Exceeding Watt rating of CC

snuffy
snuffy Solar Expert Posts: 72 ✭✭✭✭
in General Solar Power Topics #1
What's the problem with exceeding the wattage rating of a CC with a solar array? As long as the Voc of the array doesn't exceed the CC limits I'd think the CC would just limit the current irregardless of what the array was capable of producing. Outback cautions against exceeding the wattage their controllers are rated at but Xantrex seems to have no such limitations other than saying the current is limited to the max rating of their CC.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #2
    Re: Exceeding Watt rating of CC

    Well I have just over 1000 watts pv connected to my MX-60 feeding a 12 volt system for 3 years now and so far, it has handled it. I know I'm pushing it though. In "normal" operation it does limit output current to the adjustable limit. In my case just a wee bit over 60 amps. HOWEVER - - especially in winter with sun reflecting off snow, or cloud edge effect conditions, and during sweep mode, I've seen the NX pumping out over 70 amps! I think one time I saw 74 amps! Granted that only lasted for a few seconds while the sweep mode was passing through that max point, but it does happen, and indicates to me that the current limit is not in effect during sweep mode. Certainly the MX is well designed as it handled such an over current without issuing smoke, but if it gets killed, I'll have no one to blame but myself.
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  • snuffy
    snuffy Solar Expert Posts: 72 ✭✭✭✭
    #3
    Re: Exceeding Watt rating of CC

    Thanks! Apparently the current limit is not independent of the processor then. I was thinking there would be an independent current clamping circuit. From your experience then the mx 60 anyway, has some tolerance for overcurrent. The most I've seen so far on my FM80 is 76 amps but I expect when winter hits I'll see it hit 80 or more.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Exceeding Watt rating of CC

    The maximum charge current level can be programmed to values less than peak so that if for some reason your array is capable of pushing 60 Amps output from the MX/FM and your battery is only 200 Amp hours you don't melt the plates in one go. It's in the manual. Somewhere. :roll:
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #5
    Re: Exceeding Watt rating of CC
    Cariboocoot wrote: »
    The maximum charge current level can be programmed to values less than peak so that if for some reason your array is capable of pushing 60 Amps output from the MX/FM and your battery is only 200 Amp hours you don't melt the plates in one go. It's in the manual. Somewhere. :roll:

    And with the MX at least, adjusted in the other direction as well. Mine if I remember right, can be adjusted to 65 amps max, although it's not really recommended.
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  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    #6
    Re: Exceeding Watt rating of CC
    Cariboocoot wrote: »
    The maximum charge current level can be programmed to values less than peak so that if for some reason your array is capable of pushing 60 Amps output from the MX/FM and your battery is only 200 Amp hours you don't melt the plates in one go. It's in the manual. Somewhere. :roll:
    Yes I was reading the manuals I have for 2 unused MX 60 charge controlers I have. In the setup menu you can set the output amp output limit up to 70 amps if you want to or limit it to less than 60 amps. :Dsolarvic:D
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Exceeding Watt rating of CC
    waynefromnscanada wrote: »
    And with the MX at least, adjusted in the other direction as well. Mine if I remember right, can be adjusted to 65 amps max, although it's not really recommended.

    There are different revisions, of course.
    I remember laughing when reading my manual: it says something like "The maximum output current can be adjusted up to 70 Amps for those who know what they're doing and actually read the manual." :p

    The initial start-up involves "the Vulcan mind-probe method" too!
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  • snuffy
    snuffy Solar Expert Posts: 72 ✭✭✭✭
    #8
    Re: Exceeding Watt rating of CC

    Must be something to do NEC then.
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  • GreenerPower
    GreenerPower Solar Expert Posts: 264 ✭✭✭✭✭✭
    #9
    Re: Exceeding Watt rating of CC

    That is to keep the heat generated within design parameters of the CC (its heat dissipating capacity including running fan).
    If the bulk charging at 14.5V@60A, that's 870W (CC spec'ed at 800W for 12V). If the charging current limit is set for 70A, that's 1015W, the CC would get hot real quick. 48V system is much worse.
    So, if your PV is capable to deliver such bulk charging current, you might want to program to limit the bulk charging current down.

    GP
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  • snuffy
    snuffy Solar Expert Posts: 72 ✭✭✭✭
    #10
    Re: Exceeding Watt rating of CC
    GreenerPower wrote: »
    That is to keep the heat generated within design parameters of the CC (its heat dissipating capacity including running fan).
    If the bulk charging at 14.5V@60A, that's 870W (CC spec'ed at 800W for 12V). If the charging current limit is set for 70A, that's 1015W, the CC would get hot real quick. 48V system is much worse.
    So, if your PV is capable to deliver such bulk charging current, you might want to program to limit the bulk charging current down.

    GP

    Kinda what I was thinking too. Control the heat/current at the CC and not at the current source. If I can control the current flow through the CC what difference does it make how much current the current source can supply?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #11
    Re: Exceeding Watt rating of CC

    actually, the nec wants you to limit the current to under 80% of the mx60's rating. it is approved for the 70a when one performs the mod so this is .8 x 70a = 56a. without the mod the nec wants it limited to 80% of 60a or 48a even though the 70a is a software mod. i can see limiting to the rated current of the cc as they are meant to handle that much without the need to derate. the wattage rating these controllers handle is arbitrary and a rough guess based on the current and nominal battery voltage and need not be taken literally as being overpowered when at 14.5v or 29v. the real limit is the specified current limit of the cc and it does hold it fairly well to its own abilities without the nec's need to interfere and regulate it, but should they feel this need it should be the full limited current rating of the cc as it was designed to handle and not to be derated because the nec feels the need to inhibit the industry with some of his (yup, his) asinine rule making.
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