Another question on combining panels of different wattage

michaels
michaels Registered Users Posts: 5 ✭
in Solar Beginners Corner #1
This is gonna ramble on and I apologize in advance - hell of a first post I guess.

I didn't think/know to check the actual panel voltages when I bought them, I assumed all 24v panels were created equal so I have:

2x Astronergy 245W 24v panels at 37.98v
2x Astronergy 230W 24v panels at 29.3v
1x Trina Solar 180W 24v Panel at 36.8v (my first panel which was free)

I run 2 feeds of #6 stranded into my house - one feeds the micro grid tie inverters and usually has all the large panels connected to it in parallel.

The other has the Trina Solar 180w panel and it feeds the charge controller which keeps my batteries topped off.

Batteries are: 6 12v deep cycle batteries tied in 24v series parallel.

Charge controllers are (1) 24v 40A WMA (I assume, I can't read Chinese) and (1) 24v 150A WMA controller made by Missouri Wind and Solar and seems pretty nice although I've only had it working 2 days now but really like it.

The Smaller charge controller used to be my only controller but I bought the larger one and here's why:

I decided on some days I needed to charge the batteries quicker so I hooked all the panels up to the original charge controller and it just shut down cold. It was nowhere near 40 amps as it was a cloudy day and I was barely generating 300w according to the charge controller.

I ass/u/med it was just a faulty charge controller so I bought the larger one - but when all the panels are connected to it and generating upwards of only 10 amps it starts doing some weird stuff as well like the overheat light flashing but it's NOT overheating...I can feel the heat sink for the FETs and it's barely even warm if at all.

This is pretty much a home made setup and I never noticed anything out of the ordinary I suppose because everything was hooked up to the grid tie inverters with minimal monitoring meters on them.

In reading other posts concerning panels of different wattages in parallel it's generally accepted that's fine as long as the VOLTAGES are pretty close to the same. Would the voltage differences in those panels actually cause my charge controller to shut down? And if so can I solve this by grouping the panels together with like voltages...i.e. The Trina together with the 245W panels doing grid tie duty and the other two that are the same feeding the charge controller and in the future being mindful to get 24v panels of the exact or very close specs?

Sorry if this was confusing!!

Michael
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #2
    Re: Another question on combining panels of different wattage
    michaels wrote: »
    This is gonna ramble on and I apologize in advance - hell of a first post I guess.

    I didn't think/know to check the actual panel voltages when I bought them, I assumed all 24v panels were created equal so I have:

    2x Astronergy 245W 24v panels at 37.98v
    2x Astronergy 230W 24v panels at 29.3v
    1x Trina Solar 180W 24v Panel at 36.8v (my first panel which was free)

    In reading other posts concerning panels of different wattages in parallel it's generally accepted that's fine as long as the VOLTAGES are pretty close to the same. Would the voltage differences in those panels actually cause my charge controller to shut down? And if so can I solve this by grouping the panels together with like voltages...i.e. The Trina together with the 245W panels doing grid tie duty and the other two that are the same feeding the charge controller and in the future being mindful to get 24v panels of the exact or very close specs?

    Sorry if this was confusing!!

    Michael

    OK. First of all, the results of paralleling will be different depending on whether you are using a PWM or MPPT Charge Controller. If even one of your CCs is MPPT, then you cannot run two CCs off the same panel array, even if they are going to the same battery bank. They have to be wired separately to their own set of panels.

    If you are charging 24 volt batteries and have a 24 volt PWM CC, then it should not matter that the individual panel voltages (Voc or Vmp, you do not specify which you listed) are not equal. The output currents from the panels should add up. But your 245 watt 24 volt panel will not produce 10 amps output. If Vmp is 36 volts, it will produce only 6.7 amps best case in full sun. Your 230 watt panels with a lower Vmp should actually be able to deliver more current (a little under 8 amps).

    You may have exceeded the maximum voltage input to your CC when you connected all of the panels, since the voltage under load would have gone closer to Voc than Vmp if the batteries were not able to absorb 40 amps in their current state of charge. Look at all of the specs of the CC (if you can find them).

    If you are using an MPPT controller, then you need to match the Vmp of parallel strings to within 10% to avoid losing a lot of power. That means that your two 38 volt panels could not be paralleled with your two 30 volt panels. And trying to put mixed pairs in series would also not work well since they have more than a 10% difference in Isc/Imp.

    Your Trina could be combined in parallel with your 38 volt Astros with no problem in one subgroup and the two 30 volt Astros could form another group.
    Is your GTI one of the fleaBay plug in GTIs? If so, I cannot give you any advice except not to use it.
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Another question on combining panels of different wattage

    There's about a 20% difference between the low and high Voltages of these panels. That in itself should not cause the controller to shut down, but it just may be that the controller isn't designed for these Voltages.

    The first two panels will have a Voc of nearly 50, which could be a problem. Their Imp should be about 6.
    The second two will have an Imp around 7.5, but are so low in Vmp compared to the others that they may actually contribute nothing.
    The 180 Watt panel would have an Imp around 4.5 to 5.

    If you add all the Imp ratings you get something around 32, which shouldn't affect a 40 Amp controller at all.

    Test them in turn, feeding only matched panel sets to the input on a sunny day and see what you get. Keep in mind that a charged battery will not demand much current, so the panels would not produce much. You may have to apply some load to pull maximum Amps. Do not be surprised if the output from the cheap Chinese controller is less than the input to it; it wouldn't be the first time this has happened.

    Your six 12 Volt batteries (three parallel strings) isn't a good idea either.
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  • michaels
    michaels Registered Users Posts: 5 ✭
    #4
    Re: Another question on combining panels of different wattage

    Sorry - yes both cc's are PWM. The GTI's are indeed off fleabay, those 1kw ones. I always liked them, at least they seemed to work from what I can tell from the little light on the front. As best I can tell I've lowered my elec. bill by around 80.00 a month using them. What's wrong with them, I didn't do a whole lot of research but quite a few people eemed to like them...in fact they just came out with a 2500w one I was eyeballing.

    I'll go out now and reconfig the string - thanks for the advice!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Another question on combining panels of different wattage

    Well plug-n-play GTI's are illegal for one thing.
    So is back-feeding the grid without the knowledge & consent of your utility. In fact if you've got the wrong kind of meter and you actually do produce surplus power you will be charged for it instead of credited.
    Then there's all those issues with proper wiring.

    If you buy cheap, non-certified equipment and install it in an unsafe fashion don't be surprised by the results. Which could include your house burning down (no joke).

    When it comes to electricity, do it right or don't do it at all. The stuff is dangerous.
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  • michaels
    michaels Registered Users Posts: 5 ✭
    #6
    Re: Another question on combining panels of different wattage

    Not that it matters maybe but I'm in Arkansas lol - I did call my electric co-op and asked about the meter, they said it was fine for the amount I was generating at that time as I wasn't putting a lot back into the grid and only when I was at work and the the house was quiesced. They did ask about the GTI's and I told them exactly what they were, and they asked about island protection so I just faxed them the data sheet on them and they didn't say anything or call me back regarding them...I assume they'll get more interested next week when I get the 4 other panels possibly.

    The one thing they DID say that interested me when I asked about getting charged for the elec I generated if I had the wrong meter installed was that's a myth generated by the smart meter hating people..dunno if it's true or not but the guy did have a good laugh. For the record I *don't* like the smart meters but oh well, no one asked me!

    Again, this is Arkansas so I believe only about half of what I hear or read or am told by people around here.

    BTW I went and configured the panels as you suggested but it's getting dark so I'll have to check tomorrow - thanks for all your help though!!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Another question on combining panels of different wattage
    michaels wrote: »
    The one thing they DID say that interested me when I asked about getting charged for the elec I generated if I had the wrong meter installed was that's a myth generated by the smart meter hating people.

    Nope. No myth. And it has nothing to do with smart meters as typically they are bi-directional. It's the older digital and mechanical meters that read current flowing either way as current flowing the same way. A mechanical meter is pretty much just an AC motor; it doesn't care which way you plug it in it still runs in the same direction. :D

    It's good that you contacted the utility about this. If they're happy, that's half the battle. I hope those inverters have a UL listing at least. The installation regulations are another matter, which varies from one jurisdiction to another. Although it shouldn't, really.
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  • michaels
    michaels Registered Users Posts: 5 ✭
    #8
    Re: Another question on combining panels of different wattage

    Well I got curious and went down there and looked and they're NOT UL listed but rather CE listed (CE?!?!). What brand GTI would you recommend?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Another question on combining panels of different wattage

    "CE" is Conformance European, meaning it's certified in the EU.

    Sort of makes me wonder what the output specs are. ;)

    Considering what your panels are, your best solution may be to use micro-inverters. You've got less than 1 kW of panel there (discounting the 180) so a big central inverter isn't necessary. The small ones like an SMA SunnyBoy 700 are pretty expensive per Watt (just over $1,000) and you've got a bit more panel than that. An Enphase 215 is about $170 each so you'd have 'full capacity' plus money left over for the special wiring. Could be a better way to go. Expandable too.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #10
    Re: Another question on combining panels of different wattage
    Cariboocoot wrote: »
    Nope. No myth. And it has nothing to do with smart meters as typically they are bi-directional. It's the older digital and mechanical meters that read current flowing either way as current flowing the same way. A mechanical meter is pretty much just an AC motor; it doesn't care which way you plug it in it still runs in the same direction. :D
    .
    Sorry Coot, but not true. Since a watt meter has to measure actual watts and not just VA, it uses two magnetic fields, one driven by the current and one driven by a voltage coil.
    The product of the two is what makes the disk spin. An old or new mechanical meter will make the disk spin the other way during sell-back. But the drive between the disk and the dials is what changed. To stop people from just turning the meter upside down, the mechanical linkage was changed so that either direction of disk rotation would cause the dial reading to go up. Just like the case of the people who put a reversing gear on the odometer of their car.

    For digital meters, smart or otherwise, the circuitry adds up actual power transmission until it gets to a bucket size and then generates a pulse. The standard smart meter just counts pulses, the bi-directional smart meter counts UP pulses and DOWN pulses separately (and in fact usually keeps both counts for electronic readout even thought it displays only one total which goes up and down.)
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Another question on combining panels of different wattage

    Well you must have different meters than we ever did.

    Besides which, and this is the point, the analogy is correct for purposes of explaining the problem.

    I've about had it with nit-pickers.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #12
    Re: Another question on combining panels of different wattage
    Cariboocoot wrote: »
    Well you must have different meters than we ever did.
    Quite possibly. I have never seen a mechanical meter in which the disk would not turn the other way when the power flow was reversed.
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: Another question on combining panels of different wattage

    Well we only got ours here changed to the new style last year. No I'm not joking. The house we moved from had a new bi-directional digital smart meter installed the year before, and now they'd "updated" this house to a meter that should have been in place 50 years ago.

    Welcome to Canada. We catch up eventually.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #14
    Re: Another question on combining panels of different wattage
    Cariboocoot wrote: »
    ... and now they'd "updated" this house to a meter that should have been in place 50 years ago.

    Welcome to Canada. We catch up eventually.
    But if it ain't broke, don't fix it! The old meters, like old telco equipment, were designed to last 20-50 years if not abused. And it was hard to abuse them!
    The meter sockets (jaws and supporting insulators) would fail long before the meters.
    SMA SB 3000, old BP panels.
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