Partial shade on Jayco solar panels

Hi,

We have two 120watt monocrystaline solar panels mounted on roof of Jayco Eagle and connected to two 100amp AGM batteries.
We also have two Jayco boat rack bars mounted for and aft which are clear of both panels.
My question is when we are driving, if the sun is at an angle so as to cast a narrow shadow from either of the boat racks onto the panels, what is the effect on current produced and to what extent.

Thanks
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Reply By: Battery Value Pty Ltd - Sunday, Mar 14, 2010 at 17:32

Sunday, Mar 14, 2010 at 17:32
If they're 2x12V panels wired in parallel, the partially shaded panel will drop its output voltage meaning, that you could lose up to half of your 'normal' solar current.
But this depends on the state of charge of your batteries.
If they're almost at 100% SOC, then the consequences of partially shading of one panel will be nil.
Only if the batteries want to take in more current than the unshaded panel can provide at that time, then you'll notice the lack of current from the shaded panel, i.e. the batteries will take longer to recharge.
If both panels are in the shadow, your current will be reduced a lot more.
So it would be good to ensure that at least one panel receives full sun at any time.

Hope this explains it?

Best regards, Peter
AnswerID: 408795

Follow Up By: sastra - Sunday, Mar 14, 2010 at 18:07

Sunday, Mar 14, 2010 at 18:07
Thanks Peter,

What I should have added is we run our Dometic 3 way fridge on 12v while mobile drawing 10 amps approx.
If there was a thin shadow from the boat rack bar it would affect both panels as they are side by side and yes 12V wired parallel.
There are shadow variables like change of direction but I suppose if at start of journey the batteries were at 100 per cent then in say 10 hours day travel in clear sunshine I'm thinking the battery voltage should still be acceptable at days end but depending on charge loss from shade.

Mike
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FollowupID: 678752

Follow Up By: Battery Value Pty Ltd - Sunday, Mar 14, 2010 at 18:25

Sunday, Mar 14, 2010 at 18:25
that's ok Mike,

so the fridge drawing 10 amps while travelling....hmmm
Say you're on the road for 10 hours, then your panels (unshaded) will give you 8.7 amps during an 8 sun hours period.
So your batteries will see a slight reduction in charge during the day.
I guess at the end of the day you recharge them by means of a mains powered charger, or a gennie?

It's hard to predict the amount of charging current loss from the partial shading, but it could be significant.

In case you're using a mains powered charger at the end of the day, it doesn't really matter because it'll recharge the batteries overnight, no matter what.
Different story if you had a gennie, because you want to keep run times down...

What about getting some charge from the alternator while travelling?

Best regards, Peter
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FollowupID: 678756

Reply By: jdwynn (Adelaide) - Sunday, Mar 14, 2010 at 17:57

Sunday, Mar 14, 2010 at 17:57
I don't have any experience with monocrystalline which are supposed to be better with shade, but with polycrystalline, if you have 10% (perhaps even less) shade across width of panel you lose 90% odd of amperage. Shade is a killer!
AnswerID: 408798

Follow Up By: sastra - Sunday, Mar 14, 2010 at 18:15

Sunday, Mar 14, 2010 at 18:15
If the charge loss is this great then the racks will be coming off as they are not used and are there because they came with the camper when purchased.
Obviously I should do the test at home with fridge on and simulate boat rack shadow across both panels.

Mike
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FollowupID: 678753

Follow Up By: jdwynn (Adelaide) - Sunday, Mar 14, 2010 at 18:32

Sunday, Mar 14, 2010 at 18:32
Mike, good to test but rather than fridge just get yourself an ammeter (2/120w panels might be a bit beyond a multimeter) if your regulator does not show amp flow - easy to do and very informative. Cheers
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FollowupID: 678763

Follow Up By: PeterInSa - Sunday, Mar 14, 2010 at 19:20

Sunday, Mar 14, 2010 at 19:20
Mike,
Why not top up the van battery via an anderson plug and the vehicles alternator, then the van batteries will be in good shape.

Peter
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FollowupID: 678779

Follow Up By: dbish - Sunday, Mar 14, 2010 at 19:44

Sunday, Mar 14, 2010 at 19:44
Why compromise a good setup? if the racks arent used Id remove them & get the best out of the panels. Cheers Daryl
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FollowupID: 678784

Follow Up By: Mark Howlett - Sunday, Mar 14, 2010 at 23:12

Sunday, Mar 14, 2010 at 23:12
Hi Mike,

Peter is on the money with the Anderson plug off the car - your alternator will be pushing a lot more amps than your solar panels. Make sure your wiring from the Anderson plug to the fridge is heavy too.

Cheers,

Mark.
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FollowupID: 678827

Reply By: Maîneÿ . . .- Monday, Mar 15, 2010 at 06:23

Monday, Mar 15, 2010 at 06:23
sastra,
You say: "My question is: *WHEN WE ARE DRIVING* if the sun is at an angle so as to cast a narrow shadow from either of the boat racks onto the panels, *WHAT IS THE EFFECT ON CURRENT PRODUCED* and to what extent"

When you are driving the Alternator will be charging any battery attached to the vehicle charging system.

When you are NOT driving with the sun at the low angle mentioned it will depend on what type of solar panel you have as to what amount of current will be sent to the batteries.

Maîneÿ . . .


AnswerID: 408865

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