24 volt to 12 volt

Hermit1

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.

Estragon

Typically, you would use a 24v inverter to power AC loads, and a separate 24v->12v DC buck converter(s) for 12vdc loads.

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).

mike95490

You can use a 24V bank for the main inverter, and use a Morningstar MPPT controller as a step-down battery charger for your 12V house lighting.  Figure a size that can recharge the house bat in 12 hours, not sized to run your largest 12v load.
 Most of the MS controllers are unique in their ability too be powered by a battery to charge a lower voltage battery

BB.

As Estragon said... 60 amps * 12 volts = 720 Watts. No where near the 3,000 Watts you asked about.

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

Hermit1

WOW! Thanks guys! Give me a few days to absorb all that info, and I'll pick your brains some more! Thanks again!

Hermit1

 OK, more about my situation. I rent a 1/2 acre in Kansas from a farmer in the country, really an idyllic spot, shade, so minimal AC use, but 50-75' away a sunny spot for the panels. (I envision a wheeled racking system that could be swiveled w/ a riding mower in the event of hail or to mow) I will do little if any traveling but $75 a year for tags is all the tax I pay here. (retired) My Motor Home is 32', with lots of basement storage, 1 compartment amidships is 4'x6' and I would cut out a lid from the floor and put my battery bank (maintenance free type) there, with a grate over them to allow heating in winter.             My fridge, water heater stove and furnace are gas, so little power draw there (furnace fan mainly) My present DC converter (junk) is 45 amp, 745 watt. As I say, my AC use is minimal, I can run the generator for that, so upon further reflection, I think you're right, 3000 watt inverter is too big, but I would like to able to run coffee pot, microwave, TV and computer at one time if I want, so maybe 2000? 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?                                                   I am now thinking 5/250-300 watt panels for a total of 12-1500 watts of charge possible?                                                       I appreciate your advice very much, so please feel free to tell me if I am misguided in any way! Thanks again!                       

mcgivor

@Hermit1 said 
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.

Hermit1

What I was talking about was a 24 to 12 converter off the 24v bank. Is this not a good idea?

mcgivor

A buck converter is fine as long as its ability it support the intended loads is within its design capabilities.

Estragon

With your larger loads being fairly short duration, one way to handle them is to have a large inverter servicing just the larger loads which is otherwise turned off when the coffee's made and the toast is done.  Smaller, more constant loads like TV and computer would be run off a smaller inverter with lower self-consumption.

BB.

If you are planning on using AGM batteries--They will support higher discharge currents (C/2 or even more)... AGM tend to be more expensive and have a bit shorter life (it seems to me).

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
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 52° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.76	4.01	4.83	5.19	5.37	5.56
Jul	Aug	Sep	Oct	Nov	Dec
5.93	5.68	5.44	4.84	3.90	3.54

Say you want a minimum usage of genset, pick December as your "break even" month (need to run genset during bad weather):

• 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

Hermit1

Thanks for all the good info, Bill. Once again, it will take me a little while to get my mind around it all, but I get the basic gist of what you are saying. I'm sure I'll be back with more questions in a while, but the one that comes to mind immediately after reading your post is the idea of running the 120 volt line from the shed to the RV. Isn't that why you would run a 24 volt DC line from the panels as far as possible? Or would a 120 volt AC line not lose all that much power in say 60 feet? if that is the case I can see where it would be a lot easier to set it up that way. Thanks again.

Estragon

Basically, lower voltage needs higher current to power a given load, and higher current will have higher losses on a given sized wire.

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.

Hermit1

Thanks, Estragon. Got it. That 120 run is looking better to me. 

BB.

I think you understand how this all works... But to give you an example. Normally, we suggest a 1% to 3% voltage drop. For longer cable runs a 1% drop is usually too expensive (lots of heavy copper wire).

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&amperes=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&amperes=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

Hermit1

Bill, thanks so much for taking the time to explain this to me. Seems like the more I see, the more I have to learn. I'll definitely have to study this latest info for a while to understand some of the voltage terms, but I think I can grasp it given time. I'll check in again when I get a handle on it. I appreciate it.

PNW_Steve

I am going 24v on my system. 

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.