June 20, 20215 yr Hi, Newbie here, We have purchased the Mecer ivr 1200mppt. it is recommended quite a low dc and that we put the pvs in parallel. See attached. My question is why would it want such a low dc when the mppt has quite a high range? was liking the idea of having the panels in series, running higher dc , amps low (around 6 amps) and not having to buy 20m of 8 gauge wire. is there something I’m missing here. we have a 100ah 12v lfp battery
June 20, 20215 yr 20 minutes ago, sunset1 said: Hi, Newbie here, We have purchased the Mecer ivr 1200mppt. it is recommended quite a low dc and that we put the pvs in parallel. See attached. My question is why would it want such a low dc when the mppt has quite a high range? was liking the idea of having the panels in series, running higher dc , amps low (around 6 amps) and not having to buy 20m of 8 gauge wire. is there something I’m missing here. we have a 100ah 12v lfp battery What is the Voc of the panels you want to connect to it?
June 20, 20215 yr 31 minutes ago, sunset1 said: Hi, Newbie here, We have purchased the Mecer ivr 1200mppt. it is recommended quite a low dc and that we put the pvs in parallel. See attached. My question is why would it want such a low dc when the mppt has quite a high range? was liking the idea of having the panels in series, running higher dc , amps low (around 6 amps) and not having to buy 20m of 8 gauge wire. is there something I’m missing here. we have a 100ah 12v lfp battery Note they said "best" Vmp range, not min and max Vmp range. That's because the system/battery voltage is 12V. So if your charging voltage is 14V and your panels are tracking at 15V the charge controller barely has to do any work and you get max efficiency. The higher your tracking, the more work it has to do to step down the incoming voltage, so your efficiency gets lower. Edited June 20, 20215 yr by tetrasection
June 21, 20215 yr Author Hi tetra. Thanks very much. In terms of efficiency how much would I be loosing doing roughly 90v? trying to keep costs down hence series and 20 amp cable.
June 21, 20215 yr Author 8 hours ago, Achmat said: What is the Voc of the panels you want to connect to it? This is the one right now. See attached but I am thinking of swapping it out and doing 2x 450 watt Canadian etc in series which puts me at roughly twelve amps and about 90 dc. the 450 watt panel are roughly 12 amps each and 40 odd dc.
June 21, 20215 yr 19 minutes ago, sunset1 said: Hi tetra. Thanks very much. In terms of efficiency how much would I be loosing doing roughly 90v? trying to keep costs down hence series and 20 amp cable. I can't say for sure but it shouldn't be too much. I would also series those panels.
June 21, 20215 yr 2 hours ago, sunset1 said: In terms of efficiency how much would I be loosing doing roughly 90v? It says the "operating" voltage max is 80 V. I would not exceed that voltage.
June 21, 20215 yr Author 2 hours ago, Coulomb said: It says the "operating" voltage max is 80 V. I would not exceed that voltage. hmmm... I thoight it was 100v. manual says Max PV array open circuit voltage = 100v what is the difference between operating and open circuit?
June 21, 20215 yr 14 minutes ago, sunset1 said: hmmm... I thoight it was 100v. manual says Max PV array open circuit voltage = 100v what is the difference between operating and open circuit? You do not look at that one, you have to look at the operating voltage ranges of your inverter which is 15v min and 80V max, according to the manual. Think of the Voc as max Voltage you should not exceed when you have a fault condition.
June 21, 20215 yr Just now, hoohloc said: You do not look at that one, you have to look at the operating voltage ranges of your inverter which is 15v min and 80V max, according to the manual. Think of the Voc as max Voltage you should not exceed when you have a fault condition. Operating= normal running of the system, day to day Open circuit = Fault condition, you don't operate the system when you have an open circuit
June 21, 20215 yr 1 hour ago, hoohloc said: You do not look at that one, you have to look at the operating voltage ranges of your inverter which is 15v min and 80V max, according to the manual. Think of the Voc as max Voltage you should not exceed when you have a fault condition. The operating voltage as I understand it in this case is the MPPT operating voltage, so that would correspond more to Vmp. The manual specifically says max Voc is 100V, so if his panels are 90V Voc it is below the limit and within the usage parameters. As such if anything does go wrong he is well within his rights to claim warranty. However, generally it is recommended to stay at 80% or below maximum specifications. The US electrical code even requires that you multiply the Voc by 1.25 before matching it to the inverter's max Voc. I do not know if such regulations exist in SA though...
June 21, 20215 yr 7 minutes ago, tetrasection said: The operating voltage as I understand it in this case is the MPPT operating voltage, so that would correspond more to Vmp. The manual specifically says max Voc is 100V, so if his panels are 90V Voc it is below the limit and within the usage parameters. As such if anything does go wrong he is well within his rights to claim warranty. However, generally it is recommended to stay at 80% or below maximum specifications. The US electrical code even requires that you multiply the Voc by 1.25 before matching it to the inverter's max Voc. I do not know if such regulations exist in SA though... To make life easy for the Newbie and not cause too much confusion, also assuming that his inverter has integrated MPPT, he needs to look at the operating ranges to determine what panels to use or how to connect them. He must not exceed the inverter/MPPT operating max and must not be under the minimum. Follow this and you will not go wrong
June 21, 20215 yr Author 28 minutes ago, hoohloc said: To make life easy for the Newbie and not cause too much confusion, also assuming that his inverter has integrated MPPT, he needs to look at the operating ranges to determine what panels to use or how to connect them. He must not exceed the inverter/MPPT operating max and must not be under the minimum. Follow this and you will not go wrong From the newbie to hoohoc I thank you for the advice and the new name Edited June 21, 20215 yr by sunset1
June 21, 20215 yr Just remember to add disconnect fuse box DC Switch between the PV and Inverter. extra safety is not wrong. Edited June 21, 20215 yr by Barryv
June 21, 20215 yr 43 minutes ago, sunset1 said: From the newbie to hoohoc I thank you for the advice and the new name I didn't mean any disrespect @sunset1 , it was just to lighten up the conversation. Please don't take it the bad way 😆
June 21, 20215 yr 2 hours ago, tetrasection said: The manual specifically says max Voc is 100V, so if his panels are 90V Voc it is below the limit and within the usage parameters. Ah, ok. I was just going on the part of the manual that you copied and highlighted. "Operating" could be taken either way, but with the above, you must be right. Power converters are more efficient when the voltage ratio is closer to unity, so that's something to keep in mind.
June 21, 20215 yr 35 minutes ago, Coulomb said: Power converters are more efficient when the voltage ratio is closer to unity, so that's something to keep in mind. On that subject, say your output voltage is 14V, is it more efficient to have a 28V input or a 25V input? In other words is it more efficient when it only has to halve the voltage compared to more "complex" transformations, even though 25V is closer to 14V than 28V is? Edited June 21, 20215 yr by tetrasection
June 21, 20215 yr Author 3 hours ago, Barryv said: Just remember to add disconnect fuse box DC Switch between the PV and Inverter. extra safety is not wrong. One of these guys?
June 21, 20215 yr Author 3 hours ago, hoohloc said: I didn't mean any disrespect @sunset1 , it was just to lighten up the conversation. Please don't take it the bad way 😆 But of course!
June 21, 20215 yr Author 4 hours ago, tetrasection said: The operating voltage as I understand it in this case is the MPPT operating voltage, so that would correspond more to Vmp. The manual specifically says max Voc is 100V, so if his panels are 90V Voc it is below the limit and within the usage parameters. As such if anything does go wrong he is well within his rights to claim warranty. However, generally it is recommended to stay at 80% or below maximum specifications. The US electrical code even requires that you multiply the Voc by 1.25 before matching it to the inverter's max Voc. I do not know if such regulations exist in SA though... 80% rule it is. Don’t want to start a fire!
June 21, 20215 yr 3 hours ago, sunset1 said: 6 hours ago, Barryv said: One of these guys? The keto disconnect is very good product and I like to install them by the batteries to the inverter. This small fuse holders work well and you get dc breakers for pv.
June 21, 20215 yr 5 hours ago, sunset1 said: 80% rule it is. Don’t want to start a fire! What mm2 cable do you currently have?
June 22, 20215 yr Author 6 hours ago, tetrasection said: What mm2 cable do you currently have? 2.5mm2 or 2mm2 Doesnt say on the cable but the electrician says it would handle 20 amps.
June 22, 20215 yr 1 hour ago, sunset1 said: 2.5mm2 or 2mm2 Doesnt say on the cable but the electrician says it would handle 20 amps. Must be 2.5mm2... If you connect the panels in series using your current cable you will get around 4.32V voltage drop. which is 4.8% power loss. So your panel Voc in series will be 90V minus the 4.32V your sitting at about 85V Voc. I would say that is plenty far enough away from the 100V max. Alternatively, if you buy 2x 20m of 6mm2 cable and connect them in parallel you will get 3.6V voltage drop which (at 45V) is 8% power loss. So even with the thicker cable you will be loosing almost double as much power than connecting in series. Also, these voltage drop calculations are based on Voc. Since you will actually be seeing Vmp when the sun is shining on them, the percentage power loss will be even higher. If you want to get power loss comparable to connecting them series but with a parallel connection, you will need at least 10mm2 cable.
June 22, 20215 yr 18 hours ago, tetrasection said: say your output voltage is 14V, is it more efficient to have a 28V input or a 25V input? 25 V would be more efficient. Power converters don't care about easy maths 🙂 It may not be linear: 28 V is probably not 28/25 as bad as 25 V. I think it's because of the high peak currents involved, so I²R losses increase more than linearly. For example, stepping down from 42 V to 14 V involves 33% Pulse Width Modulation, meaning that the peak current is three times the average current. But that means that the I²R loss (not the switching losses) is 9x higher when conducting. It only conducts for ⅓ of the time, so that's 3x the copper loss for a 3:1 voltage ratio. The switching loss would be about the same though, since you're still switching twice per cycle.
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