OFF GRID POWER; various thoughts on...

[MountainDon]

I spoke with an Outback Power Systems engineer recently. I had some specific questions about one of their inverters. He answered them quite clearly. I've always thought their products to be top line, if not the best. Anyhow he confirmed my main concern about the inverter I had a chance to buy on a deal. He also clarified one point that I was also wondering about.

The question relates to inverter/chargers, like Outback, Magnum Energy, Trace/Xantrex. Plain vanilla inverters, like most square wave inverters, are just inverters. They do not have battery charging circuitry incorporated within them. You need a separate charger to recharge the batteries from a generator or other AC power source when you have an economy inverter. Inverter/chargers have a battery charger incorporated into the unit. In the case of my Outback VFX3524M the charger has a maximum charge rate of 85 amps at 24 VDC.

These inverter/chargers all are setup so when the connected generator is fired up, the AC power to the inverter causes the inverter section to switch out. The AC power to the cabin then is coming from the generator. The generator power is also powering the battery charger section. I have wondered why this was? Why was the generator power not just used to power the battery charger section, while any AC loads would still be supplied by the inverter section?    

Since I have a small generator this was of concern. By past experience I know that when my 2800 watt generator (derated by altitude to about 2300 watts) is working hard enough to power the old 75 amp Iota charger, it is not a good move to use the microwave at the same time. There's just not enough power available from the generator. Buying a larger generator would be a solution, but that's not a part of this scenario. Ideally I would like to be able to use the generator power to charge the batteries and leave the inverter to supply any cabin electrical needs.

Outbacks engineer supplied the answer. Keep in mind I have a planned 24 VDC system. The Outback 3524 inverter is designed to operate with an input voltage maximum of 32 volts. There are similar limits with other brands. Some are even less, like 30 volts maximum.

Batteries should be subjected to an 'equalization charge' about once a month to assist in achieving maximum battery life. An equalization charge is a higher than usual voltage, low current, charge rate. An equalization charge is done at 2.57 to 2.68 volts per cell (in a wet lead acid cell battery). In a 24 VDC system that is 31 to 32 volts.

Do you see the issue? The voltages are getting too close for the comfort of an engineer. In a real world situation it might happen that the charging voltage could spike higher than 32 and thus it might cause damage to the inverter/charger. Hence, the inverter manufacturer usually elects to program the inverter to drop out when the AC power in, is active.

I am not certain, but I do believe I killed my Exeltech 1100 watt inverter when I was equalizing the batteries. The Exeltech does not have a charger built in. I left it connected to the batteries. At the same time I bumped my stand alone charger into high voltage mode. It might have exceeded the design input voltage for the inverter. Poooof!    Or maybe it was 'just one of those things'. 

Anyhow, I thought it was interesting.

I think this would be a good place to talk about good things, bad things, odd things, that we have encountered in alternative power systems.

[glenn kangiser]

My Trace inverters go off at about 21 volts low or  30 volts max. I have two 4024 Trace sine wave inverters stacked to make 240v.  They are also over-current protected internally so that any short will trip them out and a manual reset is required if a short happens.

I know as I sometimes equalize and bulk charge with my DC welder.  I doubt I am treating my batteries as well as possible but when doing it I monitor them pretty closely.  I generally charge at between 60 to 100 amps but the welder does not taper off on charge as the batteries get full therefore monitoring is necessary.  This is across 12 375ah batteries or 1125ah at 24v.

[Okie_Bob]

Excellent information guys and maybe Glenn, you could start up a separate section on off grid power? I love this idea.
Just some FYI, I had some free time between Christmas and New Years and decided to take a little road trip, just drive alone for hours and see some country to try to get rid of all the crap in my brain from the past year. Anyway, as I traveled west on old Route 66 as I was coming into Tucumcari, NM, I saw this huge wind turbin and as I got closer I could see that I could get right up to the base so I did. This thing was huge, I'm guessing 20' or more at the base and at least 150' tall with blades that had to be 100' long.
I took a bunch of pics if anyone is interested. On my way home a few days later, I was a billboard on I-40 eastbound that said it was the largest wind turbin in the world!!! Suspect that maybe it was when installed but, maybe not anymore. It is 1.6Megawatts!
I had heard that these big turbins were quite noisy and that is way some people object to them. Well, let me tell you this monster was so quiet I couldn't hear it running at all.
I was also amazed at the 100's of wind turbins I saw in Oklahoma, Tx, NM and even AZ. Someone sure believes in them!
PS: I went all the way to the Grand Canyon. Highly recommend the trip on the old Mother Road.
Okie Bob

[glenn kangiser]

Lets just go ahead and add our thought's , pix and everything to this thread, BoB.    It will be easy to find here and this thread can expand endlessly on off grid power.

[glenn kangiser]

Getting ready to change my solar tracker cylinder to a larger one so I have to rotate the panels to the sun in the morning after which it tracks all day.

Readings .... set 1 in light but 160 degrees away from the sun at 8 AM(back of tracker toward the sun) -  1 amp  immediately change to 16 amps when toward the sun.

set 2  .5 amps away     10 amps when toward the sun.

[soomb]

So happy to see this thread start up.

[MountainDon]

My planned solar PV installation is probably fairly unique. I say that because the favored cabin spot is not at all close to the favored PV panel spot. Three hundred and twenty five feet separate the two.   

Running low voltages like 12 or even 24 volts over a distance like that would suffer huge power losses to electrical resistance unless the cables were huge. Huge copper cable costs a huge amount of money. What to do? Run the panels in series to up the voltage and keep the amps low. The problem with this is that many charge controllers cannot handle high input voltages. Some of the popular charge controllers only accept up to 24 VDC, some 48 VDC. The three panels I'm buying put out about 86 VDC maximum power voltage when connected in series. With the 7.3 amp maximum power output I can get away with using 4 gauge copper wire from the panels to the cabin and charge controller and have less than 2% voltage drop. That's pretty good.

The catch is most charge controllers can not handle that voltage. The Outback MX-60 or the newer replacement FlexMAX 60 can handle it. So even though the FlexMAX60 will cost about $500, I'll save a bundle on wire costs.

Some of you may have noted I seem to be big on Outback. I don't work for them and none of my relatives do. They make an excellent product, that's all.

The Outback charge controller can also be programmed for just about any voltage combination you could come up with for both input and output (battery bank) voltages.

[glenn kangiser]

More trouble for monitoring etc, but you could build a battery house at the panels and just send 120 or 240 underground to the cabin, but I'm sure you considered that already.  Thought I'd bring it up for pro and con discussion.

[MountainDon]

Some other things to be aware of when choosing PV panels and charge controllers....

PV panels get hot with the sun shining on them and they are producing power.

PV panel output voltage drops as the panel heats up. A panel laid flat or nearly flat on a roof will run at a higher temperature than a panel mounted so air can freely move around it, like on a pole top mount.

Conversely, as a panel is cooled, as on a -20 degree day, it's output voltage rises. Cold weather performance is enhanced. This is good as cold weather usually means shorter days, fewer hours of sunshine. That won;t make up for the greater number of hours os summer sunshine, but it helps.

However, along with cold weather and the higher output voltages comes a danger. All electronics have a design range for input voltages; a lower limit and a higher limit. Drop below the lower limit and the unit may just stop functioning until the voltage rises. Exceed the upper limit and the unit may stop functioning permanently. Toast.

What this means is that on a frosty January morning as the temperature hits its low point and the sun peeks over the hill and strikes the PV panels, the voltage may spike well over the normal operating range. With panels in series this could easily overrun the maximum voltage of the charge controller. Therefore this low temperature voltage condition must be taken into account when designing a system where low temperatures may be encountered.

There is a specification called 'open circuit voltage' or Voc listed for all PV panels. This is supposed to be the maximum output voltage but cold temperatures can cause this to be exceeded. A general rule of thumb is to take that figure and multiply it by 1.3 to obtain the maximum likely voltage output and select the charge controller, or the PV panel strings with that figure in mind. Destruction of the charge controller is possible, maybe even likely if that voltage maximum is exceeded for even a split second.

My location in the Jemez has reached a lowest temperature of -32 degrees in the past ten years according to the hour by hour weather records I researched. Running the numbers on my prospective setup indicates I should not have any cold weather over voltage problems. Hurray!

There is a 'tool' on the Outback website that can assist in checking whether or not a particular arrangement of panels can be safely used with Outback charge controllers. Go to   http://outbackpower.com/resources/string_sizing_tool/.   The tool allows you to choose your panels from a list or to enter custom data in case you panel is not listed. You select battery bank voltage and the maximum and minimum temperatures for the location. The graph chart then indicates whats okay and what is not.

[MountainDon]

More trouble for monitoring etc, but you could build a battery house at the panels and just send 120 or 240 underground to the cabin, but I'm sure you considered that already.  Thought I'd bring it up for pro and con discussion.

Excellent point. 

I ran the numbers for 120 VAC. I didn't run 240 VAC because my installation does not need 240 VAC (and two inverters stacked).

I found there was little difference. By inverting the DC panel power to 120 VAC at the panels I could have changed the wire size to 6 AWG, instead of 4 AWG.

However, that would have meant locating the batteries down at the panels, or very near them. Somehow I prefer to have the batteries up at the cabin. It would have also entailed running some cat5 cable from the batteries to the cabin, where the remote control panel would be, to allow remote control of the inverter; on-off functions and programming of the charge controller from the Outback Mate2.

It didn't seem to be a great net gain to me, in this particular situation. However, this is something that should be considered for any installation where the PV panels are not located right nearby the point of use.

Another factor in deciding against placing the batteries and inverter down at the batteries, and running 120 VAC up to the cabin, was the placement of the generator. The generator would be used about once a month to equalize the batteries. If the batteries and inverter were down the hill at the solar PV and battery location it would make sense to place the generator down there as well. If I didn't do that I'd be faced with running 325 feet of proper sized wire from cabin to panel/inverter/battery location. I didn't really want the genset down the hill. I'd rather have it up near the shed or cabin. Maybe that's just my personal quirks showing.   

So there's another chapter in the mysterious workings of my brain. 

[glenn kangiser]

I like the stuff nearby also.  You just don't know what those little machines are up to when they are down there by themselves.

I wanted this information on my solar tracker on the Underground Cabin Update but I will link it here for those who don't get over there.  http://countryplans.com/smf/index.php?topic=151.msg78994#msg78994  Info and pix on the previous page also.  Will update pix soon.  I hope...

[MountainDon]

Plus like right now, according to my spies in them thar mountains  there's three feet of snow.   

With my planned installation of the batteries and related peripherals at the cabin it is much handier. More so since it didn't really make all that much difference on wire size.

[glenn kangiser]

The cost of the larger wire would likely have been exceeded by the building materials for a secure battery house.

[NM_Shooter]

Conversely, as a panel is cooled, as on a -20 degree day, it's output voltage rises. Cold weather performance is enhanced. This is good as cold weather usually means shorter days, fewer hours of sunshine. That won;t make up for the greater number of hours os summer sunshine, but it helps.

However, along with cold weather and the higher output voltages comes a danger. All electronics have a design range for input voltages; a lower limit and a higher limit. Drop below the lower limit and the unit may just stop functioning until the voltage rises. Exceed the upper limit and the unit may stop functioning permanently. Toast.

1.3 should be plenty of safety factor.  Totally unloaded, I think that the theoretical maximum for a PV cell is .7V and it goes down from there with temperature.  The other good thing is that as soon as you apply any load to the cell, the voltage drops dramatically.  That's why you need a much higher PV voltage to charge batteries... 18 to 28V for a 12V battery. 

Don, I'd be surprised if the nice inverters don't have overvoltage input protection, and overcurrent output protection.  Anyway... 1.3xpanel voltage seems like a good rule of thumb.

-f-

[MountainDon]

First, I must correct an error. I realized as my brain woke up this morning, that I posted some slightly incorrect info in the post regarding where to place the inverter in my situation and the wire sizes.

I had stated that for running AC from the panel/batteries/inverter location down the hill the wire size would have been 6 AWG. That is wrong. 6 AWG is the maximum size wire the VFX3524 inverter will accept on the AC connections.

When I originally ran the numbers the wire size fir the DC or AC run up the hill turned out to be the same; 4 AWG. However that was with a smaller output inverter. With the VFX3524 capable of putting out 3500 watts the actual computed wire size would be 3 AWG, which is a bit larger. That's calculated at full capacity which it is true, would likely never be reached.   

My apologies for any confusion. 

[MountainDon]

You are right on Frank, with a couple of footnotes.

It depends in part how close to the input limit your setup is running during normal operation. Many installations would never have a problem. In my case I'm getting close to pushing the limits with the panels I want, but according to Outback still within bounds.

During those periods of early morning very cold weather times there may not be enough of a load to help bring the voltage spike down. The charge controllers do not act instantly when they receive the wakeup call from the power coming from the PV panels. That is, they receive the power, wakeup, scan the batteries to see what they need, then turn the input into output. Enough of a delay to potentially cause problems. The 'nice' ones go to sleep during the hours of darkness. That could allow plenty of time for a spike to zap something. At least that's what the Outback tech guys have told me.

Although I don't have any figures on this, altitude can play a factor in the output power of a PV panel. At higher altitudes there is less air between the panel and the sun. This could have a small effect on over volting.   

There is a device available that can help with cold morning start up over volt protection for panels connected in series. The Apollo Low Temperature Overvoltage Bypass will cut one panel in the string out of the circuit at either 32 or 50 degrees F (panel temperature). It automatically reconnects when the temperature rises. More $$ to spend though.

[MountainDon]

This topic is not meant to be an organized coherent lecture series; it's just like any other topic here. Everyone can pitch in, query and discuss. Much of what may be presented may be slightly opinionated, but I try to keep it factual. I will be trying to keep anything I present within NEC guidelines. If you are off in the boonies or simply don't feel the need to "toe the line" by all means follow your own drummer.


Wiring, that is wiring in the DC areas of an alternative power system, is an area where many folks make errors. One of the first and most common errors is in the colors of wires selected for the DC side. The confusion, or errors, come from our familiarity with DC automotive wiring. When looking at the automobile battery the positive or hot lead is usually red and the negative, or ground lead is black. Using red and black like this would cause your system to fail an electrical inspection.

The NEC makes no distinction between AC and DC wiring as far as color codes are concerned. The neutral or negative wire is permitted to be white or grey. That's all. No black neutrals or negatives. Black is reserved for a hot or positive wire. Red may also be a hot or positive wire.

Also automotive battery cables do not have the necessary UL approval and are not approved by the NEC for any wiring, including battery hookups. Ditto for welding cables;not approved. This may only be important to you if your installation has to be inspected by an inspector who follows the book.

More on wires for specific tasks will come later in another post.

Fuses and Circuit Breakers and Switches may be listed for only AC use, some can be used with both. This is because AC power and DC power react differently. The AC power we use changes direction 60 times per second (60 Hz). This change of state makes it easier to break the circuit cleanly. One thing that distinguishes a DC switch are the much heavier contacts. Using an ordinary home wall switch (AC only) to operate a DC powered device will lead to switch failure. The contacts may weld themselves shut in a worst case scenario.

For DC circuit protection there are acceptable fuses and breakers. Square D makes a line of household circuit breakers that are approved for use in circuits up to 24 VDC. The actual breaker info says it can handle up to 48 VDC, but the NEC limits it to 24 VDC as under some conditions a 48 VDC system may go over the 48 volt limit. So the Square D  QO  series panels and breakers are ideal for many DC circuits that may be used in cabin off grid systems. Don't try to mix AC and DC in the same panels though.

Inverters should be wired with a disconnect and a fuse or circuit breaker in the DC supply line. The wiring and fuse or breaker must be sized according to the inverter requirements. If you use a circuit breaker as protection they will usually be accepted as a suitable disconnect as well. I have found solarseller.com to be a good source for many off grid power items, including high amp capacity breakers.

Proper boxes and panels for enclosing these breakers may need some fabrication. Whether or not they meet NEC approval is another matter.  UL approved enclosures are expensive. Let your conscience and the inspector be your guide.

Gotta go for now

[JRR]

Okie Bob,
We also rode a bit of Route 66 last Feb.  My first drive out in that area.  My first look at American windfarms.  Quite impressive ... as was the whole 9k mile driving vacation. 

I wonder how many windfarms that we saw were owned/intangled by Enron?  I know they owned a number of them.

[MountainDon]

I had a question under my cabin topic that I thought I would answer here.

I had heard that you wanted to keep an even number of panels.  Is that incorrect? 



Yes, I am using an odd number of panels. Panels are used in even numbers quite often. There is no real reason to do that though. Panel mounts do very often come in 2x2 or 2x3 configurations. That could be one reason. The number of panels should be driven by the total number of watts that are required from the panels.

In my case I decided that I wanted about 500 watts total. More would be alright too. In a location where the cabin and panels are close together there are many configurations that could supply that easily. My 325 foot distance renders some solutions less practical.

For example many panels are basically putting out 12 VDC. Many folks here with small systems have based their systems on such panels and use 12 VDC battery banks. They may have several batteries in parallel connections to maintain that basic 12 VDC configuration.  Right now I won't bother with getting into the pros and cons. Rather I want to explain the reasoning behind my choice of three panels, an odd number.

Panels come in many different base voltages; 12 VDC, 16 VDC, 20 VDC, 24 VDC and even higher like 50 VDC. Some panel combinations would have required both series and parallel connecting. Some of those would have resulted in 48 VDC at 12 amps or more. Raising the amps transmitted to to that level resulted in wires that would have to be larger.


The three Sharp 208 watt panels in series have a Vmp (maximum power voltage) of 85.5 volts. Each panel puts out an Imp (maximum power amperage) of 7.63 amps. Connected in series the amperage (Imp) stays at 7.63. Keeping the amps low and bringing the volts high results in being able to use a smaller wire.

Using as small a wire gauge as I could was important to my situation where the panels and batteries are separated by 325 feet. At maximum output  I will have less than a 2% voltage drop which is what I was aiming for. If I parallel connected the panels the wire would need to be much larger to avoid excessive power loss. It would have been very expensive as well.. As it is I have $450 worth of wire and I got a good price on it.

Series stringing like that is only possible with a charge controller like the Outback MX or FlexMATE 60 or 80. They allow virtually infinite voltages up to their maximum of 145/150 and also work with virtually any battery bank voltage. They are extremely versatile. There are some other charge controllers that could be used with higher voltage arrays. One other reason I'm going with Outback equipment is all their components can be interconnected to one central indoor remote control for programming and monitoring.

I hope that is clear.

[MaineRhino]

Great post Mtn.Don! One I will be sure to refer to many times in my quest for solar power.  Has anyone mentioned Home Power magazine? I recently subscribed to it, and it seems to fit the bill for the DIY'er, with lots of buyers guides.

[soomb]

I can't help but feel overwhelmed by the calculations that go into solar.  I work with Ohms law now and then at work, but solar gets me worried every time.  Is there any books or links you would suggest to combat fear with education?  Thanks

[MountainDon]

Most of what I've learned about solar PV has been gleaned from a variety of sources. That and simple application of what electrical/electronic knowledge I already had.

The following websites do have information, sometimes you have to search and read through the entire site. They also sell a variety of solar equipment.

I have purchased from the first four businesses listed and have no fault with any of them.

Affordable Solar, Albuquerque, NM

TheSolar.Biz, in SW NM

solarseller.com    Long Beach, CA

Northern Arizona Wind & Sun
Northern AZ also has a pretty good info section.


Backwoods Solar  I've not bought from them, but there's useful information.

[soomb]

Don,
Thanks for the links.  What have you read about using microwaves and other such appliances with such a heavy draw at start up?  Is there anything special that need to be taken into consideration in your opinion.

Thanks,

Craig

[glenn kangiser]

Depends on the size of your inverter and additional loads, soomb. 

I can run my 5 horsepower rock crusher motor on mine but it likely is only drawing about 2 or 3 horsepower most of the time actual load.  Forgot --- I'm actually running it on 240v or two 4000 watt inverters. 

Mine and others have no problem.  I don't know that microwaves have such a surge as just plain heavy current draw, but since it is usually for a short time, they can work out fine.  I'd say a 2000 watt inverter with some surge capability would handle it though.

[soomb]

Glenn,

Thanks for the input.  I need to sit down and get to reading.  I want to be able to prove to my wife that when we build a cabin she will not be sitting in a cold corner reading by oil lamp light (I like the rustic idea, but I like having my wife around more).

Thanks again.