Jump to content

Mppt dc question


sunset1
 Share

Recommended Posts

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 

C5980BED-A9F8-4059-A663-913BCBDE776C.jpeg

Link to comment
Share on other sites

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 

C5980BED-A9F8-4059-A663-913BCBDE776C.jpeg

What is the Voc of the panels you want to connect to it? 

Link to comment
Share on other sites

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 

C5980BED-A9F8-4059-A663-913BCBDE776C.jpeg

 

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 by tetrasection
Link to comment
Share on other sites

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.

EB575D85-A079-4AA9-93AC-BEA57E912095.jpeg

Link to comment
Share on other sites

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?

Link to comment
Share on other sites

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. 

Link to comment
Share on other sites

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

 

Link to comment
Share on other sites

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...

Link to comment
Share on other sites

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 

 

Link to comment
Share on other sites

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 by sunset1
Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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 by tetrasection
Link to comment
Share on other sites

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!

Link to comment
Share on other sites

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. 

 

CD83F906-B89F-430B-8D64-87224A9A5631.jpeg

Link to comment
Share on other sites

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.

 

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...