Which MPPT Charge Controller for a 24 volt system?

RickeyH
RickeyH Solar Expert Posts: 39 ✭
in Off Grid Solar & Battery Systems #1
I'm in the process of buying 6 Daqo 190w solar panels (specifications as below).

My power needs will be approx. 1.0kwH per day but will increase in the future to around 1.5kwH per day. The panels will be on a shed and will initially be used for lighting (12v LED mainly) and TV with some water pumping required at a later date.

Exact details of solar insolation for our site are not available but using Melbourne which I'm guessing is similar gives the following. I'm actually located in Cassilis, Victoria, Australia.

Month Insolation,*kWh/m²/day
1 6.15
2 5.65
3 4.34
4 3.07
5 2.12
6 1.66
7 1.81
8 2.52
9 3.40
10 4.60
11 5.63
12 6.07

The pitch of the roof is a bit flatter than ideal and we also have issues with the sun disappearing behind a mountain around 2:45pm this time of year. This is the reason I have gone for more panels than our power needs would suggest is necessary. We are also trying to avoid running a generator as much as possible. (Panels are $200 each, so very reasonably priced). My thinking is that by increasing the number of panels relative to my power needs I will also need less battery storage.

Daqo DQ190MFAa Series, 190W mono-crystalline module

Electrical Characteristics
Maximum power at STC - Pmax 190 W
Open-circuit voltage - Voc 45.5 V
Optimum operating voltage - Vmp 36.5 V
Short-circuit current - Isc 5.3 A
Optimum operating current - Imp 5.21 A
Operating temperature 45 °C
Series Fuse Rating 10 A
Maximum system voltage 1000 VDC

Temperature Coefficients
Short circuit current temperature coefficient - 0.06 % / K
Open circuit voltage temperature coefficient - 0.31 % / K
Peak power temperature coefficient -0.40 % / K

Are you able to advise what size Charge Controller I will need (I'm thinking 60amp). I do not expect to be adding more panels in the future.

Please let me know if you need further information.

Thanks

Rickey
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Which MPPT Charge Controller for a 24 volt system?

    G'day, mate! :D

    A couple of things. First being; are you sure about those loads? 1.5kW hours per day is not much for most people.

    Second, normally you size the battery bank to meet the load requirements and then size the array to recharge the batteries. Trying to rely on "surplus panel power" is not a good idea. You generally want that power to be available "on demand" not just when it's available.

    From that perspective you'd be looking at about 250 Amp hours of battery for a 25% DOD. That would require about 780 Watt array.

    What you've got (or are going to get) is 1140 Watts, which we'd normally expect to be good for 365 Amp hours @ 25 Volts.
    Keep in mind that the PV Watts figures are really designed for GT systems where every Watt can be harvested and sent somewhere. This is not the case with off-grid systems.

    At any rate, your proposed array (or the one I suggested) will not exceed 40 Amps charge current. If you do not add more panels the TriStar 45 MPPT would work for that. Any of the 60 Amp controllers would also work. But if your load numbers are correct and you "downsize" a bit you could even use the Rogue 3024.

    You can probably overcome the roof pitch issue by simply adjusting the mounts so the panels are closer to ideal insolation. The loss of power from the mountain should not be an issue if you use the battery-based calculation: the bank will supply the needs, the panels should be able to recharge it before 2:45 PM.

    The 12 Volt light: will this be DC or AC fixtures? Usually if you have 120 or 240 VAC for some things it is easier to use it for everything.

    Water pumping: advance warning! That can be a big power user, depending on the rate of flow, pressure, and mostly depth the water has to be drawn from.

    How we doing so far?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: Which MPPT Charge Controller for a 24 volt system?

    Welcome to the forum Rickey,

    Will this be a 12 volt battery bank or 24/48 (have you chosen an inverter yet)?

    Using are typical deratings and assuming an MPPT charge controller:
    • 6 * 190 watt panels * 0.77 panel+controller losses * 1/14.5 volts charging = 60.5 amps @ 12 volt bank
    • 6 * 190 watt panels * 0.77 panel+controller losses * 1/29.0 volts charging = 30.3 amps @ 24 volt bank
    • 6 * 190 watt panels * 0.77 panel+controller losses * 1/58.0 volts charging = 15.1 amps @ 48 volt bank

    So, the higher the battery bank voltage, the lower the current required--And smaller/less expensive charge controller.

    Normally, I like to work from loads and then design the system. Say you want to run 1.5kWH per day, and run without a genset for ~9 months of the year (note that PV watts does have an hour by hour calculation, so you can see how much power you may lose after 2:45 pm at this time a year).

    Assuming you are running an inverter with 0.85 losses, 1-3 days of "no sun" (assume 2 days nominal for this calculation), and 50% maximum battery bank discharge using a 12 volt battery bank (since it appears that is your current choice):
    • 1,500 Watt*Hours * 1/12 volt battery * 1/0.85 inverter eff * 2 days of no sun * 1/0.50 max discharge = 588 AH @ 12 volt battery bank

    Next, size the array two different ways. First on 5-13% rate of charge for battery bank, 0.77 typical panel+controller deratings:
    • 588 AH * 14.5 volts charging * 1/0.77 panel+controller losses * 0.05 rate of charge = 554 Watt "minimum" solar array
    • 588 AH * 14.5 volts charging * 1/0.77 panel+controller losses * 0.10 rate of charge = 1,107 Watt "nominal" solar array
    • 588 AH * 14.5 volts charging * 1/0.77 panel+controller losses * 0.13 rate of charge = 1,439 Watt "maximum cost effective" solar array

    Your array of 6*190watts= 1,140 watts falls in the nominal/healthy sized array.

    Next, how much power will such an array generate... Using your "hours of sun" per day, toss the bottom 3 months, we get 2.52 hours of sun per day (assuming shading by mountain is not significant for September). Use 0.52 as panel to charge controller to battery bank to inverter to load losses (yes, 52% is typical "worst case" for a flooded cell + inverter system in warm weather):
    • 1,500 Watt*Hours per day * 1/0.52 system derating * 1/2.52 hours of sun per day = 1,146 Watt solar array minimum

    So, assuming around a 588 AH 12 volt battery bank and 1.5kWH of load with ~1,140 watt panel--Every thing looks very nicely balanced.

    With battery banks, there is a recommended continuous discharge and a maximum surge discharge rate... So, using C/8 as the max continuous discharge and C/2.5 as the max surge (for flooded cell battery bank):
    • 588 AH * 12 volts * 1/8 rate of discharge * 0.85 inverter eff = 750 Watt max continuous AC load
    • 588 AH * 12 volts * 1/2.5 rate of discharge * 0.85 inverter eff = 2,339 Watt max surge AC load

    Many good quality inverters will supply upwards of 2x rated output for a few seconds... So your Optimum AC inverter (without regard for your actual loads) would be around 750 watts to (2,400/2=) 1,200 Watt inverter range...

    And, for a 12 volt system, I would be recommending about a 1,200 watt maximum output inverter anyway (if you need to run an efficient AC fridge, you may need ~1,500 watt inverter minimum).

    Anyway, those are the rules of thumb to start your design... If you need to make changes (AGM batteries can supply more surge, are a bit more efficient, and can tend towards 5% rate of charge if you are not using as much power per day).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • RickeyH
    RickeyH Solar Expert Posts: 39 ✭
    #4
    Re: Which MPPT Charge Controller for a 24 volt system?

    Thanks Cariboocoot, this is exactly the sore of information I'm after. Loads are reasonably accurate - the shed is 1 bay of a 3 machinery shed so lighting requirements are not huge. Cooking is via a butane stove or on top of the wood heater. No heating or cooling requirements - a 40litre Engel fridge and a small TV.

    The water pumping at later will be from a tank to a header tank approx. 80 metres away but 15 metres higher and can be timed to occur when we have plenty of sun.

    The battery system will be 24 volt but I will use a DC to DC converter to run 12 volt lighting. (24 volt to minimise cost of wiring and 12 volt due to ready availability of 12V LEDs etc. ) The only time we would use a small inverter (400 watt) is to charge some cordless tools.

    We are planning on living in the shed for up to 3 days at a time whilst we work on building the house.

    I'm thinking later on if we are struggling with power from the array we can then look at putting tilt legs to optimise the angle.

    As with most people we are trying to do this as cheap as we can.

    Are you suggesting at we could even go down to a 40amp MPPT?

    Thanks for your help.

    Rickey
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  • RickeyH
    RickeyH Solar Expert Posts: 39 ✭
    #5
    Re: Which MPPT Charge Controller for a 24 volt system?

    Thanks Bill, we are looking at a 24volt battery bank using 8 Trojan 105REs to give us a 450ah battery bank. What do you think of that configuration?

    We have a small 400 watt inverter that we use for charging cordless power tools and don't envisage on needing much more than that. Our fridge is a 40 litre Engel 12/24V

    Do you concur with Cariboocoot regarding the size of the charge controller?

    Regards

    Rickey
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Which MPPT Charge Controller for a 24 volt system?

    What we'd normally expect from an 1140 Watt array for 24 Volts:

    1140 * 0.77 (typical derating) = 877 Watts / 24 Volts (minimum battery Voltage) = 36 Amps, which is 10% peak charge current on a 360 Amp hour 24 Volt battery bank. Not likely to produce over 40 Amps ever.

    Using the same array/battery bank and applying some of the other rule-of-thumb formuli:

    1140 Watts * 4 hours (minimum) equivalent good sun = 4560 Watt hours * 0.52 over-all system efficiency = 2371 Watt hours AC daily potential.

    360 Amp hours * 0.25 (25% DOD) = 90 * 24 Volts = 2160 Watt hours DC * 0.90 (inverter efficiency) = 1944 Watt hours AC (this is actually good to have the battery potential lower than the array potential).

    This all looks like it will meet your needs, but remember that it's just rules-of-thumb; hot panels/batteries produce less, loads can be higher than anticipated, and the weather may not co-operate every day. As such, it's always good to have a back-up plan (generator).
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    Re: Which MPPT Charge Controller for a 24 volt system?

    Going back to the rules of thumb, changing from 12 volt to 24 volt battery bank would become a 294 AH @ 24 volt battery bank (double the voltage, 1/2 the AH for the bank... Power=Volts*Amps).

    A 450 AH @ 24 volt battery bank is about 1.53x larger--So the size of the solar arrays should be about 1.5x larger too.

    I am a fan of using single string of batteries when ever possible... A Trojan L16RE-A 325 AH @ 6 volt Deep Cycle Battery would be a suggestion.

    If you want the larger AH, then a Trojan L16P-AC 6 Volt, 420 AH Deep Cycle Battery would be a possibility.

    A single string of batteries--fewer cells to water. Less wiring. Less to go wrong (fewer connections, paralleling battery strings need to be wired to keep current flow evenly shared between strings, each string should have a fuse/breaker, cell failures (open/shorted) are easier to diagnose with a single string, etc.).

    If you do install parallel strings, and just in general, I would suggest a DC Current Clamp Meter (Sears in the US sells one that is good enough for a reasonable price).

    Personally, I would only use the larger battery bank if you need more power and install a ~53% larger solar array (assuming using flooded cell battery bank).

    Also, I would suggest some sort of Battery Monitor (Victron is also a good product) if your loads are "variable" and/or others will be using the system. Much easier than measuring specific gravity and/or monitoring bank battery voltage (day/night/loaded/unloaded all have different voltage operating points).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • RickeyH
    RickeyH Solar Expert Posts: 39 ✭
    #8
    Re: Which MPPT Charge Controller for a 24 volt system?

    Thanks Bill, a Trojan L16RE A 325AH battery in Australia costs around $600. A T105RE 225AH costs $225. I'm thinking 2 parallel strings of 4 T105REs in series to give me 450AH at 24 volts might be a more economical way to go.

    Rickey
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #9
    Re: Which MPPT Charge Controller for a 24 volt system?

    cost considerations are always a factor and i can understand that. if you go with parallel battery strings, be sure to make each string identical to the other to keep things equal. i often advise to oversize the battery interconnections (much thicker wire) as this does help the bank in many ways, but keeping the same identical resistivity between each string helps to insure that one string won't be doing more of the charging/discharging than the other, which would lead to the earlier demise of the overworked string. this is accomplished by making sure if, for example, one string has a total length of wire at 2.3ft then the other string must have that same length for the same gauge of wire.
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  • RickeyH
    RickeyH Solar Expert Posts: 39 ✭
    #10
    Re: Which MPPT Charge Controller for a 24 volt system?

    Thanks Niel, what gauge wire do you suggest for the battery interconnections and between the batteries and the charge controller. I will ensure that lengths and gauges are identical for each string.

    Rickey
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #11
    Re: Which MPPT Charge Controller for a 24 volt system?

    good question and i find that there's no easy way for me to answer this for the batteries and it could be that i'm just too tired right now to think properly. i can say that the general idea is to make the individual batteries you are arranging seem as one battery with very very little loss so that it is part of the total circuit allowing current to pass.

    i found this chart,
    http://www.powerstream.com/Wire_Size.htm
    and you can probably go by the max amps for power transmission section to cross reference for the wire size, but i like to give extra room so go to the next larger wire size for yours. i should also mention this should be as short as possible with total wire lengths from bus to bus (where combining both strings' - and +) of less than 10ft of wire. this would be less than about 3 meters of wire and preferably less than 2 meters of wire noting that i keep forgetting you are metric. the link also mentions metric sizing too, but the standard sizes the metric system may utilize would have to be looked up. round up to the larger size of wire when applicable to insure headroom.

    i should also mention that the interconnecting wires for the batteries should not be smaller than the largest wire used anywhere else on the system. often times for us it is specified for the inverter what that size is, but you will have multiple loads and a higher current requirement leading to the need for a much larger wire than the inverter calls for.

    i also thought i should say that the charge amps can sometimes exceed the amps needed by the loads and in this case you should go by the charging amps which is the higher of the 2 ampacity amounts.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #12
    Re: Which MPPT Charge Controller for a 24 volt system?

    as far as the length from the batteries to the cc goes you can't just consider this for you should consider the whole thing from the source to the load (pv to battery and some prefer pv to ac load or converter load) and keep it under 2% voltage dropped with some possibly compromising to 3% v drop. this is a rough compromise between losses and costs, but if any future expansions are planned for then plan for them ahead of time and go with the larger wire you'd need down the road now, instead of later, and if one can go with larger wire that affords even smaller v drop %s then it is to your advantage as far as efficiency goes.

    v drops are an important consideration in any system and are most critical for cc to battery lengths and interconnected batteries. (these interconnections should be considered as part of the extended length added to the path to the cc from the batteries) we often advise to keep v drop percentages from the cc to the batteries at 1% or less due to v drops fooling the cc into thinking the batteries are at a higher voltage than they actually are due to the lost voltage in the wires. it does get complicated for some instances and it can seem very complex to somebody unfamiliar with electricity or electronics.

    the link i gave before for power stream consisted of a v drop calculator, but there are other ones out there that may be better to use. i have one in my sigline i believe,
    http://www.nooutage.com/vdrop.htm
    but both of these are not geared to metric sizing.
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  • RickeyH
    RickeyH Solar Expert Posts: 39 ✭
    #13
    Re: Which MPPT Charge Controller for a 24 volt system?

    OK, I'm looking at getting the TRACER-4215RN 40A, 12/24V MPPT Solar Charge Controller. http://www.apolloenergy.com.au/Renewable-Energy-Components/Charge-Controllers/TRACER-2215RN_2 Have actually found it at AUD$175.40 excluding the meter.

    What do you think? Will it do the job?

    The next question I have is regarding the wiring of the solar panels. What do you suggest? 2 strings of 3? 3 strings of 2? Series/Parallel? Can you point me to a thread or link which discusses the science behind the different configurations. Trying to make sense of this for myself rather than just be given answers.

    Thanks

    Rickey
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #14
    Re: Which MPPT Charge Controller for a 24 volt system?

    I`m not familiar with the Tracer product so can`t offer an opinion on it.

    As far as panel configuration goes, the input limit on that controller (and many others) is 150 Volts. Your panels have a Voc around 45; three in series would be very close to the input limit. One cold day and the controller shuts down, or worse. So that alone limits your configuration to no more than two in series.

    Besides that factor there is the efficiency of the controller to consider: the higher the array Voltage is above system Voltage the more work the controller has to do to down convert and the lower the efficiency. Normally you`d go for a nominal array Voltage no more than 2X the nominal battery Voltage.

    As it happens I did write a thread post about this: http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller
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  • RickeyH
    RickeyH Solar Expert Posts: 39 ✭
    #15
    Re: Which MPPT Charge Controller for a 24 volt system?

    So, just to check that I've got it. 2 panels in series = 3 strings. Each string will have it's own circuit breaker. 10amp? Connect the 3 strings in parallel through a combiner box and then to the charge controller.

    2 panels = 90 VOC. Battery bank will be 24 volts. I know the charge controller will handle this but 'Normally you`d go for a nominal array Voltage no more than 2X the nominal battery Voltage.' Will this be a problem?

    Thanks for you help
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #16
    Re: Which MPPT Charge Controller for a 24 volt system?

    90 Voc isn`t an issue on the controller efficiency, only for staying below it`s input maximum.
    Your panel Vmp is 36.5 which qualifies as a `real`24 Volt panel. The array Vmp would be 73, and should be fine for a 24 Volt system (it would work for 48 also).

    So three strings of two in series looks like the right configuration to me. Since the panels give you a spec of 10 Amps on the series fuse rating that`s the value to go with.
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  • petertearai
    petertearai Solar Expert Posts: 471 ✭✭✭✭
    #17
    Re: Which MPPT Charge Controller for a 24 volt system?

    Looks similar to my system, I would certainly go 24 volts if starting again.Also go for a outback fm80 or similar as it gives you more options. I pump similar to you, it works well.I have a timer set to pump for 2 hours midday, tu wed and Thursday while not in residence. If there for a week or 2 I will pump if needed and once the batterys are in float.
    My pump is a 12 volt sureflow, been running for 7 years now. They make a 24 volt one as well. All the best with your solar. Regards Peter
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home 
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  • RickeyH
    RickeyH Solar Expert Posts: 39 ✭
    #18
    Re: Which MPPT Charge Controller for a 24 volt system?

    Thanks Peter and Cariboo. I appreciate all the assistance I get from this forum. Great to see people prepared to share their knowledge.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #19
    Re: Which MPPT Charge Controller for a 24 volt system?
    Cariboocoot wrote: »
    Besides that factor there is the efficiency of the controller to consider: the higher the array Voltage is above system Voltage the more work the controller has to do to down convert and the lower the efficiency. Normally you`d go for a nominal array Voltage no more than 2X the nominal battery Voltage.

    As it happens I did write a thread post about this: http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller

    RickeyH, I confirmed Cariboocoot's advice with some specific calculations that are not too different from your setup:
    http://forum.solar-electric.com/showthread.php?15907

    The key to making the calculation is the resistance of the cable between the combiner and the charge controller. If that cable is thick enough and short enough (i.e. low enough resistance) you are better off with 3 parallel strings of two. If you have a high resistance cable you may be better off with 2 parallel strings of three, but that is just making the best of a bad situation.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • RickeyH
    RickeyH Solar Expert Posts: 39 ✭
    #20
    Re: Which MPPT Charge Controller for a 24 volt system?

    Thanks vtMaps. The cable that is attached to the panels is 4.00mm squared and 900mm (3 feet) long. So, between 10 and 11 AWG. The distance between the panels and the combiner box will be about 20 feet. What gauge wire do you suggest to connect the panels to the combiner box. I was thinking 6AWG but given that the first 3 feet is only 11AWG is there any point going the thicker cable?

    Rickey
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #21
    Re: Which MPPT Charge Controller for a 24 volt system?
    RickeyH wrote: »
    Thanks vtMaps. The cable that is attached to the panels is 4.00mm squared and 900mm (3 feet) long. So, between 10 and 11 AWG. The distance between the panels and the combiner box will be about 20 feet. What gauge wire do you suggest to connect the panels to the combiner box. I was thinking 6AWG but given that the first 3 feet is only 11AWG is there any point going the thicker cable?

    A two panel string is 20 ft from combiner. 40 ft of 11 gauge copper has a max resistance of 0.0514 ohms. At full power you would have 5.2 amps running through that cable. Thus volt drop at full power could be 0.267 volts and lost power could be 1.391 watts per string.

    How far is your combiner from the controller? You will have 15.6 amps running through that cable.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • RickeyH
    RickeyH Solar Expert Posts: 39 ✭
    #22
    Re: Which MPPT Charge Controller for a 24 volt system?

    My intention is to have the combiner on the same 'board' as the controller. So, within a matter of inches.

    So, if I upped the size of the wire from the panel connector would that help in terms of power loss or would the first 900mm make that pointless?

    Thanks

    Rickey
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #23
    Re: Which MPPT Charge Controller for a 24 volt system?
    RickeyH wrote: »
    So, if I upped the size of the wire from the panel connector would that help in terms of power loss or would the first 900mm make that pointless?

    Thicker wire will help with power loss, but your loss with 11 gauge is not a big deal. Your loss for three strings is about 4.2 watts out of 760 watts, and that's only at full power. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • RickeyH
    RickeyH Solar Expert Posts: 39 ✭
    #24
    Re: Which MPPT Charge Controller for a 24 volt system?

    Thanks vtMaps. Appreciate your help.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #25
    Re: Which MPPT Charge Controller for a 24 volt system?
    RickeyH wrote: »
    My intention is to have the combiner on the same 'board' as the controller. So, within a matter of inches.

    BTW, it is more typical for the combiner to be very near the panels, then the 'home run' from combiner to controller is a single heavy cable. Most panel cables are not very thick (they are not designed for long runs to the combiner). --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • RickeyH
    RickeyH Solar Expert Posts: 39 ✭
    #26
    Re: Which MPPT Charge Controller for a 24 volt system?

    OK, so it would be a better idea to have the combiner in the 'attic' (not that I will have one as such) and then run down to the controller on the wall mounted panel?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #27
    Re: Which MPPT Charge Controller for a 24 volt system?

    although it is customary to combine near the pvs, this does not always have to be if it's too inconvenient. i say this because the breakers/fuses will also be there and if it should pop or trip one would want it to be somewhat accessible to re-fuse it or flip the breaker back on. all that would need be done is to provide a good wire size that is now extended to keep v drops down and a properly sized raceway for the wires to travel in. this allows a more convenient place for combining string outputs and fusing/breakers.

    the only time this could be a problem is when ordinances mandate a single cutoff switch at the pvs, which is sometimes the case.
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  • ywhic
    ywhic Solar Expert Posts: 621 ✭✭
    #28
    Re: Which MPPT Charge Controller for a 24 volt system?

    Put the combiner up in the attic and a properly rated cutoff box or switch near the charge controller.. solves that problem..

    Heres my setup.. controller dead next to the combiner box.. both will be inside.. my wire run will be a max of 18' to the combiner and pushing 8 amps per string at 24v though... I have 6 breakers.. though only 3 will be used at this time (for 3 strings of 2 panels).. (others for expansion)..

    Controller.jpg
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  • RickeyH
    RickeyH Solar Expert Posts: 39 ✭
    #29
    Re: Which MPPT Charge Controller for a 24 volt system?

    So if I put the combiner next to the controller for ease of access (which makes sense to me), 'The cable that is attached to the panels is 4.00mm squared and 900mm (3 feet) long. So, between 10 and 11 AWG. The distance between the panels and the combiner box will be about 20 feet. What gauge wire do you suggest to connect the panels to the combiner box. I was thinking 6AWG but given that the first 3 feet is only 11AWG is there any point going the thicker cable?'

    Thanks

    Rickey
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  • ywhic
    ywhic Solar Expert Posts: 621 ✭✭
    #30
    Re: Which MPPT Charge Controller for a 24 volt system?

    If your doing 2 panels in series.. you'd be at 48v and 5.X amps..

    With 10 AWG wire and 20 feet and 6 amps.. you'd be 0.51% voltage drop.. well below the 2-3% most talk about..

    at 40' feet you would be at 1.03% loss.. still below the 2-3% loss most talk about...

    If you just doing 6 panels at 24v each into a combiner..

    At 20 feet you would be at 1.03% loss.. below the 2-3%..

    at 40 feet you would be at 2.05% loss. below the 2-3%..

    In my opinion.. keep the 10/11 AWG wire and get some more and be done with it.. especially if you can keep the run between 20-40 feet..
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  • RickeyH
    RickeyH Solar Expert Posts: 39 ✭
    #31
    Re: Which MPPT Charge Controller for a 24 volt system?

    Thanks for the quick reply. Sounds like a plan.
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