing south:http://www.solarelectricityhandbook.com/solar-irradiance.html
Hutchinson Kansas
Measured in kWh/m2/day onto a solar panel set at a 52° angle from vertical:Average Solar Insolation figures
(For best year-round performance)
- 3,300 WH per day * 1/0.52 off grid AC system eff * 1/3.54 hours of sun per day = 1,793 Watt array minimum (Dec break even)
I would suggest an array from 1,793 Watts minimum. And optimally 2,486 to 3,231 Watt array (batteries are "expensive" these days and easy to kill--Extra solar panels makes batteries "happier" and you don't have to manage your loads on a daily basis (run the washer unless cloudy weather is forecast, etc.). As well as allowing you to use less genset time/fuel.The above is just a stick in the ground to set expectations. If you only need 1,500 WH per day, the above would be cut by ~1/2.
Anyway, the basics on designing a system. Once you have your energy needs and the basic system math penciled in, then start picking the hardware. Don't buy anything until you have done a few paper designs first.
-Bill
In this application, you can minimize wire losses (and/or avoid using larger wire) in one of two ways; you can run higher voltage dc (eg 100vdc) strings of panels over a long run to the controller, which would need a more costly mppt type controller, or wire panels in parallel to pwm controller and batteries located close to panels, and invert to higher voltage ac for the longer run to the trailer.
If the controller and batteries can be suitably housed near the panels, the second option may work out cheaper overall in terms of equipment, but at the cost of a suitable shelter and added complication if you want to move.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
Let's do a 2,486 Watt array. One at Vmp-array=36 volts (used with PWM charge controller on a 24 volt battery bank). And a 120 VAC run.
- 2,486 Watt array / 36 volts Vmp = 69 Amps Imp-array @ 36 Volts Vmp-array
- 2,486 Watt array / (3 panels * 36 volts) = 23 Amps Imp-array @ 108 volts Vmp-array (may be too high for some controller on a sub-zero F day)
Using a generic voltage drop calculator, enter 60 feet one way run, and adjust AWG until we get around 3% voltage drop:https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.4066&voltage=36&phase=dc&noofconductor=1&distance=60&distanceunit=feet&eres=69&x=59&y=13
1 AWG for Vmp~36 volt array and 3% drop maximum:
Voltage drop: 1.03
Voltage drop percentage: 2.85%
Voltage at the end: 34.97
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=108&phase=dc&noofconductor=1&distance=60&distanceunit=feet&eres=23&x=38&y=20
10 AWG for Vmp~108 volts and 3% drop:
Voltage drop: 2.76
Voltage drop percentage: 2.55%
Voltage at the end: 105.24
Every 3 AWG change in wire size is a 1/2 (or 2x) change in square inches of copper (copper face of wire in square inches/mm^2/etc.).
10-1 = 9 AWG steps or 3 factor of 1/2 = (1/2)^3 = 1/8th the amount of copper.
This is a "worse case" design comparison (worse PWM vs best 108 Vmp for MPPT controller).
120 VAC would be a little better (slightly less losses).
If you have 120/240 VAC split phase inverter and can run (for example) a 240 VAC will pump or spread your 120 VAC loads between L1 and L2 (with neutral/white lead)--You could go down 3 AWG to 14 AWG or so.
Many people like the batteries and charge controller next to their home... Saves trudging out in all weather to check the local equipment gauges. The more expensive MPPT controllers frequently have an Ethernet server, so that makes remote monitoring/control much easier.
Anyway--Why we ask the questions we ask.
-Bill
I have managed to find 24v solutions for most of my DC loads. I am using 12 volt LED lights and 12v fans installed in series pairs to run from 24v.
24v USB chargers are readily available. As are fresh water pump and hydronic circulating pump.
I will need a small 24-12 DC-DC converter for the Webasto.
Equipping for 24v house batteries has turned out easier than I had expected.