Sizing a MPPT charge controller - upgrading from PWM

wiseacre

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I feel like I'm on the verge of understanding but I still don't quite get how to size a MPPT charge controller.
While my solar panels will remain the same I will be adding more.
I will be upgrading my (2) Gel 12v, 100Ah batteries in series (24v) to a LiFePO4 24v, 200Ah battery (EG4 formally known as Gyll
The battery bank will be added to as my budget allows

In total my array will become over time 12 panels (8 now) ~ 195w panels
solar panel specs.jpg

Panels are paired in series going to a combiner box - ? 2s4p ? (future array 2s6p)
I would like to keep that configuration because I've built an adjustable frame for tilt with each series pair as a unit. It's possible to reconfigure the panels as long as I watch out for the cables as I adjust each pair.

Off hand I calculate each panel's volts as 25v to compensate for -40 winter temps.
12 panels:
x 25v = 300v -(this seems high so I figure I'm wrong here)
x 195w = 2340w -(I've seen watt limits on chargers and that confuses me too, I thought the main consideration was the panels' voltage going in)

Now what to do with those numbers? or am I on the wrong tract entirely?
Using the combiner box is what really confuses me. I love the fuses on each string and the breaker for the output (plus the other doohickeys in the box)
I kind of know what's going in but what comes out is beyond me at the moment.

My rough figure is each string 2s = appx 50v, 10a
I currently have 4 strings (with room for 6 total I plan on taking advantage of)

Any thoughts, comments and explanations are welcome. Who knows what tidbit of info will click in my brain

Nurse! Nurse! My brain hurts!
 

brewmatic

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MisterSandals

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Off hand I calculate each panel's volts as 25v to compensate for -40 winter temps.
The panel Voc thermal coefficient is .38%
Which means for ever degree C it drops below 25C the voltage will rise .38%
-40C is 65 degrees C below rated Voc

65 x .0038 x 21.6Voc = 5.33V
So at -40C, the temp compensated voltage is 21.6 + 5.33V = 26.9Voc
 

wiseacre

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So for both 2s2p and 2s3p you need a controller with at least 50V input voltage. For 2s3p you will have up to 30Amp.
Thanks I think you jumpstarted my brain.

I think I get the 50v input.
2 panels in series adds to 50v input (period) - since each string at 50v is being paralleled in the combiner box (correct?) the voltage will remain at 50v no matter how many strings I parallel.

with the same understanding of parallel the amps of each string will add.
1p -10a
2p- 20a
3p - 30a
until I get to 6 (2s6p) = 60

from those numbers I calculate a 100v, 60a SCC would be sufficient. At 50v input it cuts too close, 75vgoing to input would actually be sufficient.

More room for thought, adding even more panels to my array
going to 3s, 6p ~ input voltage goes to 75 and amps remain the same
5s would push the input to 135v (27v x 5 revised from 25v thanks to MisterSandals)

So, if I really wanted to plan for a big expansion of my array a 150v input 60amp SCC would be adequate.
Would a 80amp be better, less stressful on the charger?
 

wiseacre

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The panel Voc thermal coefficient is .38%
Which means for ever degree C it drops below 25C the voltage will rise .38%
-40C is 65 degrees C below rated Voc

65 x .0038 x 21.6Voc = 5.33V
So at -40C, the temp compensated voltage is 21.6 + 5.33V = 26.9Voc
I appreciate the math. It helps to see what the process is instead of just getting the number.
I'll round that up to 27 in future calculations.

double check:
5s would total 135a with plenty of leeway for a 150v input SCC
 

mikefitz

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Your 12 panels will. as you point out, give a possible 2340 watts, that with an ideal MPPT controller would put 98 amps into a battery at 24 volts.
Assuming a single MPPT controller you need a unit rated at 100 amps.
Victron offer a controller with either 150 volts input, or 250 volts input, units with this output current.
3 strings of 4 panels ( solar input max volts 108, max current 32 amps) with the 150/100 controller or 2 strings of 6 panels (solar input max volts 162, max current 21 amps) with the 250/100 controller.

Ideally you want to keep the current the current as low as possible in the solar cables to reduce losses and keep within the MC4 current limits.

In the event of exceeding the 2340 watts input, the controller will limit the output current to 100 amps.

Can the battery tolerate a charge at 100 amps?

Mike
 
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wiseacre

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Your 12 panels will. as you point out. give a possible 2340 watts, that with an ideal MPPT controller would put 98 amps into a battery at 24 volts.
Assuming a single MPPT controller you need a unit rated at 100 amps.
Thanks Mike,
That actually answered a question I didn't know how to express.

I take the 2340 watts and divide the system/battery voltage to get the 98v
It makes some sense to me.
I guess there's some magic smoke contained in the SCC to convert volts or watts or something into additional amps. To go from 60a at the panels into the SCC to 98 out is a lot for me to get my head around.

I'll have to look at SCCs again, I see they do list max wattage and I'll play with the numbers and think about this a bit more.
 

DJSmiley

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And even if the battery can't tolerate a 100A charge, you can still use the 150/100 controller, and limit the output to eg 80A.
That won't give you full power with maximum sun, but depending on your location that's not always the case anyway.

No problem to oversize your panels for the controller, and not fully using it's potential during a summer day. Benefits are in the spring/autumn and especially winter: with (much) less sunlight available, still beeing able to topup the batteries.

Personally, 8 or even 12 panels might be slightly overkill for a 200Ah bank, but you can do so.
 

mikefitz

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have to look at SCCs again, I see they do list max wattage
Most MPPT controllers can tolerate panel watts in excess of the rated maximum output watts. What is important is not to exceed the rated input volts and current.
With regard to available controllers, the Victron smart range is head and shoulders above the rest, in terms of performance, reliability and ease of setup.

Mike
 

wiseacre

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And even if the battery can't tolerate a 100A charge, you can still use the 150/100 controller, and limit the output to eg 80A.
That won't give you full power with maximum sun, but depending on your location that's not always the case anyway.

No problem to oversize your panels for the controller, and not fully using it's potential during a summer day. Benefits are in the spring/autumn and especially winter: with (much) less sunlight available, still beeing able to topup the batteries.

Personally, 8 or even 12 panels might be slightly overkill for a 200Ah bank, but you can do so.
I'm upgrading slowly. Batteries first but with long term thinking to what I'll need. Who knows, a windfall could happen too.

I'm also using an ATS in order to use whatever I can make whenever I can.
I'm retired so this is another new hobby. I'll never recoup the investment but I do like to make my own.

Hell, considering maple syrup goes for $40/gal, I'm making $340 per gallon Maple Syrup considering how much I sunk into that hobby.
 

wiseacre

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With regard to available controllers, the Victron smart range is head and shoulders above the rest, in terms of performance, reliability and ease of setup.
I be leaning that way. I don't like apps and the addition cost of a program display tends to put me off a bit. I just wish they had a built in programable display. Regardless, they are top of the list so far.
 

MisterSandals

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I be leaning that way. I don't like apps
When i first fired up my Victron 100/30 and then the app i was in for a surprise.
The app i downloaded indicated it was out of date and asked if i wanted to update it... pressed "OK" and it was done all by itself in 30 seconds.
I relaunched the app and quickly found my 100/30 displayed. I clicked on it and the app indicated that there was a new firmware version for the 100/30 and asked if i wanted to update it... pressed "OK" and it was done all by itself in 30 seconds.

I am an user-interface snob and was REALLY impressed with this. I cannot imagine how long this would have taken on other systems or what cables or whatever i would need and have to figure out. Mighty impressed.

If you have an option to get the bluetooth version, it is worth the extra $20, many many times over. I'd bet you'll be fan of the app, checking your system from the couch with the press of a couple buttons. Its sweet.
 
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