Cable required for two solar panels

[TheCrowsNest]

Hi,

My narrow boat came with two solar panels:

1. Uni-Solar US-64 MC - http://www.energoberen.hr/index.php/en/Fotonaponski-Moduli/Uni-Solar-US-64-MC.html

2. Suntech STP065-12/Sb - http://www.arcmansolar.com/products/53.aspx?cid=7-12-11

 

I took apart the cabling used by the previous owner while painting the boat and I am just about to put it back together.

 

I noticed the + and - cables on both the panels are approx 4mm2 but the previous owner had connected both cables up to 2.5mm2 cable and run it to the solar panel controller (Pro Star 30 - http://www.morningstarcorp.com/pro-star).

 

I was just wondering if this is safe and/or able to receive the maximum amount of charge?

 

i.e. Should I buy some new cable, if so what thickness or should I just use the 2.5mm2 cable the previous owner used?

 

Many thanks for any information and advice you can provide!

[BEngo]

Crudely you are dealing with less than 10 A- that's well within the safe capacity of 2.5mm2 but depending on the length of the cable between the panels and the battery there may be a voltage drop which will reduce the effectiveness. How long are A the 4mm2 cables and B the 2.5mm2 cable? How long and what size are the cables from the controller to the battery?

 

N

[TheCrowsNest]

BEngo, thanks for the quick reply. The 4mm2 cables from the panels are about 50-60cm long, the cable required to go from those cables to the control unit is about 3-4m and the cable going from the control unit to the battery is about 40-50cm.

 

All in all, not that long.

[BEngo]

There is 8m of 2.5 mm2 cable (out and back), so you will lose about 0.8V in that at full output. That is quite significant when looking at charging voltage differences so I would uprate the 2.5 to 4mm2 or even 6mm2 which will reduce the drop to about 1/3 volt. The drops in the 4mm2 will be about 0.07V in each piece from the panels and 0.14 V between the controller and the batteries.

 

Cable loss calculator: www.calculator.net/voltage-drop-calculator.html

 

The connectors are another potential voltage loss point. Make sure they are sized for the cable and keep them clean and watertight.

 

 

N

[smileypete]

Bear in mind with a PWM controller it can't turn those surplus volts into extra amps like those magick MPPTs can.

 

So reducing volt drop may well not increase the current into the batts. It may do in certain conditions, like a large bank at a reasonably cool temp just reaching absorption voltage while the panels at high summer temps, but on the whole I'm not sure its justifies uprating the cable.

 

With MPPTs it's a question of, do you spend more £££ on bigger cables to save some watts, but would spending that £££ instead on solar instead create more extra watts the first place?

 

cheers, Pete.

~smpt~

Edited June 9, 2013 by smileypete

[TheCrowsNest]

Thank you both. As I have loads of 2.5mm2 cable left over from my electrics install, I think I will just use it for the time being. Perhaps I will upgrade before next summer.

 

As long as using the 2.5mm2 cable isn't dangerous.

[Supertramp]

If you have lots of 2.5mm - why not double it up - use all 3 cores joined together (Twin+earth) for the positive, and another similar for the negative?,

This may help with future core failures, as 2.5mm T+E cable is solid core, and not so tolerant of movement over time. Ideally you should use stranded.

 

ETA - re-read you OP - not sure if it's 2.5mm T+E or 2.5mm singles, either way, - double it up !

Edited June 18, 2013 by Supertramp

[trackman]

Whatever the controller in use, the voltage loss does matter. Not in bright sunlight perhaps, but when the conditions are less good. Then, the lost voltage may make the difference between some useful current to the batteries and none.

To make the best out of the panels you need to reduce that voltage drop. 0.8 V is a big loss, you would really notice that much loss off your alternator voltage, and the same applies to solar panels.

[smileypete]

If the OP has used some surplus arctic then they could double up the + or - with the third core.

 

That said, in cool overcast conditions the panel and PWM will act more like a constant current source, in hot sunny conditions it's likely to just start the timed absorption charger a little earlier. Could always try with/without the third core and see what difference it makes to batt charge current.

 

cheers, Pete.

~smpt~

[lexicon]

Slightly off topic but still solar wiring related.

Looking to add a additional panel to improve performance In non perfect conditions.

100v 20a controller with one 30v panel at present.

I have 2 wires one positive one negative with mc4 connectors.

Would it be best to connect In series or parallel ?

And does anyone have a good link to a cable I could use to connect the two panels to my single mc4 cables + and - ?

[matty40s]

30V panel?? , is this a 24V panel on max V or do you mean a 30W panel??If you mean a 24V panel at 200W, then that computes.

 

I guess your controller is an MPPT tracer type,?

I would stick your extra panel in series, attach the pos of one panel to the neg of the other and then the two remaining cables back to controller, if you havn't got enough stretch in the cable for this, just add a 4mm2 pholtovoltaic cable in between with a male and female MC4connector at each end to connect the two panels.

[smileypete]

Slightly off topic but still solar wiring related.

Looking to add a additional panel to improve performance In non perfect conditions.

100v 20a controller with one 30v panel at present.

I have 2 wires one positive one negative with mc4 connectors.

Would it be best to connect In series or parallel ?

And does anyone have a good link to a cable I could use to connect the two panels to my single mc4 cables + and - ?

Sounds like you have an MPPT controller, (Tracer?), in that case best buy an identical panel and daisy chain them in series, shouldn't need any extra cables if the panels are side by side.

 

If it's not poss or economical to source an identical panel post back or start a new thread for help with full details of the first panel.

 

Bear in mind a 20A MPPT charging 12V batts will only be use up to 250-280W of solar in doing so, any extra available will go unused.

 

So if the combined output from 2 panels will be way above this, then probably best get another 20A MPPT to handle the new panel separately, then connect both MPPTs to batts.

 

cheers, Pete.

~smpt~

[lexicon]

Probably a 24v panel as the controller shows 31v at times from the panel. Tracer mppt type.

Main reason is my mooring is shaded by trees so the flat mounted panel is not putting out anywhere near full charging, so hoping extra panel will give more charging from sun up till 1pm when panel starts to get full sun. If the extra panel ends up having the controller sat at its limit of 20a then I would swap it for a larger controller at a future date.