Voltage drop calculations pre and post of the charge controllers

[Growly bear]

Learning is so much fun!
Pre controller, do I use the open circuit voltage of the panel?
Post controller, do I use a nominal voltage of 12.8 (LiFePO4)?

How do I total the losses on each string? I can't see how say a 0.6% loss at 33V can be added to a 2.55% loss at 12.8V

 

[CAB96]

What are you hoping to achieve?

 

[Lenny HB]

From the panels I work on 17.5v as that is what the output will be when delivering current.
For the output 14.2v for the same reason.

 

[fishplug]

The only reason I ever calculate voltage drop is in order to size my cables correctly for a given appliance/device. This also then determines the fuse required for that circuit.

For solar I always just go for the thickest cable that the MPPT controller terminals can take. The price difference is minimal in a van so why bother figuring it all out?. 6mm all round for me.

This keeps my voltage drop to the minimum it can be with the kit I've got. Worst case scenario I've wasted £1.67* compared to the 4mm cable.

I can live with that.



* obviously I'm being flippant, but have a look for yourself for the difference in cost for your cable run length. It's unlikely to be a significant sum.

 

[Growly bear]

What are you hoping to achieve?

The only reason I ever calculate voltage drop is in order to size my cables correctly for a given appliance/device. This also then determines the fuse required for that circuit.

For solar I always just go for the thickest cable that the MPPT controller terminals can take. The price difference is minimal in a van so why bother figuring it all out?. 6mm all round for me.

This keeps my voltage drop to the minimum it can be with the kit I've got. Worst case scenario I've wasted £1.67* compared to the 4mm cable.

I can live with that.



* obviously I'm being flippant, but have a look for yourself for the difference in cost for your cable run length. It's unlikely to be a significant sum.

There are 2 routes i can take. The 6mm is 2 metres longer than the 4mm (due to space for 6mm cables).

I can also put one of the strings in series rather than parallel but prefer parallel due to shadowing .

Trying to work out what's best that's all.

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[Jimbohorlicks]

Just take the 6mm cable route and your covered for any voltage drop or configuration you may change subsequently.

 

[Raul]

Learning is so much fun!
Pre controller, do I use the open circuit voltage of the panel?
Post controller, do I use a nominal voltage of 12.8 (LiFePO4)?

How do I total the losses on each string? I can't see how say a 0.6% loss at 33V can be added to a 2.55% loss at 12.8V

Pre controller you use VMP or working, operating voltage, not the open circuit VOC

Post controller, average voltage from minimum 12.8v and max 14.2v =13.5v this is the voltage that most of the charge is received.

After you calculated the loss on each side, yes you add the percentage together to give you a total loss. Losses on 12v systems are best kept at or under 1% , on 24v 2% , on 48v 3%. On 240v AC losses should be kept at or under: 3% for lighting, and 5% for anything else, including 3phase.

 

[Lenny HB]

There are 2 routes i can take. The 6mm is 2 metres longer than the 4mm (due to space for 6mm cables).

I can also put one of the strings in series rather than parallel but prefer parallel due to shadowing .

Trying to work out what's best that's all.

Hymer's normally have the cables installed they will be 4mm.

 

[fishplug]

If cable size causes you problems with routing then go as per Raul's calculations.

Chances are that 4mm will be fine - It is for my setup, but I've over engineered it in case I ever choose to upgrade my panels.

Nothing worse than trying to add cable runs to a fully build conversion! All my cables are a couple of sizes bigger than they need to be for this reason. I can always spur off one of them if I need to add something later.

 

[Growly bear]

Pre controller you use VMP or working, operating voltage, not the open circuit VOC

Post controller, average voltage from minimum 12.8v and max 14.2v =13.5v this is the voltage that most of the charge is received.

After you calculated the loss on each side, yes you add the percentage together to give you a total loss. Losses on 12v systems are best kept at or under 1% , on 24v 2% , on 48v 3%. On 240v AC losses should be kept at or under: 3% for lighting, and 5% for anything else, including 3phase.

That's really good information. Thanks

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[Growly bear]

Hymer's normally have the cables installed they will be 4mm.

Yes, you are correct! However, I've decided to go for 2 controllers, one for the 200W and 1 for the 2 x 120W and siting the controllers in a small locker where the hydraulic levelling pump is.

I am a numpty though as after the inline fuse I can go straight to thin wall 6mm2 and get rid of the thick PV cable so there's loads of room! Sometimes it pays to step away. It was your comment which made me think differently

 

[Growly bear]

If cable size causes you problems with routing then go as per Raul's calculations.

Chances are that 4mm will be fine - It is for my setup, but I've over engineered it in case I ever choose to upgrade my panels.

Nothing worse than trying to add cable runs to a fully build conversion! All my cables are a couple of sizes bigger than they need to be for this reason. I can always spur off one of them if I need to add something later.

That's good forward planning. I've left an odd unconnected wire running between the floors as a pull through in the future. I've just got to remember it's there!

 

[Lenny HB]

I am a numpty though as after the inline fuse I can go straight to thin wall 6mm2 and get rid of the thick PV cabl

Thin wall cable sizing is often quoted including insulation so 6mm² is not 6mm²

 

[Growly bear]

Thin wall cable sizing is often quoted including insulation so 6mm² is not 6mm²

Thanks for the heads up. That is rather naughty though...

 

[Growly bear]

Thanks for the heads up. That is rather naughty though...

84/0.3mm
84 * Pi *0.15^2 = 5.937mm2

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[autorouter]

How do I total the losses on each string? I can't see how say a 0.6% loss at 33V can be added to a 2.55% loss at 12.8V

Yes, you can just add the percentages, as Raul says

After you calculated the loss on each side, yes you add the percentage together to give you a total loss.

It's not mathematically exact to do this, but for percentage losses of 5% or less, or even 10%, it makes almost no difference. In your case 0.6% + 2.55% = 3.15%. The exact calculation gives 3.13%, so no practical difference.

 

[Growly bear]

Yes, you can just add the percentages, as Raul says


It's not mathematically exact to do this, but for percentage losses of 5% or less, or even 10%, it makes almost no difference. In your case 0.6% + 2.55% = 3.15%. The exact calculation gives 3.13%, so no practical difference.

You can't do that and tease me that there is a more accurate method! I'm a mechanical engineer and retired maths teacher

 

[autorouter]

You can't do that and tease me that there is a more accurate method! I'm a mechanical engineer and retired maths teacher

OK if you really want the details...
If 0.6% of the power is lost, then 99.4% of the power is remaining.
If a further 2.55% is lost, that is 2.55% of the 99.4% of the remaining power.
So there is 97.45% of the remaining power still remaining after the 2.55% loss.
That's 97.45% of 99.4%.
Which is 99.4 x 97.45 / 100 = 96.8653%
So the total percentage loss is 100 - 96.8653 = 3.1347%
That's not much different from the 3.15% that you get from just adding the percentage losses.

An easier example is a 10% loss followed by another 10% loss. The first reduction goes down to 90%, the second reduction goes down to 90% of 90%, ie 81%. But the quick estimate of just adding 10% + 10% gives 80%, which is probably accurate enough for most purposes.

 

[Raul]

OK if you really want the details...
If 0.6% of the power is lost, then 99.4% of the power is remaining.
If a further 2.55% is lost, that is 2.55% of the 99.4% of the remaining power.
So there is 97.45% of the remaining power still remaining after the 2.55% loss.
That's 97.45% of 99.4%.
Which is 99.4 x 97.45 / 100 = 96.8653%
So the total percentage loss is 100 - 96.8653 = 3.1347%
That's not much different from the 3.15% that you get from just adding the percentage losses.

An easier example is a 10% loss followed by another 10% loss. The first reduction goes down to 90%, the second reduction goes down to 90% of 90%, ie 81%. But the quick estimate of just adding 10% + 10% gives 80%, which is probably accurate enough for most purposes.

I think he was puling your leg.

 

[BreweryDave]

…………Anyone fancy a pint?

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[Lenny HB]

Putting it in simple terms so a Maths teacher will understand.
In mid summer if your panels are pumping out 20 amps and you lose 5% due to cable size, that is a loss of 1 amp, that is not going to worry you.
In mid winter your panels are only producing 4 amps so a 5% loss will only be 200ma again not enough to worry you.

 

[Growly bear]

OK if you really want the details...
If 0.6% of the power is lost, then 99.4% of the power is remaining.
If a further 2.55% is lost, that is 2.55% of the 99.4% of the remaining power.
So there is 97.45% of the remaining power still remaining after the 2.55% loss.
That's 97.45% of 99.4%.
Which is 99.4 x 97.45 / 100 = 96.8653%
So the total percentage loss is 100 - 96.8653 = 3.1347%
That's not much different from the 3.15% that you get from just adding the percentage losses.

An easier example is a 10% loss followed by another 10% loss. The first reduction goes down to 90%, the second reduction goes down to 90% of 90%, ie 81%. But the quick estimate of just adding 10% + 10% gives 80%, which is probably accurate enough for most purposes.

Great explanation!!
That's straightforward then. Just like depreciation over a number of years!

 

[Growly bear]

I think he was puling your leg.

errrrrr no

 

[Growly bear]

Putting it in simple terms so a Maths teacher will understand.
In mid summer if your panels are pumping out 20 amps and you lose 5% due to cable size, that is a loss of 1 amp, that is not going to worry you.
In mid winter your panels are only producing 4 amps so a 5% loss will only be 200ma again not enough to worry you.

mmm what about temperature coefficients?

 

[Growly bear]

…………Anyone fancy a pint?

you're too far away

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[Growly bear]

I've enjoyed this thread chaps. Thank you. All parts on order now. 6mm2 to MPPT and 10mm to the fuse/busbars

I've not yet done the % loss calcs though...

 

[autorouter]

Putting it in simple terms so a Maths teacher will understand.
In mid summer if your panels are pumping out 20 amps and you lose 5% due to cable size, that is a loss of 1 amp, that is not going to worry you.
In mid winter your panels are only producing 4 amps so a 5% loss will only be 200ma again not enough to worry you.

But what about the poor Physics teacher? He'll be saying that if it's only 4A of current, then the voltage drop will be 1/5 of the drop at 20A, so that's only a 1% loss. Only 40mA, so definitely not enough to worry you.