Help me with these calculations please. (Solar panel string theory)

[Nevadablue]

Please check my calculations, I’m not sure if I did all this right.

I’m getting ready to install solar panels on one of my systems. The setup will include six Chins 100 amp Lithium batteries, a Renogy 2000 watt pure sine wave 12 volt inverter charger and a Renogy 40 A Li charge controller. The Renogy 40 A Li charge controller is rated at 1040 watts @24 volts and max of 100 volts input.

I have quite a selection of panels available and these appear to be the best choices for this system. It appears that the Solargy panels are unused.

Does it look like this will work OK? Did I did the calculations correctly?

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4 Solargy Alpha Series 185 watt 24 volt panels. (740 watts at 24 volts) (62”x32”)

(Voc 44.7, 36.2 Vmp)

(5.14 amps)x4=20.56 amps


4 Kyocera 125 watt 12 volt panels. (250 watts at 24 volts) (56”x26”)

(Voc 21.7, 17.4 Vmp)

Or Voc of 43.4 using 2 sets of 2 in series and paralleling the 2 sets

(7.2 amps)x2=14.4 amps


Totals:
36.2 Vmp x 20.56 amps = 744 watts
And
17.4 Vmp x 14.4 amps = 250 watts

(35 amps total) Or 994 watts.

For a total of 990 (label) watts at 24 volts (less than 45 volts Voc) which should work with the controller.

 

[Northernchateau]

Before you get to far, you say you have a 12v inverter and a 24 volt charge controller.
Are you going to build a 12v system or 24 volt system?
What you need to look at is the Input spec of the charge controller and make sure your panel or panels fit in that spec.
Important numbers, VOC, Vmp, Total Watts.
Careful mixing panel voltage with controller output voltage.

 

[Nevadablue]

Thanks for the reply.

Yes, 24 volt input to the controller, 12 volt battery strings. There will be two parallel strings of 3 batteries for a 12 volt bank.

I think I included all the necessary numbers, I just need help deciding if I did it correctly.

Specs for the controller:

 

[OffGridInTheCity]

KEY ISSUE - Its the battery voltage - e.g. 12v vs 24v - that determines the 12v@520w vs 24v@1040w mode of operation. If you go 12v on the battery side, the controller won't be able to do more than 520w of PV. You'll have to go 24v battery if you want to get 1040w OR get a different charge controller.

The "Max Solar Input Voltage" is 100vdc. Allowing for temp changes etc, the panel strings shouldn't exceed 80vdc or so to leave headroom for increased panel voltages at cold temps.

PANELS:
** 4 Solargy Alpha Series 185 watt 24 volt panels. (740 watts at 24 volts) (62”x32”) - (Voc 44.7, 36.2 Vmp) (5.14 amps)x4=20.56 amps
If you do 2s2p here, we have 36.2v * 2 = 72.4v (good) and 2p = 5.14a * 2 = 10.28a. This is 10.28a * 72.4v = 744w.

**4 Kyocera 125 watt 12 volt panels. (250 watts at 24 volts) (56”x26”) (Voc 21.7, 17.4 Vmp)
If you do 3s1p you get 21.7v * 3 = 65.1v @ Xamps. 4s would be 21.7v * 4 = 86.8v @ Xamps. Neither 65.1v or 86.8v match very well with 72.4v above, but I'd go 4s at 85.8v and it will be pulled down to 72.4v as I understand it (the lowest voltage that you parallel together). Let's say you wind up with 400w.

Recap:
If you parallel 2s2p the Solargy Alpha(s) with 4s1p of the Kyocera(s) you get in the neighborhood of 744w + 400w = 1,144w.
This will overpanel (exceed max watts) of you're charge controller a bit should be be OK for 2 reasons. 1) You don't get 100% of the on-paper-specs from panels most of the time and 2) for most controllers, as long as you don't exceed max voltage (which will burn things up) you can over wattage the input as the charge controller will only use what it needs. In the 24v battery case, up to 1040w.

It's like a 20a home plug that offers up to 2,400w. If you plug in a 100w light bulb, it will only use 100w. Similarly, if you're PV array succeeds in offering more than 1,040w, the charge controller will only use up to it's max ability of 1,040w.

 

[Nevadablue]

One issue - you can't do 24v/1040w of PV input and have a 12v battery. Its the battery voltage - e.g. 12v - that determine the 12v@520w vs 24v@1040w. Suggest you go 24v battery if you want to get 1040w. If you go 12v battery, the PV input will be limited to 520w.

You're "Max Solar Input Voltage" is 100vdc. Allowing for temp changes etc, the panel strings shouldn't exceed 80vdc or so.

PANELS:
** 4 Solargy Alpha Series 185 watt 24 volt panels. (740 watts at 24 volts) (62”x32”) - (Voc 44.7, 36.2 Vmp) (5.14 amps)x4=20.56 amps
If you do 2s2p here, we have 36.2v * 2 = 72.4v (good) and 2p = 5.14a * 2 = 10.28a. This is 10.28a * 72.4v = 744w.

**4 Kyocera 125 watt 12 volt panels. (250 watts at 24 volts) (56”x26”) (Voc 21.7, 17.4 Vmp)
If you do 3s1p you get 21.7v * 3 = 65.1v @ Xamps. 4s would be 21.7v * 4 = 86.8v @ Xamps. Neither 65.1v or 86.8v match very well with 72.4v above, but I'd go 4s at 85.8v and it will be pulled down to 72.4v. Let's say you wind up with 400w.

744w + 400w = 1,144w total. This will 'overpanel' (exceed max watts) of you're charge controller a bit but will probably be OK as you don't get 100% from panels most of the time.

Hmmm… the manual says the controller auto senses battery voltage and sets charge voltage to match. Now to go back and read the manual again.

Why not parallel the 185 watt panels, all 4 for a 24 volt bank?

The plan for the 125 watt panels is two strings of 2 in series for 24 volts, and those two strings connected in parallel for 250 watts at 24 volts. Is that not workable?

I see 744 watts plus 250 watts.

Not sure where the total of 1144 comes from. I agree that the output won’t be ‘label’ output for sure, the newest of these was made in December of ‘09.

My goal is to input 24 volts to the controller (and the Voc of both strings are pretty close when both are running 24 volts).

 

[Supervstech]

Input voltage is not the parameter the controller uses to calculate amps, it is battery voltage that counts.
You have twice the wattage the controller can process to a 12V battery…

 

[Supervstech]

Since you have two different types of panels, you could get an additional controller and tie the other type of panel to it, getting more wattage into the batteries.

 

[OffGridInTheCity]

Hmmm… the manual says the controller auto senses battery voltage and sets charge voltage to match. Now to go back and read the manual again.

Sure but that means if it senses a 12v (nominal) battery, it will max out at 520w regardless of PV offered. If it senses a 24v (nominal) battery it will go up to 1040w. The PV input has nothing to do with the max on this side of the charge controller. The PV input as to be compatible (voltage > battery, voltage < max specs) but it's the battery that determines the max the controller will try to charge.

Why not parallel the 185 watt panels, all 4 for a 24 volt bank

** 4 Solargy Alpha Series 185 watt 24 volt panels. (740 watts at 24 volts) (62”x32”) - (Voc 44.7, 36.2 Vmp) (5.14 amps)x4=20.56 amps
If you do 1s4p you'll have the same power as 2s2p, just a lower voltage = larger wire. The incoming voltage must be higher than the max 24v battery (of about 29v) for charge current to flow -> battery.

The plan for the 125 watt panels is two strings of 2 in series for 24 volts, and those two strings connected in parallel for 250 watts at 24 volts. Is that not workable?

Its when you try to parallel in these OTHER panels that you need to consider.
**4 Kyocera 125 watt 12 volt panels. (250 watts at 24 volts) (56”x26”) (Voc 21.7, 17.4 Vmp)
You'll have to do at least 2s2p for 43.4v and this will be pulled down to the 36.2v when you parallel these into the ones above - less efficient.

So that's why I suggested what I did.

I see 744 watts plus 250 watts.

4 * 125w = 500w. And the voltage mismatch paralleling them in will make them <100% so that's where I get the 400w estimate.

Not sure where the total of 1144 comes from. I agree that the output won’t be ‘label’ output for sure, the newest of these was made in December of ‘09.

4 * 185w = 740w.
4 * 125w = 500w (400w estimate)

740w + 400w = 1140w.

My goal is to input 24 volts to the controller (and the Voc of both strings are pretty close when both are running 24 volts).

You listed " (Voc 44.7, 36.2 Vmp)" - so that's where I go the 36.2v spec for 2s of Solargy Alpha Series 185. I could have read it wrong.

The KEY ISSUE here is you need to go 24v battery if you want to take advantage of 1040w of PV input.

With a 12v battery, you can only do 540w on the one charge controller....
which is is 3s or 4s of the 185w panels or 4s of the 125w panels (total voltage <80vdc or so).

As mentioned above, you can use as many charge controllers to the same battery as you want, so you can stick with 12v but you'll need at least 1 more charge controller.

 

[Nevadablue]

Ok, I think I got it now. I have a second identical controller. Sigh… I just read the entire manual for the controller, and went to the Renogy site and none of it says anything about battery voltage being the deciding factor on input. I’ll be talking to them, that should be included in the documentation if it is correct.
Thanks everyone.

 

[OffGridInTheCity]

Ok, I think I got it now. I have a second identical controller. Sigh… I just read the entire manual for the controller, and went to the Renogy site and none of it says anything about battery voltage being the deciding factor on input.

It's not about "input" - it's about max output.

These lines are the definition that you're looking for....


Try to think of a charge controller as a battery charger. This "Max" is not like the amps and vdc, it's a max of what it can do given enough PV input. It's like the 100w light bulb - 120v @ 100w is all it can do regardless if the wall socket has potential to do much more.

The reason battery voltage changes the overall power is that the circuits(wiring) in the charge controller can only carry up to 40a of current.
If you double the voltage (12v -> 24v) battery, then the same current = twice the power. 43.3a * 12v = 519.6w. 43.3a * 24v = 1,039.2w. The 43.3a is the same. The charge controller internals will 'burn up' if you do more than 40a.

The PV input is how it's powered (like the 120v wall socket for the light bulb) and these specifications are given as well. The "Max" in this case is like the max 40a current, the unit will be destroyed if you exceed the 100vdc.



It's understood that if you want 1000w of charging output you have to put in a little more input - e.g. 1020w - due to efficiency losses. BUT, you can do less. If you only give 100w of PV, then it will do 100w of charging. If you give it 300w of PV it will 300w of charging. If you give it 1000w of PV potential it will use up to 520w at 12v (e.g. 43a) - but stop there.

PV power is not forced into the controller. The controller will only draw as much as it needs. Just because you provide 1000w or 2000w of solar panels doesn't mean the controller must use it all.

I'm writing this to try to help you ask the right questions as you pursue this. I'm not bothered that you want to talk to Renogy or others - but trying to help you so you can understand the right concepts which will help you have good conversations. It took me quite a while to grasp some of these concepts myself so you have my sympathies

 

[Doggydogworld]

Your basic idea of matching 4S1P Solargy with 2S2P Kyocera is sound. But you've been misled by some confusing nomenclature. To wit:

I’m getting ready to install solar panels on one of my systems. The setup will include six Chins 100 amp Lithium batteries, a Renogy 2000 watt pure sine wave 12 volt inverter charger and a Renogy 40 A Li charge controller. The Renogy 40 A Li charge controller is rated at 1040 watts @24 volts and max of 100 volts input.

A "40A controller" can push at most 40A into the batteries. No matter what panels you use, max power is 40A * battery voltage.

4 Solargy Alpha Series 185 watt 24 volt panels. (740 watts at 24 volts) (62”x32”)

(Voc 44.7, 36.2 Vmp)

(5.14 amps)x4=20.56 amps

"24 Volt panel" is a nickname for a panel that kinda works OK if you simply wire the output leads directly to a discharged 24V battery. And that's basically what a PWM controller does*. With direct-connect, panel and battery voltages are the same (nominally 24V but more like 29V when charging).

But your MPPT does not directly connect panels and batteries. It converts voltages and can hold panel and battery at different voltage levels. So forget these notions of 12V and 24V panels, those terms only apply when direct-connect pulls panel voltage down to match the battery.

*PWM also has logic to disconnect the panels when needed to protect the battery (e.g. to prevent overcharging).

Hmmm… the manual says the controller auto senses battery voltage and sets charge voltage to match. Now to go back and read the manual again.

12V / 24V auto-sense is typically a lead-acid feature. Don't count on it working with your lithium batteries.

4 Kyocera 125 watt 12 volt panels. (250 watts at 24 volts) (56”x26”)

4 x 125 W = 500W. You always add Watts up, whether series or parallel. See below.

(Voc 21.7, 17.4 Vmp)

Or Voc of 43.4 using 2 sets of 2 in series and paralleling the 2 sets

(7.2 amps)x2=14.4 amps

Correct on Voc and Imp. But also note Vmp is 2 * 17.2 = 34.4V.

Totals:
36.2 Vmp x 20.56 amps = 744 watts
And
17.4 Vmp x 14.4 amps = 250 watts

Use the 34.4V here, not 17.4V. That's why it's 501W instead of 250W.

(35 amps total) Or 994 watts.

35A and 1245W.

You won't actually get 1245W. First, your 2S Kyocera strings will pull your 4S Solary string down to 34.4V. So you end up with:

Solargy: 34.4 V(not quite mp) * 20.56A = 707 W
Kyocera: 34.4 Vmp * 14.4A = 501 W
Total: 34.4V * 34.96A = 1208 W

That's nominal output at full 1000W/sqm sun, perfect tilt, 25 deg C, blah, blah. Your mileage may vary.

Secondly, as noted above, your MPPT can only send 40A to the battery. So for your 12V battery which charges at ~13V:
40A * 13V = 520W

If you only needed to power loads at night you could disconnect the inverter during the day and wire your six batteries up as 2S3P. Switch your MPPT to 24V mode and charge at the full 1040W. Once fully charged, put your MPPT back into 12V mode, rearrange the batteries into 1S6P and reconnect the inverter up. Maybe even wire up a bunch of relays up to do this automatically.

But if you need the inverter on 24x7, you're stuck with 520W charging. You could switch to a 24V inverter or a 80A MPPT. But I agree adding your spare 40A MPPT is the obvious choice.