planning for the future

[cindynsg]

Your panels need to be able to charge and EQ your batteries. That means your PV amps, after the charge controller, need to be about 5-10% of your battery amp capacity.


Your Array of 2506w will only produce 2,000 W under normal conditions, and with a MPPT converter to battery charging volts of 14.5V, yields 140A. So you could go up to a battery bank of 1400AH.

2000 watts / 14.5 volts = 138 amps which would be 5.2 % of my battery bank amps. Is just getting by good enough? And Is 14.5 volts the lowest an MPPT charge controller will convert higher volts down to for higher amps?

If 5.2% is too close for comfort I could afford adding maybe another 3 or 4 hundred more watts of panels.

2900 watts x 80% = 2320 watts / 14.5 volts = 160 amps.
That would be 6.05 % of battery bank amps. If 5.2% isn't good enough maybe 6.05%?

The 20% discharge overnight, means that after 3 days no sun, you will be about 60% discharged, and while that's past the 50% point, it's occasional, and not regular.

I calculated for 2 days of no sun because that's really all I need. If needed I have a serious back generator, it's 35 years old but in mint condition, I'm not afraid to fire it up if I have to in order to avoid more than 30 % discharge.

So what do you think. Is that doable? I can always add more panels later when i can afford it.

Again thanks so much for your experiential advice.

[Mike90250]

14.5V is a charging voltage for deep cyle batteries. As the batteries charge, the amps will slowly taper off, till the voltage steps up to increase the charge. The float charge is a lower voltage, and supplies little power into the battery.

Here's some links about Battery Equalization. If you don't have enough amps, you cannot get the eq process to begin working,

http://www.windsun.com/Batteries/Battery_FAQ.htm

http://www.progressivedyn.com/battery_basics.html

http://www.xantrex.com/web/id/736/docserve.aspx

hope this helps

[cindynsg]

Thanks Mike, I appreciate it. I'm looking at the links now.

[Sunking]

2000 watts / 14.5 volts = 138 amps .

Does not work like that. Use the nominal battery voltage of 12 volts. 2000/12 = 167 amps.

[Mike90250]

Does not work like that. Use the nominal battery voltage of 12 volts. 2000/12 = 167 amps.

I've got to disagree - a 12V battery is charged by a higher voltage, generally 14.5V. Sure, if a battery's' been drained to near death, the charging voltage will be lower, but an even finer point here is, there are no 150+ Amp MPPT chargers, the largest are 80A that I know of. 60A is a common limit for a charger.

This is a reason many larger systems use 24 or 48V, at 12v, the currents are so large, they become impractical to deal with.

Mike

[cindynsg]

thanks mike that is a relief. just so you know, I would never doubt you.

[Sunking]

The actual charge rate is determined by your location. A C/10 is a good rule of thumb, but may not be enough in some locations, and overkill in others.

Here is a good example. Let

[cindynsg]

You do have a solar system right? Tell me what you've got. How long have you had it? How long has your battery bank lasted? What is your calculated D.o.D? How much experience do you have with solar? Pictures?

[Sunking]

You do have a solar system right?

Wrong wouldn't have one where I live. Electricity is cheap, no Net Metering laws, and no incentives.

Tell me what you've got.

I own my own architectural/engineering design firm. PE licensure, and 31 years of electrical generation, transmission, distribution, and battery plant behind me.

How long have you had it?

Think I covered that already

How long has your battery bank lasted?

Well I assume you mean ones used for cycle service like those used in RE applications right? Of the 156 stand alone projects I have designed built never seen a set last more than 5 years before they loose 30% of their capacity.

What is your calculated D.o.D?

Depends on the client and application. In the telcom sector at remote cell tower sites where power outages cannot be tolerated 10% DOD with back-up generators. Otherwise 20% is the default design.

How much experience do you have with solar?

Roughly about 6 years designing and building them mostly for telephone companies, and a few commercial projects like Walmart. Just finished up a design for a new Walmart in Plano TX with a 500 KW grid tied system.

[Sunking]

I've got to disagree - a 12V battery is charged by a higher voltage, generally 14.5V. Sure, if a battery's' been drained to near death, the charging voltage will be lower, but an even finer point here is, there are no 150+ Amp MPPT chargers, the largest are 80A that I know of. 60A is a common limit for a charger.

Well I was not referring to the charge cycle. If you are calculating for the charge amps from a standard 12 volt panel using shunt controllers you use the Vmp voltage from the panels which is usually 17 or 18 volts.

For MPPT the charge voltage does not approach 14.5 volts until the very end of the charge cycle after the current tapers off to a C.03 rate. For charge voltage 13 to 13.8 is default number used

This is a reason many larger systems use 24 or 48V, at 12v, the currents are so large, they become impractical to deal with.

I agree. But you would be surprized how many DIY's try it cascading controller outputs to get the high amperage.

[Jason]

Sunking is a very reputable guy in this industry and we are fortunate to have him here contributing to this board. Let's not run him off!

[cindynsg]

Ok, Just couldn't figure out what MSEE or PE stands for. You seem to know what your doing, just wondering what expierience. no harm no fowl meant. Just wondering.

[Sunking]

Sunking is a very reputable guy in this industry and we are fortunate to have him here contributing to this board. Let's not run him off!

You can't run me off, my skin is thick

[Sunking]

Just couldn't figure out what MSEE or PE stands for. You seem to know what your doing, just wondering what expierience. no harm no fowl meant. Just wondering.

No problem, you did not offend me
MSEE = Master of Science in Electrical Engineering. A graduate college degree.
PE = Professional Engineer.

[cindynsg]

.

So as you can see location means everything in terms of charge rate and more importantly system cost. The rule of thumb (C/10) here would have either fallen way short or way too much overkill.

You think C/10 is no good for me? My winter insolation rate is 2 - 3 minimum but is 3 - 4 average . I'm going for 2 days no sun maximum. If need be I've got a serious NO kidding generator if I get below 70% charged.