Needing 24v install advice

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8lander
8lander Registered Users Posts: 18 ✭
in Solar Beginners Corner #1
Hello all, I'm sure it's been answered before but...
I've got 2 x 245w 24v panels, outback mppt 60 controller, Prowatt sw2000 inverter with a Samlex 300amp MRBF fuse between 4x 150ah AGM batteries.
So far I've got the batteries mounted as well as the inverter with about a foot of 3/0 copper between the two.
But now it's time to install the panels and hook up the controller.
Should I run in series?
What size of breakers do I need and where?
Thanks
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #2
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    Re: Needing 24v install advice

    Is this a 24 volt or a 12 volt battery bank?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • 8lander
    8lander Registered Users Posts: 18 ✭
    #3
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    Re: Needing 24v install advice

    The batteries are 12v 150ah run in parallel, since feeding a 12v inverter.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #4
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    Re: Needing 24v install advice

    You have 24 volt panels running through a Outback controller to a 12 volt battery bank. For best efficiency your panels should be in parallel. This assumes the run from the combiner box to the controller is not very long. If it is very long you might benefit by putting the two panels in series (less voltage drop, but lower efficiency of the controller). --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: 33,457 admin
    #5
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    Re: Needing 24v install advice

    Circuit breaker wise...

    Every cable that leaves the battery bank should have its own fuse/breaker. At a minimum ,the fuse/breaker should be rated 1.25x the maximum continuous load you expect to support--And up to the maximum rating of the wiring.

    For example a 2,000 watt @ 12 volt inverter:

    2,000 watts * 1/0.85 inverter efficiency * 1/10.5 battery cutoff voltage * 1.25 NEC derating = 280 amps minimum wiring/fusing to inverter

    For the batteries... You have 4 batteries in parallel--I would suggest that each battery has a "personal" fuse/breaker of ~140 amps (1/2 of the 280 amp inverter rating) to each battery (many people do not fuse each parallel connected battery--But, by rights, you should--especially if there are more than two parallel battery strings connected).

    3/O copper cable is rated by the NEC for ~165-225 amps max continuous current. NEC is very conservative current rating (inside conduit, ambient temperatures also affect rated current).

    The rating by the ABYC, 3/O cable is rated for 327-385 amps maximum.

    For the charge controller:

    2 * 245 watt panels * 1.25 NEC derating for wiring * 1.25 NEC derating for solar * 1/10.5 volt min battery voltage = 73 amps minimum
    60 amp maximum MPPT charge controller * 1.25 NEC derating for wiring = 75 amps

    So--for the Outback 60 amp MPPT controller, you should use a minimum branch circuit of 80 amp breaker fuse.

    From the solar array, you really do not need any fuses/breakers for 1-2 panels/panel strings in parallel. If you have 3 or more panel (or strings) in parallel, each string should have its own fuse/breaker in series with the string (combiner box with fuses or breakers).

    Some people (and code requires) a breaker or switch between the array and where the cables enters the home.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #6
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    Re: Needing 24v install advice
    BB. wrote:
    For the batteries... You have 4 batteries in parallel

    8lander, I didn't notice (in my earlier response) that your batteries are 12 volt and that you have four in parallel. Parallel batteries are problematic, and you should get yourself a DC clamp ammeter so that you can monitor them for unequal current flow. Read about some of the problems here:
    http://forum.solar-electric.com/showthread.php?14674
    I don't think it was mentioned in that thread, but all of the problems of batteries in parallel are worse with AGM batteries because their internal resistance is lower than other lead-acid batteries. Also, the problems with parallel batteries are worse with 12 volt systems than with 24 or 48 volt systems.

    If you can trade in your inverter for a 24 volt model, put your batteries in a series/parallel configuration, and put your panels in series, you will be much better off.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • 8lander
    8lander Registered Users Posts: 18 ✭
    #7
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    Re: Needing 24v install advice

    Shhhhhhh-ould of researched that parallel battery bank. Now I understand why there's BIG 2v batteries.
    Kinda stuck with what I've got, but will surely keep an eye on them.
    Is there a way I could pull power from all 4 yet only charge two at a time? Or would this not be worth the effort?
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #8
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    Re: Needing 24v install advice

    At this time all you need is to change to 24 V and buy a new inverter, all the rest is compatible.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • 8lander
    8lander Registered Users Posts: 18 ✭
    #9
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    Re: Needing 24v install advice

    I'd like to go to a 24v inverter eventually, but kinda $tuck with the 12v.
    As far as putting the panels in series or parallel, thanks for the advice for par. But what percentage of loss would I get from them being in series (just a rough guess if you've got one).
    Also an estimation of loss using:
    Parallel batteries with 12v inverter with series panels.
    VS.
    Series batteries with 24v inverter with parallel panels
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #10
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    Re: Needing 24v install advice

    What type (mppt vs pwm) charge controller?

    With Vmp>35 volts and 12 volt battery bank, and MPPT controller will only lose 1-2% efficiency at the higher voltage (at most). With PWM, that type of controller will lose about 50% when operated at Vmp~35+ volts on a 12 volt battery bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • 8lander
    8lander Registered Users Posts: 18 ✭
    #11
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    Re: Needing 24v install advice

    Ok so since I've got an mppt controller 1-2% not bad.
    Thanks.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #12
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    Re: Needing 24v install advice

    We are probably talking about ~94.5% efficiency vs ~95.5-96.5% efficiency...

    In the big picture, not a large additional loss. But you may notice the controller running hotter with Vmp-array at the higher voltage (i.e., something like 3.5% into heat sink vs 5.5% into heat sink--almost 2x more heat--worst case estimate).

    So, from an "engineer's" point of view, having good unrestricted airflow will be helpful to keep the electronics cool and happy (do not install controller in a "box", do not put controller near floor or ceiling where airflow may be blocked, etc.).

    Temperature does matter. Thermal cycling (cold at night, hot during day) can damage components. And for every ~10C (18F) increase in temperature, the device will age approximately 2x faster. (the +10C and 1/2 life, and conversely, running -10C cooler and 2x longer life, is a very common engineering rule of thumb and works for many things in day to day life).

    So, overall, running things "cool" and avoiding large temperature extremes will help keep everything working well.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • 8lander
    8lander Registered Users Posts: 18 ✭
    #13
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    Re: Needing 24v install advice

    So I guess my closet location should be rethought.
    Not to mention changing all the lead battery lugs to copper.
    What about tying the output of the inverter to the gen terminals? (Onan got removed)

    B567531F-CF1B-497F-809B-CEF49FB8EB34-5100-000005744CB75CB3_zps3bda88be.jpg
    F11C00FE-668A-4802-B72A-8F1935AA0238-5278-0000059D4ACF3498_zps27779607.jpg
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