Panel voltage vs battery voltage.

[GlockG20_10MM]

Ok, hello! Quickly I am not totally new to off-grid solar but the longer I am in it the more I realize I know so little. This particular question deals with input voltage specifically. Oh... and before anyone says throw more $$$ at it... I am on a limited budget so that is not a viable option right now. Have to make due with what I have.

Now that being said:
System:
- 5 panels 24VDC 1.24kW potential
- 1 Outback MPPT 60A charge controller (12/24/48)
- 1 Tripplight 24VDC - 120AC inverter / charger
- 8 US Batteries 24VDC @ 760AH capacity

Everything is set up for 24VDC, but I saw somewhere that you could set the panels up at a higher voltage and that would increase efficiency. I am not an elecrician so this is where I get hazy. Wouldn't my kW capacity drop?

I know that in order to do this I would have to get 1 more panel or drop one panel out of the system. I also understand from what I was able to gather from the Outback literature that I can run the panels at 48VDC and the charge controller would make the required voltage step down for the batteries.

So is this a more efficient way to run the panels? And if so... why? Thank you for learning me!

[Sunking]

Ok, hello! Quickly I am not totally new to off-grid solar but the longer I am in it the more I realize I know so little. This particular question deals with input voltage specifically. Oh... and before anyone says throw more $$$ at it... I am on a limited budget so that is not a viable option right now. Have to make due with what I have.

Now that being said:
System:
- 5 panels 24VDC 1.24kW potential
- 1 Outback MPPT 60A charge controller (12/24/48)
- 1 Tripplight 24VDC - 120AC inverter / charger
- 8 US Batteries 24VDC @ 760AH capacity

Higher voltage is more efficient because it lowers the current. You are screwed at least once, and maybe twice depending on the panel specs you have.

You screwed yourself with a Prime number of panels. With 5 panels only leaves you two configuration option of all 5 in parallel or all 5 in series. You cannot wire them in series because that would exceed the Voc input of your controller. That leave you only one option of all panels in parallel.

You want to run the panel voltage as high as possible to minimize current between the panels and controller. Doing that means you can use much smaller less expensive wire with a lot less power loss on the wiring. But that is not an option for you unless you obtain one more panel or loose a panel.

Now here is where you may get screwed twice. Check you panel specifications and tell us the Voc and most importantly Vmp. If Vmp is less than 36 volts you are screwed twice. If that is the case you have to either loose a panel or gain a panel.

Here is the funny part you are really going to like. You do not have enough panel wattage to support your batteries.

[GlockG20_10MM]

The only time I don't have enough panel to support the batteries is during extended cloudy days... those are rare where I live and I compensate with a generator. The system has been operational for 3 years now. My average daily kWh use is around 2.2kWh... the panels just today as they are currently configured produced 3.8kWh of power and when I checked the batteries were still in float. I know I need more panels for the occassional cloudy day, no argument there, but as I said... those are rare where I live.

Three of the current panels are Renesola 260 Watt JC260S-24/Bb Mono Solar Panel:
Electrical Characteristics
Panel Technology Monocrystalline
Rated Power (Pmax) 260W
Maximum Power Voltage (Vpm) 30.4V
Maximum Power Current (Ipm) 8.55A
Open Circuit Voltage (Voc) 37.7V
Short Circuit Current (Isc) 9.10A
Module Efficiency 16%
Series Fuse Rating 20A (EU) / 15A (US)
Power Tolerance 0~+5W
Maximum System (DC) Voltage 1000VDC(EU) / 600VDC(US)

The other 2 are Talesun 250W TP660M-250
Electrical Characteristics
Watts (STC) 250 W
Watts (PTC), No
Maximum Power (Vmp) 30.1 V
Maximum Current (Imp) 8.35 A
Open Circuit Voltage (Voc) 37.7 V
Short Circuit Current (Isc) 8.78 A
Power Tolerance 0~+3%
Module Efficiency 15.40%

What I am looking to do is improve efficiency of my system AND learn more detail as to why one way ismore efficient than another.

[Sunking]

You got problems, you only have 30 volts charging a 24 volt battery, but I will ignore that and just answer your efficiency question.

You only have one panel configuration available to you with 5 panels of all in parallel. Th eonly other option is 5 in series which you cannot do because excceds maximum voltag einput of 150 volts of your controller.

So you have 3 panels with Imp of 8.55 amps each, and 2 panels with Imp of 8.35 amps. When in Parallel at 100% power is 42.35 amps @ 30.1 volts for 1274 watts.

Minimum copper wire size to safely handle 42 amps is #8 AWG. So assume there is 30 feet 1-way distance between the panel combiner and controller giving 60 loop feet. #8 AWG has a resistance of .63 Ohm's per 1000 feet. So 60 feet of 8 AWG has .06 x .63 ohms = .038 Ohms of resistance. With 42 amps of current flowing through .038 Ohms you loose 1.6 volts and 67 watts of power on your wiring or roughly 5.2% of your power lost as heat.

Rearrange the panels in series using the same wire and you have 151.4 volts @ 8.35 amps. With 8.35 amps flowing through .038 ohms you loose .3 volts and 2.5 watts or roughly .2% of you power. From a design POV we shoot for no more than 2% loss. So for 8.35 amps at 30 feet 1-way all we need to use is the minimum wire size allowed by code of #14 AWG. 8.35 amps flowing through 60 feet of 14 AWG you loose 1.25 volts and 10 watts or just .7% of the power. Not only a lot less power loss but #8 AWG cost 30-cents per foot, and 14 AWG cost 10-cents per foot.

[GlockG20_10MM]

So I have the base wiring set up for good efficiency...

The panels are 10 feet from the combiner and another 8 feet from the charge controller. The wire is #8AWG from the panels to the combiner and from the combiner to the charge controller 4AWG and from the controller to the batteries is 6 feet and that is also 4AWG. The batteries are using lead bus bars so as to reduce corrosion due to dissimilar metals. The panels are mounted at a summer angle of 37deg and a winter angle of 42deg facing South with no obstructions, I did the research on this and have the mounting bracket set for summer and winter angles. So at least I am getting as much good sunlight as possible.

Anyhow... this is where I am not clear, I thought AMPS were what I need to charge the batteries and voltage was the driver to get the AMPS there. So if I increase the voltage and lose AMPS... wouldn't I also lose charge capacity?

Based on your reply I would need basically a minimum of 7 more panels wired in at least a 48VDC (actual would be 60VDC?) series parallel configuration. Would this be correct? That would put me at ballpark 60VDC input @ 51A. Which would be roughly 1.5kW. Hmmm...

[sensij]

You should read the documentation on your charge controller in more detail. An MPPT controller is essentially designed to have power out = power in. If you have 1260 W coming in (wired any way you'd like), you'll get 52.5 A out at 24 V, or the proportional amperage for whatever voltage is required at the stage of the battery cycle charge it is in. (OK, that is over simplified, there are some losses, but the basic concept holds).

What you are thinking of is a PWM type charge controller, where amps in = amps out, and any power offered by running the array at a higher voltage with panels in series is lost.

Keep in mind your charge controller is only rated for 60 A output. 60 A at 24 V = 1440 W, so that is about the maximum rating of array you can hook up, no matter how you choose to wire it (again, oversimplified, but you need to understand this before you can move on). The data sheet for your controller says you can go up to 1500 W (NEC recommendation), or 1800 W (absolute max), so maybe you could get away with adding one more panel and making two strings of 3 in parallel.

If you go to 6 panels, try to keep them balanced... adding a third Talesun panel so you have have one Talesun string and one Renesola string would probably hurt you the least with respect to mismatch loss.

If you want to get serious about this, I would suggest ditching the 24 V inverter and get one that operates at 48 V. Reconfigure the batteries for 48 V, and now your controller can handle an array of 3000 W if you choose to add even more panels. Just make sure the temperature adjusted Voc of the panels in series doesn't exceed the limit of the controller (150 Vdc). If your panels are tilted that much for optimum output, it sounds like it might get cold where you are it.

[Sunking]

Anyhow... this is where I am not clear, I thought AMPS were what I need to charge the batteries and voltage was the driver to get the AMPS there. So if I increase the voltage and lose AMPS... wouldn't I also lose charge capacity?

No Sir, you just do not know anything about controllers or electrical.

A PWM controller Output Current = Input Current. So in your case if you had a PWM controller 42 amps in and 42 amps out at 24 volts = 1008 watts out of 1280 watts or a 20% loss of power on top of wire losses.

A MPPT controller Current Output = Panel Wattage / Battery Voltage or in your case 1280 watts / 24 volts = 53 amps. If charge controllers could withstand 2000 volts input you can go in at 1280 volts @ 1 amp (1280 watts) and come out at 24 volts @ 53 amps or 1280 watts.

[QUOTE=GlockG20_10MM;145058] Based on your reply I would need basically a minimum of 7 more panels wired in at least a 48VDC (actual would be 60VDC?) series parallel configuration. Would this be correct? That would put me at ballpark 60VDC input @ 51A. Which would be roughly 1.5kW. Hmmm. [QUOTE]

No sir; where on earth are you coming up with those numbers?

There are 3 reasons to use MPPT controllers:

1. You can use much less expensive grid tied panels vs battery panels. GT panels cost on the order of $1/watt. Battery panels cost $2 to $8/watt
2. PWM controllers at best are 66% efficient, and MPPT is 95% efficient.
3. MPPT allows you to use much higher input voltage, and thus much lower current. That allows you to easily meet 2% or less voltage drop between panels and controller, and use much smaller less expensive wire. This is why utilities use very high voltages, as high as possible.

In order to take advantage of a MPPT controller two objectives have to be met.

1. Use GT panels because they are less expensive and most importantly higher voltage
2. Panel numbers need to be such so we can install the panels is a series parallel combination to obtain the maximum operating voltage possible. With your panels and controller it takes a minimum of 2 in series on a 24 volt battery, and up to 3 in series to stay within the 150 Voc input limit. That means with your panels and battery voltage it requires a minimum of 2 panels, or numbers of 3, 4, or 6 panels maximum. Do you see a 5 in that string of numbers. Here is how it works

2 panels in series. Vmp = 61 volts @ 8.4 amps = 510 watts
3 panels in series. Vmp = 91.5 volt @ 8.4 amps = 765 watts
2 panels in series with identical parallel string 2 x 2 using 4 panels. Vmp = 61 volts @ 16.8 amps = 1020 watts.
3 panels in series with identical parallel string 3 x 2 using 6 panels. Vmp = 91.5 volts @ 16.8 amps = 1530 watts

You have a PRIME number of panels which forces you to configure them all in parallel operating at 30 volts. Minimum voltage required on a 24 volt battery sytem is 36 VOLTS. You need to loose a panel or gain a panel and reconfigure as above. Take your pick.

[Bucho]

Everything is set up for 24VDC, but I saw somewhere that you could set the panels up at a higher voltage and that would increase efficiency. I am not an elecrician so this is where I get hazy. Wouldn't my kW capacity drop?

You loose less power as it travels through wires with higher voltage and lower amperage. This is why the power lines are a much higher voltage than we get out of our outlets. You can use this voltage drop calculator and see how things work: http://www.calculator.net/voltage-drop-calculator.html

However you have a different problem, your vmp is too close to your batteries voltage which will make getting them fully charged an issue.

Also 60amps, you're controllers max is right around a c/12 charging rate for your 760AH bank so I'd really recommend that whatever solution you pick involve adding and not subtracting a panel.

[Sunking]

However you have a different problem, your vmp is too close to your batteries voltage which will make getting them fully charged an issue.

Also 60amps, you're controllers max is right around a c/12 charging rate for your 760AH bank so I'd really recommend that whatever solution you pick involve adding and not subtracting a panel.

Good advice, reflects what I suggested. But me thinks he does not want to hear any of it.

[Bucho]

Anyhow... this is where I am not clear, I thought AMPS were what I need to charge the batteries and voltage was the driver to get the AMPS there.

Which is why it's bad that your voltage isn't high enough.

So if I increase the voltage and lose AMPS... wouldn't I also lose charge capacity?

Your MPPT charge controller converts the extra voltage into amperage. It's not perfectly efficient either way but there's probably a chart of it's efficiency at 24v and 48v somewhere you can look at.

[Bucho]

Good advice, reflects what I suggested. But me thinks he does not want to hear any of it.

I suspect he wouldn't be on here asking how to reconfigure his system if he didn't have a feeling that something's off.

It's probably just a question of how much he's willing to spend before it gets obvious. Would leaving the bank at 24v, configuring 4 panels for 48v and putting the 5th (and hopefully more) on a second charge controller be a viable option?

[Sunking]

It's probably just a question of how much he's willing to spend before it gets obvious. Would leaving the bank at 24v, configuring 4 panels for 48v and putting the 5th (and hopefully more) on a second charge controller be a viable option?

Stop and think about it.

The only way to use his panels at 48 volts is 3 in series, and in groups of 3 in series. Only 3, 6, 9 and 12 work in a 48 volt configuration with his 30 volt panels. 5 is a Prime number and only can be configure 5 in parallel or 5 in series. 3 is the only prime number that works or in multiple of 3's.

At 24 volts he can only use on paper 7 panels, but 7 is a Prime number and does not work because he has to use at least 2 panel in series. That only leaves him an option of 2, 4, or 6 panels. 5 will not work worth a dang.

[Bucho]

Stop and think about it.

The only way to use his panels at 48 volts is 3 in series, and in groups of 3 in series.

I get that 4 of his panels wouldn't charge a 48v bank any better than what he's got now but if he leaves the bank at 24v that's irreverent right?

With that in mind my question is:

Would leaving the bank at 24v, configuring 4 panels for "48v" and putting the 5th (and hopefully more) on a second charge controller be a viable option?

Hopefully it makes more sense with 48v in euphemism quotes.

[Sunking]

I get that 4 of his panels wouldn't charge a 48v bank any better than what he's got now but if he leaves the bank at 24v that's irreverent right?

Correct

Would leaving the bank at 24v, configuring 4 panels for "48v" and putting the 5th (and hopefully more) on a second charge controller be a viable option?

You would still be left with a panel voltage that is too low where he is at now right? He would still incur the cost of another controller and misc hardware and wiring. When for about the same cost or less just could buy the 6th panel to make things right...

Just to nick pick a little he does not have 24 volt battery panels which is part of the problem. At a minimum one needs 36 mono cells per 12 volts of battery which is a Vmp of 17 to 18 volts per 12 volts of battery. Sounds like he has 60 cell panels with a Vmp of 30 volts which is too low for a 24 volt battery. This gets back to your question of using a separate controller for the lonely single panel. The voltage would never get to a high enough state to match the output of the other string with a higher source voltage. About the only time it would ever be useful and functional is when the system drops down to Float voltage of 27.2 volts. Kind of like a 5 foot tall basketball player in a field of 7 foot players. The poor guy will never be part of the game and never see the ball 5 feet above his head of reach.

[bulldrummerbullies]

Ok, hello! Quickly I am not totally new to off-grid solar but the longer I am in it the more I realize I know so little. This particular question deals with input voltage specifically. Oh... and before anyone says throw more $$$ at it... I am on a limited budget so that is not a viable option right now. Have to make due with what I have.

Now that being said:
System:
- 5 panels 24VDC 1.24kW potential
- 1 Outback MPPT 60A charge controller (12/24/48)
- 1 Tripplight 24VDC - 120AC inverter / charger
- 8 US Batteries 24VDC @ 760AH capacity

Everything is set up for 24VDC, but I saw somewhere that you could set the panels up at a higher voltage and that would increase efficiency. I am not an elecrician so this is where I get hazy. Wouldn't my kW capacity drop?

I know that in order to do this I would have to get 1 more panel or drop one panel out of the system. I also understand from what I was able to gather from the Outback literature that I can run the panels at 48VDC and the charge controller would make the required voltage step down for the batteries.

So is this a more efficient way to run the panels? And if so... why? Thank you for learning me!

You have been getting great advice from everyone. You need more panels at less one so you can go parallel. Now let us get back to your question "So is this a more efficient way to run the panels" going to a higher voltage is more efficient if you want to run smaller wire size. Resistance is reduced that is all. You can reduce resistance by increasing you wire size. You are being mislead by the term efficient. Inverter salesman tell you your system is more efficient because you can use cheaper wire, over sizing your wire will also make your system more efficient because of reduced resistance. if you are talking to a wire sale man. not a inverter salesman.
your statement "Anyhow... this is where I am not clear, I thought AMPS were what I need to charge the batteries and voltage was the driver to get the AMPS there" it is not clear because change voltage only reduces resistance lost in the wire.