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surfynic · 14-03-2016, 01:57

Hi guys,

Ok I have an issue that has been driving me bonkers for a while now so hopefully someone here can help me out! For ease I won't go into depth about my system unless you guys require more details.

So I have 2 x 250w 24V panels mounted on my targa which are connected in parallel and then wired 12m to my mppt with 9mm (AWG 8) wire. Both panels are rated to create a maximum of 8A @24V so a potential total of 16A or 500W. Now here lies my issue, the most I have ever seen come from the 2 panels combined is 9A or 260W.

So to try find the problem I have tried a series of test all of which create some interesting findings-

• I have covered one panel up when they are producing max power and the current only changes slightly so I then uncovered that panel and covered the other and again the current does not change much. This eliminates one panel been faulty, but adds a strange thought that one panels seems to dominate the other or there is a bottle neck somewhere.

• I have disconnected the panels from my charger and connected them directly to a 24V battery bank and they still read the same amps. So this eliminates there been a problem with my voltage settings or my mppt.

• I have tried putting a large load on my batteries and this had the interesting effect of allowing my panels to work harder and at that time was seeing 12A @ 24V come in combined.

• I have checked and double checked all my float/absobtion/bulk settings and these all seem fine (also the bypass test with the 24v battery bypassed these been an issue).

So now I'm at the point of hair tearing out! would love to hear if any one has had a similar experience or any thoughts on what might be going on?

Cheers in advance!
Nic

**noelex 77** · 14-03-2016, 02:09

Hi Nic

"24v" panels are not suitable (on their own, or in parallel) for a 24v battery bank.

You need panels with a Voc of around 42 volts and a Vmp of around 35 v to be suitable for a 24v battery. These will have 72 solar cells.

The Voc and Vmp will be printed on the panel so you can check for yourself. My suspicion is the voltages for your panels will be much lower. Another check you can do is to count the number of cells.

Most so called 24v panels have output voltages that are too low for a 24v battery. They are designed for domestic instalation not 24v batteries.

If you connect your panels in series you should be able to fix the problem.

Series connection is generally slightly less efficient, but is necessary in your case (if my diagnosis is correct) to provide enough voltage. The Voc and Vmp are listed under standard conditions at a much lower temperature than will be seen in practice.

Before connecting in series check your regulator will handle the combined Voc voltages (with some reserve). Voltages higher than the maximium will generally instantantly damage the regulator

surfynic · 14-03-2016, 02:20

Thanks noelex 77 the only problem with putting them in series is that if I have a shadow on one panel this will then effect the entire array instead of just the one panel (from what I understand anyways!).

Quote:

Originally Posted by noelex 77

Hi Nic

"24v" panels are not suitable for a 24v battery bank.

You need panels with a Voc of around 44 volts and a Vmp of around 35.5v to be suitable for a 24v battery. These will have 72 solar cells.

The Voc and Vmp will be printed on the panel so you can check for yourself. My suspicion is the voltages for your panels will be much lower. Another check you can do is to count the number of cells.

Most so called 24v panels have output voltages that are too low for a 24v battery. They are designed for domestic instalation not 24v batteries.

If you connect your panels in series you should be able to fix the problem.

surfynic · 14-03-2016, 02:24

Sorry I should have been more clear, they do read about 32V on a multimeter. Ive now attached the specs. The panels actually charge a 12V battery bank after the mppt.

**noelex 77** · 14-03-2016, 02:31

Quote:

Originally Posted by surfynic

The panels actually charge a 12V battery bank after the mppt.

Sorry I thought you had a 24 v battery bank.

Your panel voltage is too low for a 24v battery bank, but is fine for 12v so the problem is elsewhere. Parallel connection is fine for a 12v battery bank.

However, this test and the conclusion is not valid (because your panels will not work properly with a 24v battery)

Quote:

Originally Posted by surfynic

• I have disconnected the panels from my charger and connected them directly to a 24V battery bank and they still read the same amps. So this eliminates there been a problem with my voltage settings or my mppt.

surfynic · 14-03-2016, 03:04

hmm I see. In the specs I attached though the VMP is 30.91V is that not suitable to put on a 24V battery as a test? The readings were the same as when they are connected to my MPPT/Charger. If what you say is the case, can you think of a test I can do to eliminate the chance of a problem with the MPPT?

Quote:

Originally Posted by noelex 77

Sorry I thought you had a 24 v battery bank.

Your panel voltage is too low for a 24v battery bank, but is fine for 12v so the problem is elsewhere. Parallel connection is fine for a 12v battery bank.

However, this test and the conclusion is not valid (because your panels will not work properly with a 24v battery)

**noelex 77** · 14-03-2016, 03:21

Quote:

Originally Posted by surfynic

hmm I see. In the specs I attached though the VMP is 30.91V is that not suitable to put on a 24V battery as a test? The readings were the same as when they are connected to my MPPT/Charger. If what you say is the case, can you think of a test I can do to eliminate the chance of a problem with the MPPT?

The voltage is too high for reasonable output if you directly connect the panels to a 12v battery and too low for direct connection to a 24v battery.bThe only option I can think of is to substitute another MPPT regulator preferably with tempory wiring to a 12v battery.

MPPT regulators do fail resonably frequently, but more commonly with no output at all.

Most of these problems are due to a poor connection somewhere. Measuring the voltage drop along the system will help pinpoint the location.

The other simple test is to feel for heat at any of the connections when the output is high.

You could also test the Voc and Isc of each of the solar panels. This only needs a multimeter and bright sun.

twinboat · 14-03-2016, 03:54

If your controller can handle it, wire them in series as a test.

If you gain output, that may be a sign of "panel to controller" wiring issues.

Is the 12 meter run, 1 way ?

Did you do the wire size calculation at 24 meter round trip ?

surfynic · 14-03-2016, 04:21

I think I will try the wiring in series test next. it is a 24m return trip on the wiring and the guys I bought the wire from did the math. I didn't think wiring would bottleneck the amps though? I thought it just created voltage drop?

Quote:

Originally Posted by twinboat

If your controller can handle it, wire them in series as a test.

If you gain output, that may be a sign of "panel to controller" wiring issues.

Is the 12 meter run, 1 way ?

Did you do the wire size calculation at 24 meter round trip ?

NahanniV · 14-03-2016, 05:03

Your third test (large load) seems to show that your setup may be working correctly.

What you are not telling us is the state of your bateries.
What controller do you have?
What mode is it in during you test? (Bulk, absorb, float)
How big a load did you use?
When you say12A at 24V: is that the actual voltage, or do you just mean curent on the panel side? Is the panel voltage ever getting to the VMPP of 30V?

The controller will only use maximum amps during the bulk phase of charging. Or possibly in in the other modes if the load is large enough; 500W at 12V is 40A, that is a big load.
Many MPPT controllers can not supply full current while in float mode, since they are working in PWM mode at that point in order to control the voltage.

Cheers,
JM.

surfynic · 14-03-2016, 05:23

Hi JM,

Thats what I thought with the third test. The results of that test show I am capable of getting the power down the wire (well so I concluded). But it just doesn't seems shy! haha

By state of the batteries do you mean what % of charge or do you mean age?

The batteries are brand new lithium batteries, 4 x 12v 160ah batteries to give 640ah battery bank. I have run the tests at all battery levels from 30% discharged right up to 99% full and always have the same results.

My charge controller is the Mastervolt Mass Combi Ultra 12/3000-150

Having tested through 30% to 99% I have done it through all the modes.

Ive had the same results using a big load i.e. a heat gun but have also has similar results when using my water maker.

I just don't know what else I can try!

Quote:

Originally Posted by NahanniV

Your third test (large load) seems to show that your setup may be working correctly.

What you are not telling us is the state of your bateries.
What controller do you have?
What mode is it in during you test? (Bulk, absorb, float)
How big a load did you use?

The controller will only use maximum amps during the bulk phase of charging. Or possibly in in the other modes if the load is large enough; 500W at 12V is 40A, that is a big load.

Cheers,
JM.

NahanniV · 14-03-2016, 06:09

Lithium changes everything. Your charge parameters become very important.

Your controller is likely working in float mode during your tests, which is PWM, so it will not be able to pull maximum amps from the panels.

In short, if your quoted "12A @ 24V" actually meant 12A @ 30V and your charger is in float mode, your system is probably fine.

Cheers,
JM.

surfynic · 14-03-2016, 08:36

the problem is that I only get 12A when I'm using high powered devices, otherwise it rarely higher than 6A @ 30V. and here in lies the problem, I want to be pulling the 12A to get my batteries up to 100% not when I'm draining them..

Quote:

Originally Posted by NahanniV

Lithium changes everything. Your charge parameters become very important.

Your controller is likely working in float mode during your tests, which is PWM, so it will not be able to pull maximum amps from the panels.

In short, if your quoted "12A @ 24V" actually meant 12A @ 30V and your charger is in float mode, your system is probably fine.

Cheers,
JM.

senormechanico · 14-03-2016, 09:32

Quote:

Originally Posted by noelex 77

Sorry I thought you had a 24 v battery bank.

Your panel voltage is too low for a 24v battery bank, but is fine for 12v so the problem is elsewhere. Parallel connection is fine for a 12v battery bank.

However, this test and the conclusion is not valid (because your panels will not work properly with a 24v battery)

My home's 12 panel / 60 cell array and Midnite Solar MPPT 150 seem to be doing just fine with my LiFePo4 24 volt system. Go figger.

I suspect his batteries are either not discharged enough to take full current, or the panels are hot or the sky isn't clear enough, there's a resistance in a connection, or various combinations of the above.