Dumb (maybe) questions from a newbie?

Rikerz
Rikerz New Member Posts: 19 ✭
in Solar Beginners Corner #1
I have the Renogy RNG-100D solar panels, my 1st question is: If the VMP is 18.9v and the VOC is 22.5v then how are these panel rated at 12V? I know this is a dumb question for most, but I can't find the answer. Is this correct: If the panels are 100w @12v then the max amps that can be received is 8.33?
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  • 6bicks
    6bicks Registered Users Posts: 7 ✭
    #2
    Re: Dumb (maybe) questions from a newbie?

    Not a dumb question at all.
    12 volt batteries are closer to 13 volts actually.

    Second, you need to input more volts to fill the battery which is why you need a charge controller to keep from "overfilling" your batteries. Most charge controllers take the extra/over voltage and convert it into amps to recharge the batteries quicker.
    Having a panel that puts out 12.7 volts could take a long time to fully charge your battery.

    That is also the reason they are for 12 volt batteries, they will not reach a high enough voltage to charge a 24V system.
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  • PNjunction
    PNjunction Solar Expert Posts: 762 ✭✭✭
    #3
    Re: Dumb (maybe) questions from a newbie?

    A 12v nominal panel usually ranges anywhere from 18 to 22 volts OCV. Yours is on the high end of the scale, which is helpful.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Dumb (maybe) questions from a newbie?

    There are no dumb questions. But I have certainly seen some dumb answers on here before. ;)

    The "12 Volt" rating is the nominal system Voltage. It means these panels are suitable for use with a 12 Volt system through a PWM controller. The Vmp needs to be higher than nominal because a 12 Volt battery actually charges at between 14 and 15 Volts, and you need a bit of "overhead" Voltage to account for losses due to heat and wiring resistance.

    If the Vmp were 16 (as some are) you could end up with too low a Voltage at the battery to achieve full charge. If the Vmp were 30 (as some GT panels are) much of the power of the panel would be lost when using a PWM controller as the Voltage would be 'pulled down' to battery level and you only get the benefit of the current (Imp) @ battery Voltage. Or in other words a panel with Vmp 30 and Imp 7 would act the same as a panel with Vmp 17.5 and Imp 7; a loss of about 87 Watts.
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  • PNjunction
    PNjunction Solar Expert Posts: 762 ✭✭✭
    #5
    Re: Dumb (maybe) questions from a newbie?
    6bicks wrote: »
    Second, you need to input more volts to fill the battery which is why you need a charge controller to keep from "overfilling" your batteries. Most charge controllers take the extra/over voltage and convert it into amps to recharge the batteries quicker.

    Actually, in the typical PWM controller, all it does is limit the absorb voltage to usually about 14.4 to 14.7 volts or thereabouts. To get there when actually attached to a battery, it needs to be at least 2 volts above this, hence the ocv of 16 - 22 volts. During bulk, it is basically doing nothing but physically attaching the panel to the battery. When the absorb voltage is reached, the controller merely limits the voltage, and in fact the battery itself is doing the regulating. By having the absorb voltage locked, the battery current naturally reduces as it finishes the absorb cycle. The controller itself is not doing any current limiting, only voltage limiting.
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  • Rikerz
    Rikerz New Member Posts: 19 ✭
    #6
    Re: Dumb (maybe) questions from a newbie?

    Ok, so my vmp on these panels is 18.9 and they are 100w panels so the amps are 5.29?
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  • Rikerz
    Rikerz New Member Posts: 19 ✭
    #7
    Re: Dumb (maybe) questions from a newbie?
    PNjunction wrote: »
    Actually, in the typical PWM controller, all it does is limit the absorb voltage to usually about 14.4 to 14.7 volts or thereabouts. To get there when actually attached to a battery, it needs to be at least 2 volts above this, hence the ocv of 16 - 22 volts. During bulk, it is basically doing nothing but physically attaching the panel to the battery. When the absorb voltage is reached, the controller merely limits the voltage, and in fact the battery itself is doing the regulating. By having the absorb voltage locked, the battery current naturally reduces as it finishes the absorb cycle. The controller itself is not doing any current limiting, only voltage limiting.

    I have a MPPT controller, Morningstar SS-MPPT-15L 12/24v and have my panels wire to 24v.
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  • PNjunction
    PNjunction Solar Expert Posts: 762 ✭✭✭
    #8
    Re: Dumb (maybe) questions from a newbie?

    Yes, according to the PIE chart. 100w / 18v = 5.29A.

    But that is under BEST conditions. Add a real-world derating factor like .77 and you get 4.07A, which is what I'd use for headroom in long term calculations. However depending on the quality of the panel, those ratings might be a minimum low figure, and you could find that yes, you are indeed getting 5.29A from them in good conditions.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Dumb (maybe) questions from a newbie?

    Panels are usually capable of their Imp rating as current is affected mainly by insolation, not so much by temperature. Voltage is what drops off as the panels heat up.
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  • Rikerz
    Rikerz New Member Posts: 19 ✭
    #10
    Re: Dumb (maybe) questions from a newbie?

    Ok, got that down. Now, wiring my panels(I have 2) to 24v (series) increases volts but not amps, so we are still at 5.29a on a good day? As you can see I am starting with my panels and working my way down in questions.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Dumb (maybe) questions from a newbie?
    Rikerz wrote: »
    Ok, got that down. Now, wiring my panels(I have 2) to 24v (series) increases volts but not amps, so we are still at 5.29a on a good day? As you can see I am starting with my panels and working my way down in questions.

    On the input side of an MPPT controller you would have 2X the Vmp and 1X Imp.
    The output side becomes a bit trickier as it depends on battery Voltage and other factors: the charge controller will adjust Voltage and current to try and achieve the best power (Maximum Power Point) for the conditions on the output at the time.
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  • Rikerz
    Rikerz New Member Posts: 19 ✭
    #12
    Re: Dumb (maybe) questions from a newbie?
    Cariboocoot wrote: »
    On the input side of an MPPT controller you would have 2X the Vmp and 1X Imp.
    The output side becomes a bit trickier as it depends on battery Voltage and other factors: the charge controller will adjust Voltage and current to try and achieve the best power (Maximum Power Point) for the conditions on the output at the time.
    Ok, thanks for the info! Next question please; I was told that I didnt really need to fuse between my 2 panels and the charge controller, correct? But when it comes to fusing, this is what confuses me the most. My panels are 24v and still around 5.29a, how do you determine the correct size fuse if I did want to fuse it? I know that 5.29a isnt much but let say when I double my panels I will then have 24v, 400w and then the amps will be near 11, should it then be fused with a 15a fuse and if so how did you come up with a 15a and not a 20 or 30a fuse? Does one fuse close to the amps that is being received without going over too much? Wheeeeew...
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: Dumb (maybe) questions from a newbie?

    The panels should have a "series fuse rating" for them. Go with that. If the two are in series it would only be one fuse because of the current rules.

    You don't really need it because the most current a panel can produce is its Isc rating (for the nit-pickers: plus a bit if insolation is increased by such means as reflection). So if the wiring is designed to handle that there can never be an over-current situation.

    Now, the reason why you do need it with three or more in parallel is because if one panel should become a conductor (i.e. short) it can still handle its current maximum but it can't handle the 2X Isc available from two panels in parallel with it.

    So say you have four panels is parallel, each with an Isc rating of 6. One shorts; it is now like a piece of wire. The other three panels can feed 3*6 Amps to it, or 18 Amps. Now if they all have their own series fuse/breaker (which would probably be rated at 10 or 15 Amps) that will trip before any part of the panel overheats and ignites. Yes, it can happen.

    Now want to take a trip to wonderland? Some panels have a much higher series fuse rating than Isc! This is because their current output is quite small but the materials they are made of can handle more current than they can produce.

    All over-current protection is based primarily on the size of the conductor and how many Amps it can handle. You want the fuse/breaker to be the 'weak link' in the chain if something goes wrong. That said, such circuit protection also needs to be able to handle the Voltage. If you put a fuse rated for 10 A @ 30 Volts on an array string that produces >30 Volts it may not safely interrupt the Voltage and arcing could occur.
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  • Rikerz
    Rikerz New Member Posts: 19 ✭
    #14
    Re: Dumb (maybe) questions from a newbie?
    Cariboocoot wrote: »
    The panels should have a "series fuse rating" for them. Go with that. If the two are in series it would only be one fuse because of the current rules.

    You don't really need it because the most current a panel can produce is its Isc rating (for the nit-pickers: plus a bit if insolation is increased by such means as reflection). So if the wiring is designed to handle that there can never be an over-current situation.

    Now, the reason why you do need it with three or more in parallel is because if one panel should become a conductor (i.e. short) it can still handle its current maximum but it can't handle the 2X Isc available from two panels in parallel with it.

    So say you have four panels is parallel, each with an Isc rating of 6. One shorts; it is now like a piece of wire. The other three panels can feed 3*6 Amps to it, or 18 Amps. Now if they all have their own series fuse/breaker (which would probably be rated at 10 or 15 Amps) that will trip before any part of the panel overheats and ignites. Yes, it can happen.

    Now want to take a trip to wonderland? Some panels have a much higher series fuse rating than Isc! This is because their current output is quite small but the materials they are made of can handle more current than they can produce.

    All over-current protection is based primarily on the size of the conductor and how many Amps it can handle. You want the fuse/breaker to be the 'weak link' in the chain if something goes wrong. That said, such circuit protection also needs to be able to handle the Voltage. If you put a fuse rated for 10 A @ 30 Volts on an array string that produces >30 Volts it may not safely interrupt the Voltage and arcing could occur.

    This part I will have to study on. Next question: When matching the solar panels to the inverter, I will have 400watts of panels and a 600w inverter, will that work ok? Right now I have (2) Exide GC-145 batteries. How is my match?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #15
    Re: Dumb (maybe) questions from a newbie?
    Rikerz wrote: »
    This part I will have to study on. Next question: When matching the solar panels to the inverter, I will have 400watts of panels and a 600w inverter, will that work ok? Right now I have (2) Exide GC-145 batteries. How is my match?

    For off-grid system the solar panels are not "matched" to the inverter: in essence they don't have anything to do with each other.

    Inverter capacity is a matter of the maximum loads that need to be supplied at any given time.

    Inverter runs off batteries, so the batteries must be able to support that maximum load and provide the daily Watt hour needs.

    Panels are there to recharge the batteries.

    Your batteries are rate as 245 Amp hours. The 600 Watts @ 12 Volts would be approximately 50 Amps, right about 20% of the batteries' capacity. This would be about the limit you'd want, and you wouldn't want it to be constant. The maximum Watt hours from the batteries would be (50% DOD) 1470 Watt hours DC. For practical purposes you'd want to limit the discharge to about 25%: 725 Watt hours DC. From that has to come the inverter's consumption and the conversion loss to AC.

    Recharging would normally be based on peak current potential of 10% of the battery capacity or 24.5 Amps. For an MPPT controller this works out to (24.5 * 12 / 0.77) 382 Watts. For an PWM controller it works out as (24.5 * 17.5 Vmp) 429 Watts. So your 400 Watts of panel is viable.

    So depending on how you use the system your equipment is not a bad choice.
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  • Rikerz
    Rikerz New Member Posts: 19 ✭
    #16
    Re: Dumb (maybe) questions from a newbie?

    WOW, thanks for the info. I will have to read and reread this over and over to fully understand it. Am I correct or close on thinking that I need a 100a fuse between the battery and inverter or maybe closer to a 80a fuse?
    Cariboocoot wrote: »
    For off-grid system the solar panels are not "matched" to the inverter: in essence they don't have anything to do with each other.

    Inverter capacity is a matter of the maximum loads that need to be supplied at any given time.

    Inverter runs off batteries, so the batteries must be able to support that maximum load and provide the daily Watt hour needs.

    Panels are there to recharge the batteries.

    Your batteries are rate as 245 Amp hours. The 600 Watts @ 12 Volts would be approximately 50 Amps, right about 20% of the batteries' capacity. This would be about the limit you'd want, and you wouldn't want it to be constant. The maximum Watt hours from the batteries would be (50% DOD) 1470 Watt hours DC. For practical purposes you'd want to limit the discharge to about 25%: 725 Watt hours DC. From that has to come the inverter's consumption and the conversion loss to AC.

    Recharging would normally be based on peak current potential of 10% of the battery capacity or 24.5 Amps. For an MPPT controller this works out to (24.5 * 12 / 0.77) 382 Watts. For an PWM controller it works out as (24.5 * 17.5 Vmp) 429 Watts. So your 400 Watts of panel is viable.

    So depending on how you use the system your equipment is not a bad choice.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #17
    Re: Dumb (maybe) questions from a newbie?
    Rikerz wrote: »
    WOW, thanks for the info. I will have to read and reread this over and over to fully understand it. Am I correct or close on thinking that I need a 100a fuse between the battery and inverter or maybe closer to a 80a fuse?

    The inverter should come with a manual which give specific recommendations for that inverter. This is important because the actual current draw on the DC side will be the output power multiplied by the conversion efficiency and the inverter consumption added in. Not all inverters are the same for those two factors.

    As far as NEC goes the circuit protection is 20% larger than the maximum continuous current. So at 50 Amps the fuse/breaker would be (50 * 1.2) 60 Amps. The wire should be able to handle more current than that.
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  • Rikerz
    Rikerz New Member Posts: 19 ✭
    #18
    Re: Dumb (maybe) questions from a newbie?
    Cariboocoot wrote: »
    The inverter should come with a manual which give specific recommendations for that inverter. This is important because the actual current draw on the DC side will be the output power multiplied by the conversion efficiency and the inverter consumption added in. Not all inverters are the same for those two factors.

    As far as NEC goes the circuit protection is 20% larger than the maximum continuous current. So at 50 Amps the fuse/breaker would be (50 * 1.2) 60 Amps. The wire should be able to handle more current than that.
    The inverter should be in early next week. I am guessing 4 awg wire should be correct as I believe it can handle up to 80amps, sound correct?
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  • Rybren
    Rybren Solar Expert Posts: 351 ✭✭
    #19
    Re: Dumb (maybe) questions from a newbie?

    Just something to watch out for...

    Some inverters have a fairly low high-voltage cutoff of ~15V. When you equalize your batteries, the voltage will be higher than 15V and your inverter will shut down, so you won't be able to run your inverter or loads while equalizing.

    You'll want to check your inverter specs.
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  • Rikerz
    Rikerz New Member Posts: 19 ✭
    #20
    Re: Dumb (maybe) questions from a newbie?

    Thanks for that info, a question about this please: Doesn't the charge controller do this (equalize)automatically and how will I know when this is happening and what can I do about this if correction is needed? thanks again for the info.
    Rybren wrote: »
    Just something to watch out for...

    Some inverters have a fairly low high-voltage cutoff of ~15V. When you equalize your batteries, the voltage will be higher than 15V and your inverter will shut down, so you won't be able to run your inverter or loads while equalizing.

    You'll want to check your inverter specs.
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  • Rikerz
    Rikerz New Member Posts: 19 ✭
    #21
    Re: Dumb (maybe) questions from a newbie?

    After rereading the manual on my Morningstar Sunsaver, it says that equalization is optional and I guess turned off when I received it, why would I not want this feature on? And it says it does it every "28 days or if the battery discharges too low the previous night" when activated, does that sound about right?
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  • Rikerz
    Rikerz New Member Posts: 19 ✭
    #22
    Re: Dumb (maybe) questions from a newbie?

    Just found this out, my inverter's high-voltage cutoff is 15.5v and my cc has a equalize rate of 14.9, so this would work ok?
    Rybren wrote: »
    Just something to watch out for...

    Some inverters have a fairly low high-voltage cutoff of ~15V. When you equalize your batteries, the voltage will be higher than 15V and your inverter will shut down, so you won't be able to run your inverter or loads while equalizing.

    You'll want to check your inverter specs.
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  • Rybren
    Rybren Solar Expert Posts: 351 ✭✭
    #23
    Re: Dumb (maybe) questions from a newbie?

    You should probably have the auto-equalization turned off. Most folks recommend to only equalize when the SG of the cells in a battery are not equal (or vary by more than ~0.01).

    On the other hand, some battery manufacturers (like mine - US Batteries) recommend an equalization every 30 days. I really don't know which is the best route to follow, but I haven't been doing a monthly EQ.

    You may find that 14.9V is a tad low for an EQ voltage.
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  • Rikerz
    Rikerz New Member Posts: 19 ✭
    #24
    Re: Dumb (maybe) questions from a newbie?
    Rybren wrote: »
    You should probably have the auto-equalization turned off. Most folks recommend to only equalize when the SG of the cells in a battery are not equal (or vary by more than ~0.01).

    On the other hand, some battery manufacturers (like mine - US Batteries) recommend an equalization every 30 days. I really don't know which is the best route to follow, but I haven't been doing a monthly EQ.

    You may find that 14.9V is a tad low for an EQ voltage.
    I dont think I have a choice in this as this is why my cc states and I dont think I can change it, can I?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #25
    Re: Dumb (maybe) questions from a newbie?

    Different charge controller would change it. That more money.

    At this point I would not worry about it as you need to see how this all works for you. You'd be amazed at how often I don't EQ my batteries. But with the lower charging Voltage of the Sunsaver and the higher charging Voltage specs of the Trojan batteries you may find you are using that extra couple of tenths more often than not.

    Trojan recommends you don't EQ until the SG readings of two or more cells differ by a significant amount. The Sunsaver is is design more for "preventive" EQ than "corrective" EQ.

    Worst case: you find the SG's drifting and have to hit them up with the generator & battery charger to push the Voltage up high enough and bring things back in line.
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  • Rikerz
    Rikerz New Member Posts: 19 ✭
    #26
    Re: Dumb (maybe) questions from a newbie?
    Cariboocoot wrote: »
    Different charge controller would change it. That more money.

    At this point I would not worry about it as you need to see how this all works for you. You'd be amazed at how often I don't EQ my batteries. But with the lower charging Voltage of the Sunsaver and the higher charging Voltage specs of the Trojan batteries you may find you are using that extra couple of tenths more often than not.

    Trojan recommends you don't EQ until the SG readings of two or more cells differ by a significant amount. The Sunsaver is is design more for "preventive" EQ than "corrective" EQ.

    Worst case: you find the SG's drifting and have to hit them up with the generator & battery charger to push the Voltage up high enough and bring things back in line.
    More advanced knowledge that I will have to brush up on, thanks for the info. What should I look for in a good charger?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,590 admin
    #27
    Re: Dumb (maybe) questions from a newbie?
    Rikerz wrote: »
    More advanced knowledge that I will have to brush up on, thanks for the info. What should I look for in a good charger?

    That is a tough question... You can get "simple" but rugged/reliable chargers such as the Iota.

    Or you can get very fancy inverter/charger units that have fully programmable charge controller subsystems that have one or two AC inputs (grid/generator/etc.) and can do all sorts of stuff (feeding power back to the utility--grid tied with hybrid inverter), share AC current between AC loads and AC Charger (i.e., program inverter/charger to only pull 11 amps @ 120 VAC and split that between AC loads and AC charging), to generator support (that same 11 amp AC input limit, and if the AC loads exceed 11 amps @ 120 VAC, inverter will actually "support" the genset up to the inverter's maximum AC output limit--make a smaller genset look larger to the loads--i.e., support short term starting currents and longer term loads such as a hot plate/induction cooker/hair driers, etc.).

    So--The options are all over the map--And these devices that "make sense" are based on battery bus voltage, Amp*Hour Capacity, and your AC load requirements (watts, 120 vs 120/240 VAC), generator autostart, etc.

    So, far, there is no "one perfect solution"--And you should put down your requirements and lay your options against those specifications (and user experience here).

    As your system voltage/AH/kWatt sizes increase, you have more (and more expensive) options.

    May times, you may end up with a mix... I.e., a smaller AC genset for day to day keeping the battery happy during the winter, and a second larger genset to run the shop and power your loads as a backup to the small genset+inverter system. Similar with inverters and chargers--If your daily power needs are small--Then a smaller AC inverter + battery bank + AC charger + generator can be the "investment".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • PNjunction
    PNjunction Solar Expert Posts: 762 ✭✭✭
    #28
    Re: Dumb (maybe) questions from a newbie?
    Rybren wrote: »
    You may find that 14.9V is a tad low for an EQ voltage.

    Correct - the idea here is that for FLA batteries, this is a "light" EQ, and is also not very long in duration - just one time each 28 days, and that is only during your daily solar insolation period, which in winter, can be as short as 1 or 2 hours depending on your location.

    Most recommend leaving it off, but I see no harm in this very light-eq, and if you aren't doing a full EQ on FLA's on a regular basis, this may help somewhat.
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