Simple question, 24v panels in a 12v system?

jianelli
jianelli Registered Users Posts: 3
in General Solar Power Topics #1
Looking to upgrade panels & wondered about using 24v panels. I have an mppt ch controller, older Xantrex inverter & 900 ah batteries, ( 2 6v). I have 3 120 w panels on the roof... Can the charge controller deal with the higher voltage from 24v panels? Thanks for any help... John
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
    Re: Simple question, 24v panels in a 12v system?
    jianelli wrote: »
    Looking to upgrade panels & wondered about using 24v panels. I have an mppt ch controller, older Xantrex inverter & 900 ah batteries, ( 2 6v). I have 3 120 w panels on the roof... Can the charge controller deal with the higher voltage from 24v panels? Thanks for any help... John

    Welcome to the forum.

    I don't understand what you're trying to do... are you adding panels to your existing three panels, or replacing your three panels? How are your three panels configured (series or parallel)? I will need to know the Voc, Vmp, and Isc of your new panels (and your old panels, if you plan to use them with the new panels).

    What brand and model charge controller do you have?

    I presume your system voltage is 12 volts... but I am curious what batteries (brand and model) you have... 900 ah at 12 volts is a lot of battery for 360 watts of panels.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #3
    Re: Simple question, 24v panels in a 12v system?

    What is the voltage produced by the panels ? 24V or 35V ?

    A 24V battery needs 35V panels. A 12v battery needs about 19V of panels

    The sticker on the back of your panel should have a number for V max power (Vmp) or some variation of the phrase.Attachment not found.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • jianelli
    jianelli Registered Users Posts: 3
    #4
    Re: Simple question, 24v panels in a 12v system?

    Sorry for the poor info. I was looking to upgrade & replace the 3 120w panels with 4 250w, while keeping the 12v batteries (they are rolls Surrette). The charge controller is by BZ Products, an off brand I guess... I am off grid, but use a propane fridge right now so my power needs are very low..
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #5
    Re: Simple question, 24v panels in a 12v system?
    jianelli wrote: »
    Can the charge controller deal with the higher voltage from 24v panels?
    jianelli wrote: »
    The charge controller is by BZ Products

    I can't find a BZ products web site to look up the voltage limits... but there are several models listed for sale by various venders, including one with a "hv" suffix, which I presume stands for high voltage.

    I suppose that if you put the four new panels in parallel you will be OK. You will need a combiner box with four circuit breakers. How far will the combiner be from the controller? You may need to upgrade the cable to handle the higher current.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #6
    Re: Simple question, 24v panels in a 12v system?

    The BZ controller needs to be replaced (enough said)--It will not do what you need anyway.

    Your aim point for charging a 900 AH @ 12 volt battery bank with 5% to 13% typical rate of charge:

    900 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 847 Watt array minimum
    900 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,695 Watt array nominal
    900 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,023 Watt array "cost effective" maximum

    Note that the largest commonly available charge controller is the Midnite Classic--Which is good for ~90 amps maximum @ 12 volt output -- Or 10% of your bank capacity.

    Some other questions... Are your batteries still in good shape? 360 Watts is pretty small for charging the battery bank, and if cycled deeply (daily use), may not be enough to keep the batteries well charged. They maybe sulfating over time.

    Also--If you are replacing the array, charge controller, and possibly battery bank--At that size of bank, it may be worth replacing your AC inverter and stepping up to 24 volts--Makes wiring/controllers a lot easier (2x the voltage, 1/2 the current in the wiring).

    But this is all working sideways--What are your desired loads (seasonally, year round?) and roughly where is the installation (amount of sun by season)?

    Sorry--more questions than answers at this point.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jianelli
    jianelli Registered Users Posts: 3
    #7
    Re: Simple question, 24v panels in a 12v system?

    Thanks. I feel like I pulled that loose thread on an old sweater...
    The batteries are in good shape, I keep after them with a Honda 2000i in winter and turn off the inverter when I go to work and sleep. The Xantrex also has an equalization mode that I do every 4 months or so.
    Yes a 24v system is in my 10 year plan, but more $ than I can spend right now.
    I will look into the midnight controller.
    Thanks again, very helpful..
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