Help sizing a charge controller

totalconfusion

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Apologies if this is the wrong section for this post. I'm after a hand sizing up a charge controller.

These are the panels I'm looking at purchasing:

320w
lsc = 9.11
voc = 44.85

My configuration will be 2S2P and if I go with the bigger charge controller, this would eventually grow to 3S3P

These are the charge controllers I'm looking at:

Victron SmartSolar MPPT 100/20 ($200)

and

Victron SmartSolar MPPT 150/35 ($520)

I have three concerns:

1. The first is not having enough safety margin running 2S2P with the much cheaper 100/20 charge controller.

2. What is "nominal PV power" What is this and is it of any concern? Or should I just be calculating off lsc and voc?

3. What is "rated charge current". Is this the current with which the battery is charged? If I went with the 100/20 MPPT, will I experience much slower battery charging than the 150/35? What are the practical limitations for charging batteries? I was thinking of running a couple of 200AH deep cycle lead acid batteries in parallel for 400AH at 12v but would consider running a 24v system.

Thanks for reading and I really appreciate your help!
 

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Wellbuilt

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for550 $ I would just get a morning star , midnite , out back 80 amp unit it’s about the same price and you have room to grow
 

rmaddy

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Math time. You are looking at 320W panels. In 2S2P that is of course 4 panels. 4 x 320W = 1280W. That's almost too much for a 12V system because 1280W / 12V = 107A. A charge controller that supports 100A or more is very big and expensive. 1280W with a 24V system would be 53A. That's much more reasonable.

Those 320W panels in 3S3P is 9 panels. That's 2880W. At 24V that's 120A. Too much. Now you need a 48V system for that much solar.

So let's assume you go with a 24V system and 4 panels in 2S2P. At 2S the Voc of the panels would be 89.7V. Voc goes up in the cold. Depending on where you are you could easily get over 100Voc in 2S. So you would need the 150/45 or maybe the 150/60.

If you really wanted 12V you could use the 150/100 with 4 panels in 2S2P.

You may find it useful to use Victron's MPPT calculator. Bring it up and select the custom panel tab and enter the specs for your panels. Enter various panel configurations (2S2P, 3S3P, etc) and different system voltages (12V, 24V) and see which controller it recommends.

 
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rmaddy

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I have three concerns:

1. The first is not having enough safety margin running 2S2P with the much cheaper 100/20 charge controller.
The 100/20 isn't even close to big enough for those panels in 2S2P.

2. What is "nominal PV power" What is this and is it of any concern? Or should I just be calculating off lsc and voc?
Nominal PV power is just the sum of the panel wattage. In this case it's 4 x 320W = 1280W.

3. What is "rated charge current". Is this the current with which the battery is charged? If I went with the 100/20 MPPT, will I experience much slower battery charging than the 150/35? What are the practical limitations for charging batteries? I was thinking of running a couple of 200AH deep cycle lead acid batteries in parallel for 400AH at 12v but would consider running a 24v system.
The second number in the Victron naming scheme is the rated output charge current. The xx/20 supports up to 20A charge current to the battery. The xx/35 supports up to 35A, etc.

Batteries usually have an indication of the max charge current that they support. You just want to keep the charge current at or below that max value.
 
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Hedges

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What are the practical limitations for charging batteries? I was thinking of running a couple of 200AH deep cycle lead acid batteries in parallel for 400AH at 12v but would consider running a 24v system.

FLA probably wants 0.12C, so 400 Ah would like 48A charge.
AGM may accept 0.2C (possibly higher) so 80A.
Lithium often accepts 0.5C so 200A. But only around nominal 25 degrees C. It tapers down to zero at 0 degrees C.

Some inverter & charger systems can have larger PV and SCC, but regulate battery current. That is probably built into most all-in-one. Advantage is more power for AC loads during daytime while not charging battery too fast.
 

totalconfusion

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Math time. You are looking at 320W panels. In 2S2P that is of course 4 panels. 4 x 320W = 1280W. That's almost too much for a 12V system because 1280W / 12V = 107A. A charge controller that supports 100A or more is very big and expensive. 1280W with a 24V system would be 53A. That's much more reasonable.

Those 320W panels in 3S3P is 9 panels. That's 2880W. At 24V that's 120A. Too much. Now you need a 48V system for that much solar.

So let's assume you go with a 24V system and 4 panels in 2S2P. At 2S the Voc of the panels would be 89.7V. Voc goes up in the cold. Depending on where you are you could easily get over 100Voc in 2S. So you would need the 150/45 or maybe the 150/60.

If you really wanted 12V you could use the 150/100 with 4 panels in 2S2P.

You may find it useful to use Victron's MPPT calculator. Bring it up and select the custom panel tab and enter the specs for your panels. Enter various panel configurations (2S2P, 3S3P, etc) and different system voltages (12V, 24V) and see which controller it recommends.


Hey maddy,

Thanks for the really quick and detailed response I very much appreciate your input!
I think I see what you're saying about needing to move up from a 12V system. The cable sizes and costs get ridiculous and amps the charge controller needs to be able to handle go up too.

So I think I get where I was going wrong. How I was getting to the amp requirements of the charge controller was just multiplying the lsc.
Whereas what we should be doing is dividing the total wattage by the nominal voltage of the battery bank.

So assuming a 2S2P setup
I would have ended up with these figures:
voc = 44.85 x 2 in series = 89.7V
lsc = 9.11 x 2 in parallel = 18.22a
Me erroneously thinking a 100V/20A charge controller might do

Where as I should have gotten this:
voc = 44.85 x 2 in series = 89.7V
320W x 4 panels total = 1280W
1280W / 12v = 106A
Me realising that even with a 24v system, I'd still need a charge controller capable of 53a AND the voc safety factor just isn't enough


When you said:
"The 100/20 isn't even close to big enough for those panels in 2S2P."

This is because of the Voc fluctuating in cold/hot conditions. Does the lsc also need room to move like the voltage? What kind of safety factor is normal to account for this? 20-25%? I'm located in Sydney, Australia.

So just to make sure I've got it all worked out here's another example with a different panel:

2S2P
315W
voc: 39.8V
lsc: 10A
nominal voltage of battery bank: 24V

315 x 4 = 1260W
1260W / 24V = 52.5A
39.8V x 2 = 79.6V + 25% safety factor = 99.5V

So for this example I would need a charge controller rated about 100V / 55A
Is this about right?


What do even use the lsc value for? I notice there is a "Max. PV short circuit current" in the charge controller datasheet.


Thanks so much guys, I'm beginning to appreciate that my assumed knowledge was actually quite dangerous. I really appreciate the help!

If anyone's got the time I wouldn't mind learning a thing or two about the battery side of things e.g: C ratings, how to calculate them and what happens when you go under/over the rating
 

Hedges

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FLA, if you don't reach 0.12C the electrolyte doesn't get stirred enough.
FLA, AGM too much current causes some kind of damage. Can be heating. Not sure if excess water loss, or if that is only at higher voltage.
Either kind, use temperature sensor and SCC that adjusts voltage.

Lithium, too much current plates out metal for permanent loss of capacity. At lower (or higher) temperature, allowed current is reduced, like around 0.1C at 10 degrees C.

If you have multiple arrays of different orientation (like 2s2p with one 2s aimed at morning sun, one 2s at afternoon sun), peak current is reduced. About 70% as much with 90 degree angle, 50% with 60 degree angle between arrays. Some reduction in total Wh/day.

Don't let Isc of array exceed max short circuit current rating of controller.
But less than that, should be OK to have more amps and watts than can be processed and delivered to battery ("Overpaneling")

Don't let Voc exceed max of controller, even when adjusted for record coldest temperature of your location.

There are also 48V inverters.
Midnight has a lower-cost "DIY" series in 24V and 48V, we haven't heard a lot of reports on but may be attractive.
 

totalconfusion

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FLA probably wants 0.12C, so 400 Ah would like 48A charge.
AGM may accept 0.2C (possibly higher) so 80A.
Lithium often accepts 0.5C so 200A. But only around nominal 25 degrees C. It tapers down to zero at 0 degrees C.

Some inverter & charger systems can have larger PV and SCC, but regulate battery current. That is probably built into most all-in-one. Advantage is more power for AC loads during daytime while not charging battery too fast.

Thanks Hedges,

So on a 12v 100AH battery:
1C would be 100A
0.2C would be 20A

If we have 2x 12v 100AH batteries in parallel (2P) for a total of 12v 200AH
1C would be 200A
0.2C would be 40A

How does this change with batteries in series?
Are there practical limitations on what the battery post can take?



I'm looking at a battery that advertises itself as being:


12v 200AH AGM
1400 cycles @ 30% DOD
Maximum charge current: 37.5A

So it's 0.1875C?

What's the relationship between charging rates and discharging rates?
 

totalconfusion

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Don't let Isc of array exceed max short circuit current rating of controller.
But less than that, should be OK to have more amps and watts than can be processed and delivered to battery ("Overpaneling")

So is this is a simple multiplication of lsc X total amount of panels
e.g: 2S2P with 10A lsc would = 40A max. PV short circuit current?

On the second example PV panel I gave it's listed as exactly 10A lsc, and the 150/35 charge controller lists a 40A Max PV short circuit current. Would it be ill-advised to sit right on the limit like that?
 

Hedges

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Thanks Hedges,

So on a 12v 100AH battery:
1C would be 100A
0.2C would be 20A

If we have 2x 12v 100AH batteries in parallel (2P) for a total of 12v 200AH
1C would be 200A
0.2C would be 40A

How does this change with batteries in series?
Are there practical limitations on what the battery post can take?



I'm looking at a battery that advertises itself as being:


12v 200AH AGM
1400 cycles @ 30% DOD
Maximum charge current: 37.5A

So it's 0.1875C?

What's the relationship between charging rates and discharging rates?

You can calculate "C" rates with Amps charging current divide by Amp-Hours capacity.
Or by Watts charging and Watt-hours capacity.

So PV panels that deliver 0.2C into 12V system (2 batteries parallel) would deliver 0.2C into 24V system (2 batteries in series)
Give or take exact voltage, 12V can drop to 10V or rise to 14V.

That's assuming PV is the limiting factor, not SCC.

Battery post? It can take what one battery can deliver. In series, no different. In parallel, need a wiring scheme which exactly matches resistance seen by each to balance current. 2 batteries is easy (diagonal corners) and that extends to quantity that is power of 2. Or use busbars.

Here's the battery series I use (4x 12V 100 Ah, now 8x 6V 400 Ah)
Tech manual indicates about 1800 to 1900 cycles at 30% DoD, a bit more but I think a bit more expensive too.




So is this is a simple multiplication of lsc X total amount of panels
e.g: 2S2P with 10A lsc would = 40A max. PV short circuit current?

On the second example PV panel I gave it's listed as exactly 10A lsc, and the 150/35 charge controller lists a 40A Max PV short circuit current. Would it be ill-advised to sit right on the limit like that?

2s2p is the voltage of 2s (2x Voc, 2x Vmp) and the current of 2p (2x Isc, 2x Imp) so 20A short circuit current (rating)

Wires and fuses/breakers should be sized 1.56x that, 31.2A
That's 1.25x for margin and another 1.25x for direct sun plus light off nearby clouds.
(But I think it's OK to use panel specs for SCC max short circuit current rating)

PWM controllers experience max current from PV, but MPPT controllers get to control the current they see. Isc is only seen if wired backwards (clamping diodes protect the rest), or some which have a feature that if PV or maybe battery voltage gets too high they crowbar PV with a FET.
Normal operation, Isc shouldn't occur.
 

rmaddy

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When you said:
"The 100/20 isn't even close to big enough for those panels in 2S2P."

This is because of the Voc fluctuating in cold/hot conditions. Does the lsc also need room to move like the voltage? What kind of safety factor is normal to account for this? 20-25%? I'm located in Sydney, Australia.
The 100 part of the 100/20 is the max input voltage. Given the Voc in 2S of the panels being 89.7V, it wouldn't take temperatures much below the STC of 25ºC for that to reach 100V. The 20 part of the 100/20 is the max output current for charging the battery. Since 4 320W panels is 1280W, even with a 24V system that is up to 53A. 20A at 24V is 480W. Basically the controller would not take any advantage of any solar above 480W.

2S2P
315W
voc: 39.8V
lsc: 10A
nominal voltage of battery bank: 24V

315 x 4 = 1260W
1260W / 24V = 52.5A
39.8V x 2 = 79.6V + 25% safety factor = 99.5V

So for this example I would need a charge controller rated about 100V / 55A
Is this about right?


What do even use the lsc value for? I notice there is a "Max. PV short circuit current" in the charge controller datasheet.
Your 25% safety factor is serious overkill. You'd have to get down to something like -40ºC (or F) for that. But otherwise your calculations are correct. Though Victron doesn't make a 100/55. You could get the 100/50. That would still support 1200W or so and you will rarely if ever get 100% out of the panels anyway.

The Isc would be used for the "max PV short circuit current". With these panels in 2P the short circuit current (Isc) would be 20A. As long as the SCC supports 20A max PV short circuit current then you are good.

But again, play around with Victron's MPPT calculator. It will really help with these calculations.
 

Hedges

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I use 20% boost to Voc for a simple calculation. To sharpen your pencil, find a data sheet for the PV panels and see what "Temperature Coefficient of Voc" is.

If your choice of panels and array configuration is getting too close to max input voltage limit, some SCC like Midnight Classic have "Hyper Voc" which allows them to tolerate (but not operate with) occasionally higher input voltage on a cold day.

Sydney record cold -8 degrees C, 33 degrees below nominal 25 C.


If panels have temperature coefficient not greater than 0.348%/degree C, 89.7Voc would just reach 100V on that record cold day.
 

totalconfusion

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The 100 part of the 100/20 is the max input voltage. Given the Voc in 2S of the panels being 89.7V, it wouldn't take temperatures much below the STC of 25ºC for that to reach 100V. The 20 part of the 100/20 is the max output current for charging the battery. Since 4 320W panels is 1280W, even with a 24V system that is up to 53A. 20A at 24V is 480W. Basically the controller would not take any advantage of any solar above 480W.


Your 25% safety factor is serious overkill. You'd have to get down to something like -40ºC (or F) for that. But otherwise your calculations are correct. Though Victron doesn't make a 100/55. You could get the 100/50. That would still support 1200W or so and you will rarely if ever get 100% out of the panels anyway.

The Isc would be used for the "max PV short circuit current". With these panels in 2P the short circuit current (Isc) would be 20A. As long as the SCC supports 20A max PV short circuit current then you are good.

But again, play around with Victron's MPPT calculator. It will really help with these calculations.
I can't thank you enough maddy, I really appreciate all your help and patience. I feel a lot more confident toying around with the values now.
For this set up I'll be using the Risen 315w panels on a 2S2P array with a Victron 100/50 MPPT. Gets me the most bang for my buck and the max short circuit current is a non-issue.

you're amazing thanks a million!
 

totalconfusion

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I use 20% boost to Voc for a simple calculation. To sharpen your pencil, find a data sheet for the PV panels and see what "Temperature Coefficient of Voc" is.

If your choice of panels and array configuration is getting too close to max input voltage limit, some SCC like Midnight Classic have "Hyper Voc" which allows them to tolerate (but not operate with) occasionally higher input voltage on a cold day.

Sydney record cold -8 degrees C, 33 degrees below nominal 25 C.


If panels have temperature coefficient not greater than 0.348%/degree C, 89.7Voc would just reach 100V on that record cold day.
Thanks for all your help Hedges! 20% sounds like a pretty fair rule of thumb. I'll definitely consider the relationship of voc and temperature in future installations.
 
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