Over Paneling Questions

verdigo · March 2015

I have found a really good deal on some more PV and I need an additional 1200 watts to reach my inverter's upper limits. I am thinking of over paneling for those cloudy days but am concerned about exceeding the inverter's 3600 watts in the sell mode if the sun comes out. Will it limit its self and continue to sell or just shut down?
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BB. · March 2015

For a hybrid AC inverter--They operate like a "Dump Load"... The inverter attempts to keep the battery bank at a fixed voltage/state of charge (i.e., float voltage/near 100% full).

For a wind/water turbine, having not enough dump load, the "unregulated" charging sources can eventually over charge the battery and cause it to have a short life.

With solar power, the MPPT charge controller is usually set for a higher voltage (like absorb voltage) and running 100%. The hybrid inverter is dumping enough current to keep the battery bank at float.

The question will be--What happens when the hybrid inverter output is less than the available charging current. Will the MPPT controller over charge (i.e., hold the battery at absorb vs float voltage when the sun is up, or can you program the MPPT controller and the hybrid inverter to somehow "share" float voltage"... Honestly, I do not know.

If you have the current shunt (Whizbang Jr.)--One would hope that the Midnite MPPT charge controller can be programmed to not over charge the battery based on current flow--But I would ask Midnite directly if boB or Ryan do not reply here:

http://midnitesolar.com/Forum/

It gets a bit interesting... If you have two controllers trying to hold "float voltage"... How does the MPPT controller know when it is OK to supply more current vs reducing current because it exceeds the hybrid inverter's ability to regulate... You don't want the MPPT controller to produce less current than the Inverter's ability to dump power the grid.

-Bill

mike95490 · March 2015

Are you backfeeding the grid ? I'm not sure I understand BB's responses.

If you are straight off grid, the charge controller(s) should manage charging and loads just fine, as long as you don't withdraw more power than can be replaced. Overpaneling will not help much on cloudy days, until you get way up there, better to run the genset and/or reduce loads till the clouds clear.

The Classic's CAN be set to current limit their output, so a overpaneled array in full sun won't boil the batteries.

BB. · March 2015

Inverter in "sell mode"... At least the way I understood the question.

-Bill

verdigo · March 2015

Sorry for not being clear. The Classic is set to stop absorb by setting end amps but it will never reach absorb while the batteries are topped off because the inverter is set to sell a volt or two below float. A volt or two is usually a guessing game because the batteries are in a unheated shed, and sell voltage is not automatically temp compensated. So the Classic is in perpetual bulk and the inverter will sell everything above its sell set point. If I am over paneled what I want to know is "will the inverter shut off if the Classic has enough PV to exceed the inverter's 3600 watt limit?" Or will it limit itself on output and let the CC voltage possibly overcharge the batteries?

verdigo · March 2015

I guess I should add that I currently do not have enough PV up to overwhelm the inverters sell set point.

Photowhit · March 2015

You do understand to produce more than 3500 watts you would need close to 5000 watt array most days?

I don't think you have a problem, but If you're worried you might ask over at the Midnite Forum (They designed the CC and understand and may have had a hand in designing the inverter)

verdigo · March 2015

Yes most days but during some of these cold clear days of late I have seen production that prompted me to ask the question. So I should be good up to 5000 watts of PV?

BB. · March 2015

I would start to worry about exceeding the GVFX at ~3,600 Watts of panel on a cold/clear day.

I would suggest not going above ~4,640 Watts on a 464 AH @ 48 volt battery bank.

And if using the 0.77 panel+controller deratings:

3,600 watts / 0.77 = 4,675 Watt array (more likely to hit the MPPT > than Inverter crossover point (if no other loads).

-Bill

mike95490 · March 2015

What's the battery water usage? Cell plates exposed after a week, 3 weeks, 8 weeks ?

verdigo · March 2015

mike95490 wrote: »

What's the battery water usage? Cell plates exposed after a week, 3 weeks, 8 weeks ?

Not much water usage. The batteries are in eternal float or a couple of volts under when the inverter is selling. The inverter takes care of battery charging with a short absorb and then goes to float for 4 or five hours. Then the sun comes up and the inverter begins selling once the Classic pushes the voltage up to the inverter's programmed sell set point. The set point is a couple of volts below float voltage.

The Classic never gets out of bulk because the inverter's sell set point is much lower than the sell set point. So it sells everything it produces. Once a month I simulate a power outage by shutting off the grid connection and discharge the batteries. Then I re-connect the grid and let the inverter do a proper absorb charge for two hours. I then check SGs and sometimes do an EQ if the SGs are off much.

Long story short the batteries are not being baked because the inverter takes care of the batteries. The question concerns the inverter, and is (what happens if I over panel and the inverter reaches its 3600 watt capacity in sell mode and maybe exceeds it). Will it fault out and shut down or will it limit its self to the 3600 watts? There are no other loads unless the grid goes down and even then I have it set up to not use the inverter's internal transfer switch.

The only reason I ask this is because I am normally not home during week days, but I would like to sell power at or near capacity. I suppose I can go ahead and over panel and just shut down a string if the weather is forcast cold and clear. Summer months probably won't be an issue.

mtdoc · March 2015

Verdigo,

My system is similar to yours. I solved the issue by using the charging amp limits on my Classics.

verdigo · March 2015

I should have thought of that. Are you using a 10% charge rate?


mike95490 · March 2015

The charge controller is going to do whatever it can to perform a BULK, ABSORB and FLOAT charge to the batteries. If the inverter is siphioning some of the power, the CC keeps supplying whatever it can, to keep up. It's looking at battery voltage, and time, enough time at 14V and it starts the absorb timer, if voltage sags from clouds, the CC trys as hard as it can. When the clouds clear, and the voltage comes back up, the timer resumes. When Absorb is done, it goes to float, and it won't hurt the batteries even if it's putting 30 amps to them, and the inverter is selling 25 of them, the battery is still at Float volts.

mtdoc · March 2015

verdigo wrote: »

I should have thought of that. Are you using a 10% charge rate?
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No, I have AGMs which can tolerate a very high charge rate if needed. But that is not really relevant unless their is a grid outage and I am in "off grid mode".

Most of the time I am selling to the grid. My "sell RE" voltage is at or near my float voltage so the Classics stay in Bulk mode and everything I make is being sold with very little (or no) net charge into the batteries.

It is a bit complicated for me since I have 2 arrays and 2 classics. I think I have the combined output amps set to something like 75 amps which with a sell voltage of about 54V puts max output at about 4000 watts which in theory could still overwhelm my GVFX3648s ability to sell but in reality it would only happen if both arrays (with combined STC rating of 4500watts) where seeing cloud edge effects simultaneously. Also one of them is on a tracker so they are rarely at exactly the same orientation.

verdigo · March 2015

I am running my sell voltage a couple of volts below float with a little guess work since sell is not temp compensated while float moves around with changing temperatures. I suppose setting my Classic with a 50 amp limit will keep me at or about 13% and I can up my array up to 5000 watts

I have been considering buying another inverter and CC. I built my system and wired with expansion in mind. Hopefully I will be able to be patient and pick up some used or on sale equipment. Anyway thanks for the info.

BB. · March 2015

Don't drop the battery "float voltage" too much... You could end up micro cycling it as the AC inverter draws enough current to drop the voltage below ~50.8 volts (12.7 volts for a 12 volt battery) and start discharging/charging the battery a 120x a second (current draw from AC inverter).

-Bill

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