Comments on my off grid solar

Started by RIjake, March 26, 2012, 06:54:22 PM

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alex trent

Can you point me to your source?  That is the buy of a lifetime.  I have a shipment coming down here soon and i'd Like to get some of these these on it.  Interstate is a great battery. The best i could do on them was 240AH, 6v for $150..so this is like a 50% savings.



NM_Shooter

I scanned this and did not see a description of MPPT... so here goes.

MPPT is a fancy version of PWM.  It basically has two or more power converters inside, plus smarts of some form such as a microcontroller.

Solar panels have complex output capabilities.  They are sometimes listed as a maximum open circuit voltage, and a maximum short circuit current.  Mostly, the manufacturers find the sweet spot of both voltage and current, under some theoretical illumination intensity from the sun, and call it XXX Watts. 

The voltage and current curves are dependent on one another.  Increase the current draw, and the voltage drops.  Need higher voltage and the available current is less.  They are dependent and non-linear.  Throw in angle and intensity of the sun, ambient temperature, age of the panel, and the sweet spot for maximum power out of the panel changes a lot. 

In simplest form, MPPT controllers vary the amount of current they draw, measure the voltage, and determine the power that they are harvesting.  Then they raise or lower the current draw to see if the power harvested is more or less.  They work through these iterations to regularly (or constantly, depending on the quality of the converter) to find the sweet spot where most power can be found. 

The converter is then tasked with maintaining this optimum load on the panel in order to get the most power, which is then re-regulated and delivered to your battery bank to charge it. 

The MPPT provides most benefit when the panel is operating at lower efficiency states... cloud cover, oblique angle of sunlight, whatever. 

It is a high tech way of squeezing more juice given any panel size. 
"Officium Vacuus Auctorita"

MountainDon

Good description.  FWIW and if I understand correctly, inside the controller the DC input is changed to a high frequency AC. The electronics then chnage the input AC into the best output AC and then it's changed back to DC to charge the batteries. Seems like a lot of changing going on and there is. The high frequencies used are very efficient.  Personally I am "sold" on the advantages of MPPT... we have many of the conditions that make it worthwhile.
Just because something has been done and has not failed, doesn't mean it is good design.

RIjake

#29
I got the batteries today.  Interstate GC2-XHD 6V   232A at the 20 Ah rating.  I'm thinking of going to go with 2/0 cables to tie the batteries together and to feed the inverter.  Thoughts?



MountainDon

What does the inverter mfg recommend for the distance you will run battery to inverter? Use that as a minimum, maybe a size larger of you are conservative.
Just because something has been done and has not failed, doesn't mean it is good design.

alex trent

Lots of different reccos.

I have looked at a lot and tried to distill from the wire guys and the others...including inverter mfgs.

For a 3,000 watt inverter most say 00 with less than 4 feet from batts to inverters.  For 4 to 6 feet  it is  generally 000, some say 0000. My inverter mfg. says 0 or better for less than 4 feet..

I guess I am going with 00.

I may derate with a fuse to 2000 watts on batteries, so I can run with 2 AWG, since my loads are so small.

Batteries are the same as inverter...but a lot say one size less.

If you are in a really safe enclousure that is OK, but then again the wire run in that case is so small.

RIjake

#32
Xantrex (inverter mfg.) says 0 cable max 6'.  I'll be under 6' no problem so I'm thinking up size to 2/0 and keep both the inverter cable and battery cables the same.

alex trent

Inverter max sustained watts is what?



MountainDon

Before you buy the wire ascertain how the wires connect to the inverter (or other equipment). Some use threaded lugs you attach cable lug ends to. Others have a connector the bare stripped cable end is inserted into and a set screw tightened. Some of these may be limited in the size of cable they accept.
Just because something has been done and has not failed, doesn't mean it is good design.

RIjake

Alex - 2000W

Don - Yup, I need uninsulated crimp on terminals.  Batteries are 3/8", Inverter is 5/16".  I've got a source on all those.

considerations

I went to a battery supply place (small, locally owned) and had them make my cables.  The connecting cables are very thick compared to normal battery cables but they put the right sized ends on to fit the same batteries you just purchased.  The sizes required were listed in the Xantrex manual, if I remember correctly. It is less than 4' from my battery bank to the inverter.

RIjake



Here's my latest design.  Comments?

alex trent

You need a fuse as close to the batteries as possible and between them and the inverter..certainly before any wires get to the vicinity of anything flammable.  Maybe it is there, cannot read everything all that well.

Ground?

Inverter.

Where does the genset and charger for it  fit in here...good to draw that in too, if for nothing else than to keep track of all the parts.


RIjake

Good points,
Yep, got the fuse, a 200A T fuse will be after the batteries.  The inverter is not on the plan but will be there.  I know I need a ground and was planning on it.  Where does that tie in?

alex trent

My ground is at my main breaker box..no solar on mine yet just batteries , inverter and a generator.  i think I really need a ground at my generator too.

The inverter is important to think about, since from batteries to inverter is where the most potential for trouble is...high amps and needs good heavy wire.  Likely 4/0 for your set up and close as you can get batteries to inverter.

MountainDon

In a 12 VDC system the NEC states you can dispense with a ground, if desired. However a ground wire is a good idea. There should be one bond between the ground wire and the negative wire just as in an AC system.

However, depending on the individual pieces of equipment being used you may find that one of them may have a bond built into it, between its negative and ground. That's common with inexpensive inverters, the type with receptacles built into them. If there is a second bond in the system that can create problems. I have personal experience with making such an error and releasing the magic smoke that made the inverter work. Get schematics for everything and determine if there are bonds in any equipment being used. Generators often do as well.



Just because something has been done and has not failed, doesn't mean it is good design.

RIjake

Quote from: alextrent on April 24, 2012, 09:36:13 PM
My ground is at my main breaker box..no solar on mine yet just batteries , inverter and a generator.  i think I really need a ground at my generator too.

The inverter is important to think about, since from batteries to inverter is where the most potential for trouble is...high amps and needs good heavy wire.  Likely 4/0 for your set up and close as you can get batteries to inverter.

Alex a previous post I mentioned the inverter manual says 0 wire max 6'.  I'm at about 4' and have 2/0.