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  #1  
Old 09-30-2015, 04:12 PM
Marine Male Marine is offline
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Default voltage for a battery bank?

Hello to all, a FNG here. I have a simple question that I'm sure won't have a simple answer. I am going to be installing an approximate 7,000 watt system of approximately 1/2 solar and 1/2 wind. The details are not important at this time. My first question is about voltage of the battery bank. I realize that 24 is more efficient than 12 and 48 is more efficient than 24 and given that I will be targeting a net 5,000 watt output I'm wondering what volt system to use. Let's say I go with a 24v or 48v bank.
What do you do with all the 12v power loads out there? I mean you have 12v light strips; little battery re-chargers that run on 12v; RV appliances that are 12v; the list can go on and on. My Simple pump DC motor (will be) is 24v.
So, again, I go with an efficient 48v bank, which will be great to power the inverter but then I have a "bunch" of lesser voltage load requirements. What do you do??
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  #2  
Old 09-30-2015, 04:31 PM
wywhitewolf Male wywhitewolf is offline
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Use a DC-DC converter.

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  #3  
Old 09-30-2015, 09:21 PM
Marine Male Marine is offline
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I may have just disproved the theory that there are no stupid questions!
I didn't know there were "step down" DC to DC.

thanks
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  #4  
Old 10-01-2015, 10:46 AM
wywhitewolf Male wywhitewolf is offline
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Not a stupid question at all. Pay attention to the efficiency rating of the converter. Cheaper (less efficient) ones will waste more power than you use.

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  #5  
Old 10-03-2015, 01:45 PM
MichaelK Male MichaelK is offline
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Quote:
Originally Posted by Marine View Post
I am going to be installing an approximate 7,000 watt system of approximately 1/2 solar and 1/2 wind. The details are not important at this time.
Oh, yes, the details are extremely important at this time.

When you say 7000 watts, please specify where that is supposed to come from. You say half solar and half wind. I assume you mean you expect to have about 3500 watts of solar panels paired with a 3500 watt wind generator, is that right? At this point, if you are even mentioning 12V, you don't have the design concepts anywhere near reality.

Let's piece this apart, point by point. Let's assume that you hook up 3500 watts of solar, and try to run that through a 12V charge controller. Realistly, 3500 watts of panels will NOT produce 3500 watts of power. There are ALWAYS losses, do to the angle of the sun, the cleanness of the panels, the efficiency of the controller, the age of the panels, and the voltage drop through the wiring. A good expectation in the real world would be 80-85% of rated capacity.

So, on a clear, sunny day, with the panels directed EXACTLY at the sun, expect to produce about 2800 watts. Now, divide that by the charging voltage (~14V for a 12V system), and you get 200amps of current. Assuming you could even find a charge controller with that much capacity, you'd need thumb thickness of copper wire (read expensive) to carry that much current.

I'm in the process of designing my second generation system now, and I am planning on installing about 3600 watts. It will be charging a bank of 8 L-16 batteries wired in series for 48V. Three panels will be wired in series to form a 110VDC array, with 4 array's in parallel to produce about 30-32 amps at 110V. This will be funneled into a Midnight Solar 200 MPPT controller that will transform the 110V DC down to the charging current of the battery bank. This is right around the maximum that a 48V charge controller can handle.

Now, let's look at your wind charger. Is it rated at 3500 watts, or something else? You should know right off the bat that a wind charger rated at 3500 watts is NOT going to produce 3500 watts all the time. Depending on your conditions, it might never produce any meaningful power at all. That's because power production goes up with the cube of the windspeed, and most wind chargers are rated with a 25-28 mph wind. Below about 7 mph, it might not produce anything at all. At 10 mph, which is a strong breeze, you might get 350 watts. In my own location, I've never measured greater than 13 mph wind outside of a thunderstorm, and I'm on ridge top. If however, you are in an area with STEADY, CONSISTENT winds of at least 15 mph, you might make wind work for you.

Please keep asking questions. You need to get a lot more answers before you start wiring anything together. At this level of power I'd suggest completely forgetting about using anything 12V. You'll get the best results just inverting your power to 120VAC and powering standard American appliances that way.
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  #6  
Old 10-25-2015, 10:28 PM
JeepHammer Male JeepHammer is offline
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Quote:
Originally Posted by Marine View Post
Hello to all, a FNG here.
HELLO!
I spent 16 years as a self propelled sandbag myself.
FNG makes more sense to me than most!

Quote:
I have a simple question that I'm sure won't have a simple answer.

I am going to be installing an approximate 7,000 watt system of approximately 1/2 solar and 1/2 wind.

The details are not important at this time.
My first question is about voltage of the battery bank.

I realize that 24 is more efficient than 12,
And 48 is more efficient than 24,

And given that I will be targeting a net 5,000 watt output I'm wondering what volt system to use.

Let's say I go with a 24v or 48v bank.
What do you do with all the 12v power loads out there?

I mean you have 12v light strips; little battery re-chargers that run on 12v;
RV appliances that are 12v;
the list can go on and on.

My Simple pump DC motor (will be) is 24v.
So, again, I go with an efficient 48v bank, which will be great to power the inverter but then I have a "bunch" of lesser voltage load requirements. What do you do??
You are no different than ANYBODY that inadvertently creeped up (Low Crawled!) into self sufficiency...

I did EXACTLY the same thing, and my 12 volt crap is all on a shelf somewhere, replaced by more economical to run 120 VAC stuff as it needed replaced,
Or I got tired of screwing with it and beat it with a hammer...
(Yup, I still have a REAL low tolerance for quitting & B.S.!)

TWO STEPS HERE, so lets cover them...

---------------------------------

1. The Panels/Wind Generator produce in WATTS.
Watts are readily converted to either Amps AND/OR Volts.

The LOWER the Amp Draw, the higher the Voltage will be at the same given 'Watts',
The HIGHER the Amp Draw, the lower the Voltage will be.

Watts Amps = Volts.
Watts Volts = Amps.

Try it this way,
5,000 Watts 24 Volts = 208 Amps
5,000 Watts 48 Volts = 104 Amps
5,000 Watts 120 Volts = 41.6 Amps,
All the same wattage, all the same amount of power...

-----

A 5,000 Watt Inverter (output) will require...
5,000 Watts Output 24 Volt Battery String = 208 Amps to feed it.
208 Amp cables will have to be 00 Gauge (2/0 Ga.) MINIMUM, with no safety margin...

To do this 'Correctly', you add 25% to the output of the inverter,
This covers punk connections, inverter/terminal losses, ect.

5,000 Watt OUTPUT inverter will require...
5,000 Watts Output + 25% = 6250 Watts Capable.

6250 Watts 24 Volt Battery String = 260 Amps.
That's 000 Gauge (3/0 Ga.) Cables! That's a BIG HONKIN CABLE to supply that inverter properly!

-----

Feed that inverter 48 Volts,
Same 5,000 Watt Inverter OUTPUT, PLUS the 25% safety margin, 6250 Watts...

6250 Watts 48 Volts Input = 130 Amp Draw by the inverter,
That's a 2 Gauge Cable between batteries and to the inverter.
MUCH SMALLER CABLES!

------

The OUTPUT from the inverter is standard 120 VAC or if the inverter supports it, 240 VAC that every home uses.
No need for all that 12, 24, 48 volt DC stuff anymore.

------

NOW, when it comes to well pumps, the same principal applies...

Say your pump draws 1,500 Watts.
1,500 Watts 24 Volts (DC) = 62.5 Amps.
That's a 6 Gauge cable for both positive and negative.
That's your older vehicle battery cable size wire,
And in a deep well pump, that's a BUNCH OF MONEY...

1,500 Watts 120 Volts = 12.5 Amps.
That's common 14 Gauge household Romex 14/2 wire...
MUCH cheaper to wire AFTER the inverter in AC than before it in DC.

--------------

NOW!
Keep in mind that a '24 Volt' battery string DOES NOT have to be 24 volts.
If your battery string is dead on 24 volts, then you have very little reserve before voltage drops.

Most inverters will tolerate an extra 6 or 12 volts without issue,
Some are rated for 48 volts, but they REALLY don't care as long as voltage doesn't drop below 24 volts or so.

That means your '24 Volt' string can be 36 Volts, instead of 4 each, 6 Volt batteries in Series,
You simply add an extra two batteries, 36 Volts Nominal,
And your string suddenly gets a lot more reserve time.

If you have more than one battery string (Series)
Then Parallel them on the mains to the inverter,
You Double the Amp Hours available.
It's still 24 volts, just two strings of 24 volts parallel to the inverter, double the capacity provided to the inverter.

If you don't fully understand this, Series/Parallel, just ask, I have some visual aids that will help explain it.

--------------

Your charging systems, either solar PV or wind,
That output will be in WATTS.

You can split WATTS either way, as volts, or as amps.
Watts charge the batteries and power the inverter.

The easiest way to explain this is,
*IF* you have enough production (Watts) to handle both the inverter load,
AND charge the batteries...
You will see Voltage rise.

If your inverter sucks up all the Watts/Amps, line voltage will DROP.
This is how you know you aren't getting enough from the wind/solar to do BOTH jobs, inverter and charging...

This is the idea behind a CHARGE CONTROLLER,
The excess power, after the Inverter gets it's fill,
Hits the charge controllers and charges the batteries.

Batteries like a SLOW, LOW AMPERAGE charge, while if your inverter is 'Idle', not doing much, so it's not drawing much,
Your solar/wind can produce a lot of excess WATTS, which the batteries will draw as AMPS.
The charge controller keeps those batteries from getting BLASTED with too much amperage and overheating/damaging them.

A charge controller also keeps your batteries from HEATING UP THE PANELS OR WINDINGS IN THE WIND GENERATOR when they are NOT producing anything.
A little electrical gadget called a 'Diode' allows power to flow from your generators to the batteries, but not the other way around...

A charge controller for each battery string, same age, size, type of batteries in a string, so they can be charged together.
A charge controller does that.

If your second string of batteries are a different size, type, age, they will NOT charge at the same rate as the first string,
So you need a second charge controller for that string.

This allows you to rotate batteries in strings as they age and fail,
And allows you to introduce new battery strings as needed.
All you need is a simple charge controller for each string to take care of charging demands for each string.

When PROPERLY charged, the strings will last a LONG TIME!
Mine average about 10 years when properly cared for.

-------------------

2. The WIND generators often produce in AC power,
And that has to be 'Rectified' to DC so the inverter/batteries can use it.
All solar PV panels are DC output, so no rectification required,
But again, you will need a charge controller for each battery string.

I go one step further,
One PV panel string for each battery string, through a charge controller, excess shunted to the inverter.

PV panels are like batteries, ONE goes bad in a string, it cripples the entire string.
If I Series Connect all panels, just ONE cripples the entire panel array.

By Stringing the panels, connected to single battery strings,
If a panel or battery goes down, I don't loose the other strings of panels or batteries.

This also makes my system EXPANDABLE with very little effort.

If you are a (Inactive) Marine, you know the power of redundancy,
Always have a 'Plan B' to fall back on.

I'm currently running 6 panel & battery strings, with 3 Synchronized Inverters.
If I loose a panel/battery string, I only loose 1/6 of my total power capability.
If I loose one inverter, I'm down 1/3 on power.

Synchronized Inverters will support each other.
When the first inverter reaches near full capacity, the second comes on line,
IF I ever max out the second, the third comes on line.

As power consumption DECREASES, the inverters shut down (Sleep) in the same order.

One fails, the others take up the slack...

I need TWO sometimes, the third is a redundant backup, as far as I know, it's never activated other than testing...
I've been here 15 years, I've never had a power failure other than when I do maintenance, and with the system the way it is, I don't lose power even during maintenance cycles.

--------------------

As for your 12 volt, 24 volt & 48 volt equipment,
(Assuming 6 Volt batteries,)
You can tap your battery strings at the 12 volt (2 batteries) or 24 volt (4 batteries) in the strings.
The issue here is, the batteries will discharge & charge unevenly,
So you will have to rotate batteries in the strings so you don't hammer on the same batteries over and over again.

If you decide on a 24 volt string, and you want 12 volt power, you simply tap the batteries at the two battery marks,
A 24 volt string can be tapped, or charged as TWO 12 volt batteries,
Or even 4 ea. 6 volt batteries, and still draw off them as 24 volts.

This is DC, and DC seeks it's polar opposite,
Where ever you tap the POSITIVE, it's going to seek the NEGATIVE at the end of that battery string,
Doesn't matter if it's at the 6, 12, 18 or 24 volt mark...

I know that's REAL hard to grasp if your experience is in AC,
But DC is an ENTILREY different animal.
(which is why AC guys get all balled up when we discuss this stuff!)
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