Sizing Charge Controller...Need Help!!

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msgieser
msgieser Registered Users Posts: 24 ✭✭
in General Solar Power Topics #1
The more I try to use the various "sizing calculator tools" available online, the more different answers I get. Can someone give me a correct answer in terms of what amp size MPPT controller I need. Here are my module specs:

(3) ET- P672280, 280 watt, 24 volt, panels connected in parallel

Vmp = 36.72V
Imp = 7.63A
Voc = 43.78V
Isc = 7.98A

Battery bank is 12 volts

I wanted to use the Morninstar 45 or 60 MPPT but do I need to go to a 80 amp (Outback) controller???

Any thoughts or suggestions? I appreciate your assistance!!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Sizing Charge Controller...Need Help!!

    You have three 280 Watt panels. Total Watts = 840. You can usually expect an efficiency of about 77%, but even if you skip that derating you get 840/14.4 charging Volts (adjust to fit your batteries) = 58 Amps peak potential current. With the derating you get 647 Watts / 14.4 Volts = 45 Amps peak potential current.

    If you're at a high elevation you can expect the panels to perform better. If you're in cold temps you can expect the panels to perform better. If you're in high temps you can expect them to perform worse. If you're in cloudy/smoggy atmosphere you can expect them to perform worse.

    But it's doubtful the output will ever exceed 60 Amps, and if it does the controller will just clip the extra current.

    The real question is: what are the batteries? You try to size the array and its controller for the battery bank capacity and charge Voltage.
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  • msgieser
    msgieser Registered Users Posts: 24 ✭✭
    #3
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    Re: Sizing Charge Controller...Need Help!!

    My battery bank is 6 Trojan 6volt, T105-RE's, connected series/parallel for a total of 675 amp-hours at 12 volts. Thanks!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
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    Re: Sizing Charge Controller...Need Help!!

    With that battery bank you'd actually want 67 Amps at 14.8 Volts as the peak charging potential. Trojan specifies the higher Voltage and recommends the 10% rate. That would be 991 Watts, or about 1287 Watt array to adjust for 77% efficiency. As it is your panels will produce a peak charge rate closer to 6%, which will drop below the minimum 5% recommended rate for any FLA is any loads draw 1 Amp or more.

    Any chance you can get another panel?

    Any chance you can drop one string of those batteries? (Three strings in parallel can be problematic in keeping current balanced too.)
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
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    Re: Sizing Charge Controller...Need Help!!

    although a 60a controller would most likely suffice for the 3 pvs adding more could put it over the ability of a 60a cc. go with either an fm80 or better yet a classic 150.
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  • msgieser
    msgieser Registered Users Posts: 24 ✭✭
    #6
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    Re: Sizing Charge Controller...Need Help!!

    So...if I only go with 4 Trojan F-105RE's, for a total of 450 ah's, I'll have a better balanced system? Is about 450 ah's the correct number for the 840 watts of panels? This thru a 60 amp charge and I should be in the ball park?

    I realize the 60 amp charge controller will not give me room for expansion, but if, in the future, I add more panels and batteries, I can add another charge controller to
    the mix? Thanks!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
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    Re: Sizing Charge Controller...Need Help!!
    msgieser wrote: »
    So...if I only go with 4 Trojan F-105RE's, for a total of 450 ah's, I'll have a better balanced system? Is about 450 ah's the correct number for the 840 watts of panels? This thru a 60 amp charge and I should be in the ball park?

    "Ballpark" math: 45 Amps (10% of the capacity) * 14.8 Volts (Trojan's preferred) = 666 Watts, less typical 77% derating = 865 Watt array. So yes the two strings of T105's is a better match with the 840 Watts of panel.
    I realize the 60 amp charge controller will not give me room for expansion, but if, in the future, I add more panels and batteries, I can add another charge controller to
    the mix? Thanks!

    You can indeed parallel charge controllers without problem. You can also up system Voltage instead, which is the preferred method of expansion as it reduces DC Amps. Your 675 Amp hours of battery would offer up to about 4 kW hours of power on a 12 Volt system. That's sort of "over the threshold" for 12 Volts. If you really need that much power capacity you should think about a 24 Volt system.

    And thus we come back to the design process: start with the loads and determine what the maximum Watts will be and the total Watt hours. That gives the info needed for picking an inverter and sizing a battery bank. Once you know the Voltage and Amp hours of the battery you can figure out how much panel you need to recharge it. Trying to design from the panels to the loads is backwards and bound to cause headache and heartache.
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  • msgieser
    msgieser Registered Users Posts: 24 ✭✭
    #8
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    Re: Sizing Charge Controller...Need Help!!

    Thanks!! Yes, I realize I'm designing this system backwards, but the panels and a Xantrex 1800 12 volt sine wave inverter were given to me. So, I needed to size a
    battery bank and charge controller to fit. The panels are 24 volt, so if the inverter was, I'd go 24 volts now. As for loads, they will be minimal as it's just a small cabin. I'll monitor my batteries and won't deplete to more than 50 - 60%. I have an 8,000 watt generator for backup and additional battery charging/equalization if needed.

    If I need to use the generator and a charger to bring up the batteries, can I safely bulk charge with an 60 or 80 amp charger...as long as I watch battery temp/voltage/amps, and back off as they come to a full charge? Thanks again!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
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    Re: Sizing Charge Controller...Need Help!!

    Don't discharge the batteries below 50%, or you may soon find they won't recharge - at all.

    There's no problem doing a Bulk charge with a generator powered manual charger - providing you watch it. There are specific chargers that have regulation built in, like this Iota: http://www.solar-electric.com/dls-45.html However you may not want to invest in one of those if future plans may include upping the system Voltage.

    In that vein, remember it can be difficult to find panels with the same specs in a few years' time. If you up the system size in any way you'll need more panels, and may have to connect them with another controller if they're too different from the first set.
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  • msgieser
    msgieser Registered Users Posts: 24 ✭✭
    #10
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    Re: Sizing Charge Controller...Need Help!!

    Coot:

    One more question...you mentioned charging the Trojan T105RE's @ 14.8 volts not 14.4..is that correct?

    I hope these Trojans are the best choice (manufacturer)..at this point I'd purchase whatever history/experience points toward...between Trojan, Surrette, Crown, Deka to size the 12 volt bank at about 450 ah's.

    Thanks
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
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    Re: Sizing Charge Controller...Need Help!!
    msgieser wrote: »
    Coot:

    One more question...you mentioned charging the Trojan T105RE's @ 14.8 volts not 14.4..is that correct?

    I hope these Trojans are the best choice (manufacturer)..at this point I'd purchase whatever history/experience points toward...between Trojan, Surrette, Crown, Deka to size the 12 volt bank at about 450 ah's.

    Thanks

    Trojan specifies 2.45 Volts per cell max for Absorb @ 77F on T105RE: http://www.trojanbatteryre.com/PDF/datasheets/T105RE_Trojan_Data_Sheets.pdf
    That's 14.7 Volts, if you have that precise of a setting available. You can charge them at a lower Voltage, but I've always used the max for the Trojan batteries - and always had good service from them.

    Remember that temperature does affect charging, so a remote temp sensor for the charge controller is a good idea. The Outback, MidNite, and Xantrex controllers (60 Amp and up, MPPT type) all have this option available.
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