# Charge Controller Sizing Questions

Solar Expert Posts: 71 ✭✭✭✭
I don't understand very well how the charge controller is sized. This is the most confusing part of the PV system for me to understand.

I went to the Morningstar website and they have a nice online app that sizes the controller for you, they call it the "string calculator" ( http://www.morningstarcorp.com/en/strings/calc.php ), you pick the CC, the panel you are using, the voltage of your battery bank, and it tells you how many panels you can use and what configuration they need to be in. I plugged in specs for a 230w panel, 24v system, and the output was as follows:

Morningstar 60a MPPT - 2 strings of panels, with 3 panels per string, 1380w total PV (v max for the cc is 150v)

Morningstar 60a Tristar PMW - 6 strings of panels, with 1 panel per string, 1360w total PV (v max for the cc is 125v)

These are the panels specs:
Pmax 230.00 Watts
Voc 37.00 Volts
Isc 8.24 Amps
Vmp 30.30 Volts
Imp 7.60 Amps

Not sure how to make sense of the calculations on how the cc system was designed, any help would be appreciated (would like to see the calculations)... There are several nice CC's, but I am looking at the Morningstar because they supposedly are very robust and can handle hot dusty climates and they do not have a cooling fan, if there are others like this please let me know if possible- thanks.

• Banned Posts: 17,615 ✭✭
Re: Charge Controller Sizing Questions

Okay, when picking a charge controller the first thing you need to know if how much current it needs to handle. This is determined largely by the amount of peak charge current you're going for, as in 22 Amps for 220 Amp hours of battery @ any given Voltage (for the 10% "middle of the road" rate). The controller must be able to handle this current or more.

Then of course you have to have enough panel to provide that current. That's when Voltage and type of controller come into play. MPPT type controllers can down-convert higher array Voltage to system charging Voltage. The formula there is basically array Watts (less derating) divided by system Voltage equals charge current. The controller makes the Amps vs. Volts adjustment as it sees fit.

With a PWM controller it can only pass the maximum current of the array as determined by the panels' Imp. Here it is important that the array Vmp be right for the system Voltage as there is no conversion possible. Higher than proper Vmp (like 29.8 on a 12 Volt system) results in wasted power.

The other thing that is important is the array Voc, which should not exceed the charge controller's maximum input Voltage.

Basically when you plug the panel numbers into a string size calculator it tries to find the best of these parameters for the particular controller and system Voltage without exceeding the controller's capacity on any specification.

So with the MPPT controller's 2 strings of 3 of those panels you get an array with specs of:
Watts = 1380
Voc = (3 * 37) 111 (below the 150 Volt max)
Vmp = (3 * 30.3) 90.9 (largely irrelevant due to the MPPT down-convert function but important for V-drop)
Imp = (2 * 7.6) 15.2 (again important to the V-drop; the controller will handle the change to output current)
Isc = (2 * 8.24) 16.48 (important for fusing if more than 2 parallel connections)
Controller output maximum current will be roughly 1380 * 77% / 24 (system Voltage) 45 Amps

With the PWM controller you can have only one panel per string because the system Voltage must be maintained (you can't change 60.6 down to 24, it would just waste half the power). And I have to point out that these panels would be no good on a PWM controller and 24 Volt system as the Vmp is low for such a system. It should be around 35 Volts. This means that under hot panel temps they may not produce enough Voltage to properly charge the system.
• Solar Expert Posts: 71 ✭✭✭✭
Re: Charge Controller Sizing Questions

So the PMW will always discard any voltage above the system voltage and not use it?

I should have added that the battery bank is 220ah at 24v (4 series 6v gold cart batteries, @ 220ah each)...

so the battery bank wants a charging current of at least 11a (5% of 220), and a max charge current of 29a (13% of 220)?

Will the battery bank be damaged if it is being charged at 45a that this system can generate using the 60a MPPT?
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Charge Controller Sizing Questions

btw, i disagree with morningstar if they say you can use a pwm cc with a single pv at a vmp of around 30v to charge a 24v battery bank. this is too low to properly charge a 24v battery bank and should be closer to the 34v or 35v neighborhood at a minimum.

now if you are unsure you can run it past the forum of a setup you might have in mind and you would need many particulars for one just does not always know what may be important. for example, somebody may want to use mppt to downconvert from much higher pv voltages that were necessary to overcome voltage drops due to long distances between the pvs and controller. that means the gauge of all wires and their lengths come into play and importance too.

45a may cause excessive maintenance and you could run risk of damages as that is a very high rate and not one i would put to a golf cart battery. you must give us the full layout with the number of those pvs into what controller and how the pvs are arranged series/parallel along with the wire gauge used and distances involved.
• Banned Posts: 17,615 ✭✭
Re: Charge Controller Sizing Questions
Caneman wrote: »
So the PMW will always discard any voltage above the system voltage and not use it?

Actually it will "discard" the power that is Imp * the Voltage difference between the panel's Vmp and the system's "ideal" Vmp. (Not precisely, but close enough for purposes of understanding why you shouldn't do it.)
For example, if you were to use one of those panels on a 12 Volt system whose "ideal" Vmp is 17.5 you get a Voltage difference of 30.3 - 17.5 = 12.8, times the Imp of 7.6 or approximately (but not precisely) 97 Watts of potential power being lost.
I should have added that the battery bank is 220ah at 24v (4 series 6v gold cart batteries, @ 220ah each)...

so the battery bank wants a charging current of at least 11a (5% of 220), and a max charge current of 29a (13% of 220)?

As Neil and I have both said, the 24 Volt system is not going to be properly charged by an array Vmp of 30.3 which is all that is available with a PWM type controller and those panels. I guess Morningstar doesn't have any "catch" for "Vmp is too low" in their calculator?

You're right on the charging current range, with the caveat that FLA batteries aren't as tolerant of higher current (in or out) as AGM's are.
Will the battery bank be damaged if it is being charged at 45a that this system can generate using the 60a MPPT?

Repeated high current charging like that will wreck flooded batteries. AGM's can take higher current better, but are more picky about Voltage. Not that this is absolutely going to happen, but if it were me, I'd "dial back" the array so that there was NO chance of hitting those current levels.

My calculations for that battery bank would be:
22 Amps * 24 Volts = 528 Watts / derating factor = 685 Watt array. Or three of the 230 Watt panels totaling 690 Watts. And you must use an MPPT type controller. In this case the Rogue 3024 would probably be a very good choice providing you are not planning on future expansion.
• Solar Expert Posts: 71 ✭✭✭✭
Re: Charge Controller Sizing Questions

you guys are great to help out a noob like me, thanks so much..

i think i get it now on the cc power coming in to the battery bank and how the cc sizing is determined...

about the current going out of this 220ah 24v battery bank:
...FLA batteries aren't as tolerant of higher current (in or out)...

I have seen the C/8 rule of thumb used on the forum, does that refer to the max advisable amp-hours going out of a battery bank? so for my 220ah 24v battery bank it should not have more than 220/8 = 28ah drawn out of it at any time when a load is attached to the inverter?
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Charge Controller Sizing Questions

c/8 is a good reference, but batteries do take small excursions beyond that. the name of the game really is battery life as you want your batteries to last for you.

btw, as to the robustness, to use your words, it matters not as a cc rating is a cc rating. if you prefer no fan, that is fine, but be aware that you do want to keep any cc out of sunlight as the sun will add to the heat the cc is already producing. excessively hot areas should be avoided too.
• Banned Posts: 17,615 ✭✭
Re: Charge Controller Sizing Questions
Caneman wrote: »
you guys are great to help out a noob like me, thanks so much..

i think i get it now on the cc power coming in to the battery bank and how the cc sizing is determined...

about the current going out of this 220ah 24v battery bank:

I have seen the C/8 rule of thumb used on the forum, does that refer to the max advisable amp-hours going out of a battery bank? so for my 220ah 24v battery bank it should not have more than 220/8 = 28ah drawn out of it at any time when a load is attached to the inverter?

Actually it is usually used in reference to charging, but you've got the formula right: capacity in Amp hours / # = current (not Amp hours which is current over time).
Strangely, most batteries can handle this as a peak discharge rate too, but not for very long. The reason being that the capacity numbers are in terms of steady discharge current over time. Usually we use the "20 hour rate". But as such the capacity actually changes according to the rate of discharge: draw more current and the capacity goes down. So if you pull 5 Amps from a "220 Amp hour" battery it will "last" for "so long" (difficult to be precise) but if you pull 10 Amps from the same battery it will last less than 1/2 that time because effectively its over-all capacity is lessened due to the increased rate of discharge.

Clear as mud, isn't it?:p
• Solar Expert Posts: 71 ✭✭✭✭
Re: Charge Controller Sizing Questions
niel wrote: »
btw, as to the robustness, to use your words, it matters not as a cc rating is a cc rating. if you prefer no fan, that is fine, but be aware that you do want to keep any cc out of sunlight as the sun will add to the heat the cc is already producing. excessively hot areas should be avoided too.

^^^ thanks, the cc rating is the cc rating for sure... i like the idea of no cooling fan, though, because then it can't draw dust inside of the controller and muck it up if i take the cover off of my computer and look inside there is dust piled up everywhere from the cooling fan drawing in air, and that is with the box inside the house, can't imagine what it would look like if it sat in a hot dusty shed...

so the C/8 rating is a rule of thumb like the 50% max DoD, to help ensure battery life?
• Banned Posts: 17,615 ✭✭
Re: Charge Controller Sizing Questions
Caneman wrote: »
so the C/8 rating is a rule of thumb like the 50% max DoD, to help ensure battery life?

Yes. We use lots of rules-of-thumb around here. They aren't 100% accurate and applicable in every situation, but they are close enough to get a design near to where it can be tailored to the particular installation. Otherwise you'd just be guessing or doing a lot more math than you'd want to. :roll:
• Registered Users Posts: 22
Re: Charge Controller Sizing Questions

BTW...the Rogue cc has max 60v input
• Banned Posts: 17,615 ✭✭
Re: Charge Controller Sizing Questions
rbtrrer wrote: »
BTW...the Rogue cc has max 60v input

Good catch.
Gee, I wish they'd improve that.
• Solar Expert Posts: 71 ✭✭✭✭
Re: Charge Controller Sizing Questions
Repeated high current charging like that will wreck flooded batteries. AGM's can take higher current better, but are more picky about Voltage. Not that this is absolutely going to happen, but if it were me, I'd "dial back" the array so that there was NO chance of hitting those current levels.

My calculations for that battery bank would be:
22 Amps * 24 Volts = 528 Watts / derating factor = 685 Watt array. Or three of the 230 Watt panels totaling 690 Watts. And you must use an MPPT type controller. In this case the Rogue 3024 would probably be a very good choice providing you are not planning on future expansion.

thanks, i am going to dial it back with the 3 panels, then i can expand it later if i add parallel battery string...
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Charge Controller Sizing Questions
Caneman wrote: »
thanks, i am going to dial it back with the 3 panels, then i can expand it later if i add parallel battery string...

know that the voltage limits on controllers are not based on nominal voltage applications, but the total voc presented to the input of the controller. if in parallel those pvs will be fine for the voc, but if you tried putting them in series you would blow out the rogue controller.