PV voltage - how high is too high?

[tiz]

I am planning a 2-200w solar panel setup on a mainship 390. Nominal boat systems are 12v throughout. 2 - 8D FLA batteries for now. Victron mppt controller to charge bat 1 with yandina charge combiner (100a) to batt 2. No inverter or additional batteries of any type right now. Boat will live on a mooring.

The question is, do I use two 24v panels in series (90.8v <voc> @ 5.83a <isc>) to achieve the lowest voltage drop? Panels on flybridge aft rail, probably 30-35ft wire run to mppt. **MIGHT** add a third panel someday....so like 135voc total at that point.

OR

Run two (possibly 3 later) 12v panels in series (48.6v voc and 10.2a isc)

OR

Run 2 (possibly 3 later) 24v panels in PARALLEL (45.4voc and 17.49a isc)

The first option has lowest voltage drop (12ga) PV to mppt (under 1%). Third option has the most voltage drop PV to mppt - closer to 3%.

Should I be concerned about voltage drop or high voltage?

What happens to charge output if the mppt sees like 3% voltage drop as input/pv voltage?

--Kevin

 

[diver dave]

Its a function of the controller spec.
fyi. Homes might be set up for around 600V. Just did some work at a utility solar site; running around 1500V. They looked like typical home panels.

 

[tiz]

Its a function of the controller spec.
fyi. Homes might be set up for around 600V. Just did some work at a utility solar site; running around 1500V. They looked like typical home panels.

I get that part. Controller has to handle input voltage and output current.

But what is considered normal/safe PV voltage going to controller? I have a question thread on the victron community where the one answer so far is that 90v is way too high on a boat even with a controller that handles 150v.

Would be good to hear of some real world setups in the 45-130v range.

--Kevin

 

[Insequent]

I recently saw a comment about ~60V being an upper limit for DIY type installs. My own set-up runs at 65V, and I'm OK with it. Much higher and I think it would be best to have a qualified electrician do it. Personally I'd dislike higher voltage.

For voltage drop concerns, run fatter wire. You could parallel the panels to a fuse/switch panel in a dry area near the install point, and then run a pair of heavy gauge wires down to the controller. In my case I'm running either 6 or 8 ga, one pair of wires for each of the four pairs of panels in parallel to 4 Victron controllers. I have a total of 2720W installed.

One aspect to plan ahead for is the controller capacity. With pairs of 345W panels I can use a Victron 100/50 controllers, close to their rated capacity. But if you pair larger panels, say 400W which are getting very common, then you need to go to the 150 series, and will end up at almost double the cost.

 

[rslifkin]

Boats typically have 120v AC on board already. And most solar panel and related wiring components are rated for 600 volts or more. So I wouldn't worry about being in the 100-200 volt range provided the cable routing is well protected and there aren't any exposed terminals at the controllers, breakers, etc.

My own setup is 1 panel per controller with big house panels, so it never gets over about 50 volts on the PV side.

 

[tiz]

Thanks much.

Ill rerun the voltage drop numbers with the largest conductor since the pv input can fit. Then try and decide on the panel layout.



--Kevin

 

[Jeff F]

I don't think voltage drop is a big concern in this particular application. Lower is better, obviously, but it's not critical to system performance. Modern controllers will optimize on whatever comes through.

 

[Jeff F]

I have four 175w panels on a Victron 100/50 controller. I used 10awg 25 foot cables with the intent of wiring the panels in series at a little under 100v. But I got an open voltage reading above 100 when I was setting them up and chickened out and wired them in a 2x2 array. Currently seeing daily max voltage of 45-48v and max power of about 450w. So 10 amps maximum current.

I don't think the working voltage falls much, so the max current assuming 42v and 700w is 17 amps.

I may change to 90-96v some time as an experiment. I haven't read anything to suggest that either is automatically better.

 

[Insequent]

I don't think voltage drop is a big concern in this particular application. Lower is better, obviously, but it's not critical to system performance. Modern controllers will optimize on whatever comes through.

I have four 175w panels on a Victron 100/50 controller. I used 10awg 25 foot cables with the intent of wiring the panels in series at a little under 100v. But I got an open voltage reading above 100 when I was setting them up and chickened out and wired them in a 2x2 array. Currently seeing daily max voltage of 45-48v and max power of about 450w. So 10 amps maximum current.

I don't think the working voltage falls much, so the max current assuming 42v and 700w is 17 amps.

I may change to 90-96v some time as an experiment. I haven't read anything to suggest that either is automatically better.

You are right in that the controller can handle the lower voltage. But voltage drop is wasted energy - thin wires heating up.

Your circuit length appears to be 2 x 25', so 50'. Using 10 ga wire for that and your expected current fits with a 10% voltage drop on the chart. So if your panels do generate 700W, your controller is only seeing 630W or thereabouts.

Using the Blue Seas chart attached you would recover quite a bit of that by using 6 ga wire. Its an efficiency gain that is pretty easy to get, so best to grab it IMO.

 

[Jeff F]

Using the Blue Seas chart attached you would recover quite a bit of that by using 6 ga wire. Its an efficiency gain that is pretty easy to get, so best to grab it IMO.

Yes, agreed. Had I planned on 45v instead of 90v I would likely have used heavier cables. But in my mid latitude cruising I'm rarely seeing more than 400w. At that real-world output with less than 10 amps flowing there is little efficiency to be gained in increased wire size.