PhotoWatt PV's with MX-60

arcandsparkarcandspark Solar Expert Posts: 63 ✭✭✭✭
This system uses ten Photo Watt 95 watt panels each rated 34.4 VMP @ 2.9 IMP. 43.2 VOC..The panles are wired up in five parrelled pairs. When the system was first installed using the MX-60 default setting for charging 24 volt batteries it seemed to be working great. The reading on the MX-60 was showing 68 to 72 volts from the PV's at 14 to 15 amps. The output to the batteries was 27 volts at around 29 to 30 amps. We could watch the battery voltage slowly moving up from 24.3 to 24.8 to 25.2 finally holding around 25.7 with a load. I put my clamp on amp meter on the positive cable from the MX-60 to the battery buss and I could only measure about 6 amps??? Now after almost six to eight weeks of use the PV system is not looking so good. It averages 52 to 56 volts @ 7 to 9 amps from the solar panels and about 15 to 17 amps output to the batteries on the MX-60 screen with full sun, some times with cloud edge sun I can see the PV output go back up closer to 62 to 64 volts only for a few minutes. I clean the panels which were not dirty, checked all connections, at the PV breaker box I turn off the breakers and check the voltage from each pair and read about 75 volts VOC, but still only about 52 to 56 volts when you turn the breaker back on. I have isolated each pair with no change seen. Can the summer heat de-rate the panel output by such a large amount? The batteries are always under load, never disconnected from the inverters for charging. On average the inverters are outputting 4 to 6 amps each during the day while the PV's are trying to chage the batteries. It just seems like for the first couple weeks the system was working great and now it does not seem to be doing much of anything noticable. The average temperature has been around 95 degrees lately. Any suggestions??? We are currently installing a second PV system using a Zome Tracker with eight BP585 panels and a second MX60 which will charge a third battery bank. Thanks, arcandspark...

Comments

  • Solar GuppySolar Guppy Solar Expert Posts: 1,973 ✭✭✭
    Re: PhotoWatt PV's with MX-60

    Photowatt VMP is about 34V @ 25C ( panel tempature NOT ambient ) 24V configuration.

    The derating is about 0.16V for each degree above standard ... in 95F temps, your panels if roof mounted can easily be @ 70C ( 50C above standard )

    So simple math shows, 0.16 * 50 = 8 volts less per panel, two in series = 16 volts loss to heat, vmp at standard is 68 , subtract heat loss 68 - 16 and you have a corrected vmp of 52 Volts.

    Your panels may not be quite this hot, but there is also losses in the wiring ... this is why most recommend in hot climates to use 3 panels in series to keep the vmp above the needed charge voltages required by the batteries.

    When the panels vmp is lower than the battery charge voltage, the panel output falls fast, its NOT linear, just look at any Si panels VI graph.

    One more not talked about loss is the MX-60's minimum voltage differential ( Vin to Vout), on the MX-60 its about 1.2 volts ( 48V ), meaning regardless of the input voltage, the MX-60 will alway output 1.2 volts less ( the internal buck can't do more than about 98% duty cycle ). At 24V its half that and half again for 12V
  • arcandsparkarcandspark Solar Expert Posts: 63 ✭✭✭✭
    Re: PhotoWatt PV's with MX-60

    Thanks for the information. I will attach a thermcouple to the the front and back side of the PV panel and use the average of the two as my panel temp and do some math using your information and see if that explains the reason for the reduction in perfomance from the system. We planned on adding two more panels soon, so I could rewire so I would have four parrallel strings of three panels. That would give me 102 VMP at about 11.5 amps input to the MX-60. I had originally figured that using two panels inseries for 68VMP would have been more than enough for the 27 volts I needed for charging the batteries. Again thanks for helping me to understand the reason for the performance changes. arcandspark
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: PhotoWatt PV's with MX-60

    try hosing them off to cool them and see if the voltage comes up along with more current. you may need somebody to watch the mx60 for you as the pvs will heat up fast.
  • arcandsparkarcandspark Solar Expert Posts: 63 ✭✭✭✭
    Re: PhotoWatt PV's with MX-60

    Thats a great idea, but there is no way to get a hose to that location. I can use a pump up garden spray rig and that might work. I will give it a try, thanks for coming up with that great little experiment. I will let you know the results when I get to try the water spray next Saturday. To see some of the pictures go to the web site below and go down to "Some informative Battery Installation Pictures."

    http://alternativeenergy.meetup.com/153/photos/126202/

    arcandspark
  • Solar GuppySolar Guppy Solar Expert Posts: 1,973 ✭✭✭
    Re: PhotoWatt PV's with MX-60

    Cooling the panels won't do anything as the MX-60 will just maintain what it thinks is the vmp.

    You will need to force a re-sweep on the MX-60 as it is a sweep and sleep mppt ( not dynamic) if you quickly cool the panels down.

    I understand your interest in seeing it for yourself, but the numbers are what they are ... heat lowers the voltage and being on top of a metal roof is about as bad as it gets

    Also, from your pictures, its hard to tell the distance from the array to the MX-60, but if it is as far as it looks ( assuming the MX-60 is in the house ) thats some serious voltage drop potential in the wiring ...
  • System2System2 Posts: 6,290 admin
    Re: PhotoWatt PV's with MX-60
    Cooling the panels won't do anything as the MX-60 will just maintain what it thinks is the vmp.

    You will need to force a re-sweep on the MX-60 as it is a sweep and sleep mppt ( not dynamic) if you quickly cool the panels down.

    An excellent point that I hadn't thought of. I've seen the same type of thing happen during partly cloudy days. A cloud blocks the sun at a critical time during a sweep and throws off the sweep calculations, locking the MX into a non-ideal panel voltage, regardless of changing conditions, until the next sweep. So, for this to work, you would have to cool the panels and KEEP them cool while the MX does a new sweep and locks onto the new, higher voltage, then continue to keep then cool while you have a look at the results.
    Wayne
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: PhotoWatt PV's with MX-60

    i forgot about that myself. sorry david.
  • arcandsparkarcandspark Solar Expert Posts: 63 ✭✭✭✭
    Re: PhotoWatt PV's with MX-60

    Can you force the MX-60 to do a re-sweep by turning off the power from the panels completely for a few seconds??? I dont know enough about the MX-60 yet to play tricks with it. Can I change the sweep time, right now I think it is set to sweep every 15 minutes. I am using 1/0 copper from the compbiner box to the MX-60 about 75 feet away. The voltage drop should not be that bad at 52 to 68 volts. I am using a 4 1/2 digit DVM and I am reading about the same voltage at the combiner buss as I do at the input to the MX-60. I do notice that using #2 gauge wire from the MX-60 to the battery buss, about 12 feet, the cable does get slightly warm with 30 amps showing on the MX-60 display. I plane to change this to #0 gauge or even larger. Still complexed with the clamp on Amprobe reading of the MX-60 cables at the battery buss of around 6 amps when the dispaly shows 29 to 30 amps output. Maybe that 2 gauge welding cable is what is hurting me the most. Thanks for the help, arcandspark
  • Solar GuppySolar Guppy Solar Expert Posts: 1,973 ✭✭✭
    Re: PhotoWatt PV's with MX-60

    I think I miss read your original post, I thought you were a 48V battery, but now see its 24V. The only way the current would be 1/2 what it is suppose to be is you have issues in the panels in series, or the MX-60 isn't tracking correctly.

    I would suggest, near solar noon to isolate the panels into a single pair at a time and verify that each of the 5 pairs is operational, based on your current drop, it would only take 2 bad panels, on either the upper or lower 5 in parrallel to knock down the current that much.

    Your clamp on is most likey an AC amp meter, not DC, your probably just measuring the ripple current. Either get a meter that is DC current rated or an external DC rated clamp on that works with any DVM.

    Wire losses are simple to calculate, there is a link here on this site for a spreadsheet. I use an old HomePower .xls file and come up with a 0.22V drop on 150 ft of 0 guage and 0.14 on your 2 gauge run @ 30ft, nothing needs changing there you did good!
  • boBboB Solar Expert Posts: 995 ✭✭✭✭
    Re: PhotoWatt PV's with MX-60
    Can you force the MX-60 to do a re-sweep by turning off the power from the panels completely for a few seconds???....... Thanks for the help, arcandspark

    If OB hasn't removed the feature, try pressing buttons 1 and 4 (the two right side buttons) at the same time for a moment. This used to force a re-sweep.
    Also, (if it's still in there), you can press and hold button 4 to raise the Mpp Voltage or press button 3 to lower the Mpp Voltage until its next scheduled sweep. This will only work if the MX is in MPPT or "B-MPPT" mode. i.e. not Absorb or Float that is.

    It would be nice if they were to add the dynamic sweep routine into the MX60, which they certainly could do I suppose.

    BTW, Mr. S. Guppy, have you played with an MF80 yet ? I'm curious what you think about their new dynamic tracking.

    boB
    :D
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