How many solar panels

Submitted: Saturday, May 17, 2014 at 23:14
ThreadID: 107822 Views:2072 Replies:3 FollowUps:5
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Need some opinions. Looking at putting solar on the Cruiser. I purchased these four thin 75w solar panels (on sale, only wanted 3 but being sold in pairs, ran out of time to ask if i could just get 3). Originally I thought I could only fit 3 sideways, but looks like I can fit the four. Wanting to run two Waeco fridges - Danfos DB35 compressor @ 3 - 4 amps. That's about 80watt hours. So perfect sunny day and panels following sun, a 100w panel might do. But with panels sitting flat, only going to get about 2-3hrs of good sun, rest being only part of the sun. Have 300 AH in battery storage.
The 3 panels will be simpler to fit, but want it there for long term, so not afraid to do the job properly. Downside of fitting the four is,
1- will go right to the edge of the pod it will be fixed to, meaning a branch from a scrub track may damage it.
2- much bigger job to fit it properly.
Thoughts, ideas, experience would be appreciated.
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Reply By: Member - Boobook - Sunday, May 18, 2014 at 07:35

Sunday, May 18, 2014 at 07:35
Firstly, Solar is like air, you can never have too much of it, especially with 2 fridges plus I imagine other stuff, especially if you are in places with mountains and shade from trees, or in the tropics. The extra 75W will be useful.

Can you mount 3 panels on the cruiser and have one as a portable panel? That way if you are parked in the shade you can still charge. ( 2 portable ones would be even better).

Also, if you wire the panels in parallel ( + to + and - to -) make sure you put blocking diodes in series with each panel. These should be schottkey diodes and rated at a higher current rating than each panel. This will prevent partially shaded panels drawing the energy from the ones in sun.

AnswerID: 532667

Follow Up By: Crusier 91 - Sunday, May 18, 2014 at 09:21

Sunday, May 18, 2014 at 09:21
I always thought that any shade on one panel would replicate that shade spot and solar decrease in all panels if strung together.
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Follow Up By: Member - John and Val - Sunday, May 18, 2014 at 09:46

Sunday, May 18, 2014 at 09:46
No Crusier. The cells on a panel are usually wired as two strings of 18 cells. Shading one cell in a string will affect that whole string, but not the other string which is wired in parallel with the affected one. Likewise, other panels wired in parallel will not be affected by their neighbours, apart from possibly a little leakage. The diodes Boobook recommends are to prevent that leakage.


J and V
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Follow Up By: Member - Boobook - Sunday, May 18, 2014 at 11:03

Sunday, May 18, 2014 at 11:03
Cruiser, further to what John is saying, your thought about the shading can occur, particularly when Panels are wired in series. Partial shading can reduce the whole output of several panels. But most solar panels have "bypass" diodes in their terminal block these days when you buy them to prevent this.

Bypass when they are in series, blocking when they are in parallel to stop the leakage.

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Follow Up By: tomtrish - Wednesday, May 21, 2014 at 07:27

Wednesday, May 21, 2014 at 07:27
Thanks heaps guys for your thoughts. I like the idea of having the portable panel, but most of our trips we are usually driving somewhere during the day stopping at night. I'm leaning toward putting the whole four up top. The blocking diodes are now my problem. Some info I've researched supports what Bookbook says while others say that as long as the charge controller has a blocking diode, that will suffice. Apparently the inefficiencies caused by having blocking diodes between the panels is worse than the loss caused by not having them. I'm not so sure, as what Bookbook says makes sense to me. So a little more research into how much loss the blocking diodes can cause as to how much they can save is now on my to do.
Thanks again for your input, much appreciated.
FollowupID: 816150

Follow Up By: Member - Boobook - Wednesday, May 21, 2014 at 10:20

Wednesday, May 21, 2014 at 10:20

Yes many websites will refer to a blocking diode not being necessary when you have a controller, and that they lead to a loss. Both of these are true. However the statement about the controller negating the need for the blocking diode is only in the case where you have one solar panel ( or several in series).

With respect to the losses, if you use a schottkey diode, you will lose about 0.4V, that is about 4 watts per panel or 16 watts in total out of your 300W

Consider that you have 4 panels in parallel and no blocking diodes. If one or two panels are shaded, then they act as a load and consume much of the current from the, working panels. ( just like a single panel draws current from the battery without a controller). A blocking diode in series with each panel stops this reverse current flow. The cost of 16 watts during full sunlight is far less than you could expect with some panels acting as a load, under partial shade conditions, when you can lease afford losses. Also it is possible that the reverse current could damage your shaded panels, ( but unlikely).

If your panels are identical, right next to each other, and will never be shaded then you could get away without them.

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Reply By: Member - John and Val - Sunday, May 18, 2014 at 09:22

Sunday, May 18, 2014 at 09:22
As Boobook says, you can never have too much.

The fridges will draw an average of about 7 amps for about 1/3 of the time (about 50-60 amphours per day) unless you run one as a freezer which might double that requirement.

Allowing for the fact that they will be mounted horizontal rather than following the sun, your 3 panels will probably average about 10 amps over a sunny 8 hour day which should be adequate for the fridges.

We actually carry our two panels one on top of the other. Only the top one produces while travelling, but the top one , (or both) slide out to be positioned on the ground when stopped. Maybe you could carry your 4th panel this way? Pretty simple to attach L shaped brackets on the long sides of the lower panel and slide the upper panel into them.


J and V
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AnswerID: 532678

Reply By: chisel - Sunday, May 18, 2014 at 09:24

Sunday, May 18, 2014 at 09:24
The waeco's should use under 24AH per day if used as fridges, not freezers ... so 2 points
(a) your 300AH of storage (say 150AH usable) will cover them for about 3 full days (probably a lot more especially in winter)
(b) your 300W solar (say 200W effective after losses) will give you over 15AH for every hour of good sun you get ... so 3 hours of sun will cover your fridge needs completely without even going to the storage.

I think you're covered. Even the 3 panels would be sufficient, assuming you're driving occasionally and recharging via the alternator when driving.
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