Solar Panel ...roof rack or portable ???

Submitted: Monday, Oct 03, 2011 at 14:39
ThreadID: 89357 Views:16658 Replies:8 FollowUps:1
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Hi, just wanted to ask a few more questions as a follow up to my recent thread
# 89255 on solar panel placement. Hope this is OK.

Two questions:

#1. If the solar panel is roof mounted and is connected to the 2nd battery in the rear of the vehicle it would be charging. If it was ALSO connected so that the vehicle's alternator was charging it would this be OK? Or would I have to choose one charging method at a time? It's very easy for me to pull the alternator charging plug, I just wanted to know whether this would be necessary to avoid any battery damage or could I just leave it connected.

#2. I am sure that only I can answer this question but I will seek your comments in any case.
Charging the 2nd battery in the vehicle by roof mounted panel is fine whilst your driving along but what happens when you arrive at your campsite, and more
to the point, the ensuing days if you decide to stay put. Do you decide to leave your vehicle in the sunlight to continue charging the battery, eventhough the fridge is now in a very hot environment in the rear of the car or do you remove the panel from the roof rack and place it accordingly thus enabling your car to be parked in the shade (hopefully).


Do you settle for a portable fold up type panel and just set it up at the end of the day in the knowledge that your 2nd battery has hopefully been charged by the vehicle's alternator.

Like I said, I'm sure I have to make up my own mind on this one but no doubt it's a quandary we face going down the solar panel charging path and for those of you who have already made up your mind upon facing this question have you been satisfied with your decision?

I do appreciate that my questions have not taken into consideration the variation in charging values between the vehicles alternator and the solar panel.

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Reply By: Robin Miller - Monday, Oct 03, 2011 at 15:28

Monday, Oct 03, 2011 at 15:28
Point 1

Its fine to have both charging at once, provided its going thru a normal solar charge regulator which prevents power back feeding into the panel.

Point 2 A lot depends on the time you have to fiddle around.

You don't normally rely on the panel to charge your second battery while driving - as most have it being charged by the car when driving.

We look at the weakness link and attack that as part of an overall approach.
So this would mean you don't want the fridge to get hot in the first place.

I'd never leave the car deliberately in the sun so the solar panels can cool a fridge, I'd sooner park in shade and run the car a bit.
The economics of solar panels for that application are not very good.

But a car in sun can provide shade for the fridge with its doors open, or a fridge can be moved outside depending on size and your camp site / security concerns.

Window tint and a seperate exhaust fan may even be viable for some - like those little solar ones that clip on a slightly not closed window.
Robin Miller

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AnswerID: 466668

Reply By: Battery Value Pty Ltd - Monday, Oct 03, 2011 at 15:35

Monday, Oct 03, 2011 at 15:35

#1: battery receiving charge from alternator and solar panel: basically ok, but max specified charging current shouldn't be exceeded for longer than a few minutes or the battery will be ageing faster.
#2: this is up to you. Just note that any rise in battery temperature will shorten its life, more so, if the solar regulator doesn't have a battery temperature sensor.
The folding panel has a number of advantages: can be adjusted several times a day, easy to keep clean, can be put in a sunny spot, 10 metres away from the vehicle (if you've got a good solar regulator which allows you to extract 24V instead of 12V, for more charging amps).

cheers, Peter
AnswerID: 466670

Reply By: Crackles - Monday, Oct 03, 2011 at 17:09

Monday, Oct 03, 2011 at 17:09
Keep them portable. Panels on the roof of a car would contribute to an insignificant % of the total charge compared to that being put in by the alternator particually if they have dust, mud or bugs splatted on them. Park your car in the shade to keep it cool thereby minimising the fridge run time & set up your panels in the sun at the correct angle to maximise solar charge.
My parants van has a panel clipped to the roof using window security locks which allows it to be easily moved into the sun. This would be an alternative way to safely secure a panel but give the option of getting maximum charge on the hots days you really need to park in the shade.
Cheers Craig...............
AnswerID: 466678

Reply By: Travis22 - Monday, Oct 03, 2011 at 17:56

Monday, Oct 03, 2011 at 17:56
G'day Wayne.

I am going through the exact same dilemma right now. A few points to consider;

Mounting a panel on the vehicle roof and wiring it to an aux battery is fine, however the solar panel regulator will not turn 'on' and actually charge the aux battery while the vehicle is running and the aux battery is receiving charge from the alternator. Depending on the specific solar panel regulator, and the voltage AT the battery when the alternator is providing charge basically the regulator is going to believe the battery is full and stop the panel from providing charge to the battery. Its one or the other, not both; unless you suffer from some serious voltage drop.

Every time you stop in the sun, sure the solar panel will try and pickup where the alternator left off but there are many variables ie. load on the battery (fridge running at the time), float voltage of the battery before it has rested from the alternators charge etc. What im getting at is if you just switch off the vehicle for 5minutes to run into a shop, the panel might not even start charging in the mean time as the regulator might see the battery again as being full.

To answer question 1 specifically, despite the above there is no problem wiring the solar panel regulator straight into the charging circuit, no need to isolate it.

With regards to question 2, as you said it really is personal preference, and picking at straws. Depending on the fridge you run and how well it is insulated the exact results will differ but generally speaking i think its fair to say that yes, the hotter the environment the fridge / freezer is operating in, the more frequently it will run and the more power it will use.

So, depending on the size panel you are choosing to buy it may or may not be able to replace what is being used by the fridge. If the panel is big enough it should still do the job fine, perhaps it wont bring the battery up to near full as quick but again, still better then no panel!


Personally, im really torn. I like the idea of mounting a panel on the roof once its done its done! Ive just fabricated a canopy for my ute but they do take up a lot of space which may or may not be required (depending on the panel wattage).

Another point to consider however not vital is any panel mounted flat is going to loose some of its efficiency. Approx 10-20% depending on the conditions. (Compared with a clean panel aimed perfectly at the sun).

The other day i went out to purchase a panel but bailed in the end. I went out to buy a folding panel, because 1 its less work for me to do in the lead up to our next outback adventure real, real soon! 2 I get to keep all of the space on the canopy roof free for stuff.

And as you have already mentioned, with a portable panel the vehicle can be parked somewhat in the shade and the panel moved around to achieve maximum performance out in the sun.

Theft of a portable unit is very real i guess, however id really like to buy a solar panel just in case we stay an extra night somewhere. Our battery's all being well on their own have enough capacity to do the job on their own. So if we decide to stay an extra night somewhere or arrive early one day, the portable panel can go out, and we will remain close buy.

Now back to the other day, i had a good look at the 120W folding panel (2x60w panels). I found it was quite a bit heavier and painful to unpack and repack then i expected. However this has more to do with the cheap bags they come with, a decent quality canvas job with a zip that went around 3 of the 4 sides would make setup / packup much easier.

So that notched up another point to a fixed mono panel on the roof, and thus im still bloody torn what to do! Fixed panel on the roof is also quite a bit cheaper. ie. 80w folding panel $300/400, 80w fixed mono $160 + reg $20-50.

As our departure date gets closer i think more and more about a folding panel, as i just buy it and chuck it in the back. One less job before going!!!

AnswerID: 466687

Follow Up By: Member - Robert R1 (SA) - Tuesday, Oct 04, 2011 at 21:36

Tuesday, Oct 04, 2011 at 21:36

I have my solar panels mounted on the roof of the canopy and I find that even when I am parked in the sun, the inside of the canopy does not get very hot. I often open up the canopy doors to allow the air to flow through.

Two of my panels are hinged and are raised by gas struts. The other can be slid out and set up away from the vehicle. I don't normally bother unless things are getting desperate. If I can I park on a sllope.

FollowupID: 740906

Reply By: Rod - Monday, Oct 03, 2011 at 18:33

Monday, Oct 03, 2011 at 18:33
I have 145W of panels on my roofrack across three physical panels. One panel typically lies flat, but the other two are mounted on a arm that allows them to come up to a 60 degree angle to catch the early morning/afternoon sun. All three can be detached from the roof within 5 mins if I want via undoing the padlocks.

I suggest trying the keep the flexibility of being able to detach them from the roof if needed. Many nice campsites are shady and you need to be able to get the sun where you can. A 15m 6mm twin sheathed extension cord with anderson plugs allows you to connect back to the vehicle/battery. Try to keep the solar controller as close as possible to the battery being charged.

I frequently park my vehicle in the sun with the fridge in the back to maximise solar charge. Just try to point it directly at the sun and leave a window and the back tailgate open for air to circulate. Been doing that for years in Qld with no issues.

In vehicle mounting solar applications, be careful mounting panels on your vehicle with some solar controllers which are positively earthed. If you inadvertantly connect the earth of the panel to the earth of the controller to the earth of the vehicle, you can destroy the controller. To keep it idiot proof, I specifically purchased a negatively earthed solar controller. Surprisingly, many solar controllers (eg Stecca) are positively earthed.
AnswerID: 466695

Reply By: True Blue - Monday, Oct 03, 2011 at 19:52

Monday, Oct 03, 2011 at 19:52
I must admit I was loathed to ask these questions, I guess because your afraid of appearing to be a bit of a dill and plenty of times replies on this forum can be "a little short and abrupt"...or worse if you know what I mean.

I will probably get a few more responses to this thread as people get a chance to respond but I just wanted to thank you all very much for seeing my problem and each giving full, well thought through responses.

Very much appreciated from you all. I can now go forward with confidence and that makes all the difference.

Thanks guys.


AnswerID: 466708

Reply By: Member - Serendipity(WA) - Tuesday, Oct 04, 2011 at 22:26

Tuesday, Oct 04, 2011 at 22:26
I have had a permanently mounted solar panel over the roof to my ute for years now. Works great.

I had the issue once whilst camping at Steep Point. I put the extra engel in the boat on the trip up which took 2 days. By the time we got to the fishing spot the boat battery was flat. I exchanged it for the auxiliary battery in the car. In about 3 days of me out fishing in the boat and the car just sitting on the shore (no shade at steep point) the battery had recharged from the roof mounted solar panel.

When camping I have a tray mounted camper with a hard roof that opens out and becomes a hard floor camper. I have mounted 2 more 85w solar panels on the roof of my camper so they can be removed when at camp. Of course in a caravan park I just plug into power but when out bush I have the two solar panels out pointing at the sun. These two panels I bought for $284 each delivered to my house. I have a 24 amp regulator in the camper which cost around $120 from eBay.

With this combination of a fixed panel on the car that is always there charging and allowing me to put things on the tray that need power or when I have the camper loaded I also have two removable panels that power all the camping needs.



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AnswerID: 466812

Reply By: S Paul - VIC - Wednesday, Oct 12, 2011 at 22:09

Wednesday, Oct 12, 2011 at 22:09
I have a 140 watt panel mounted on the Jeeps roof - controller , 100 aH AGM and Waeco CF50 in the back - 6 B&S cable link from cranking battery to the auxiliary - When the engine stops the solar kicks in and charges both batteries .
I have a 140mm PC fan sucking air away from the Waeco compressor which helps reduce the run time of the Waeco .

This has been running for 2 years so far and no dramas at all ..


AnswerID: 467478

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