Battery top up solar panels

[Heepy]

We sold our Swift Charisma 555 last year and bought a 2018 Lunar Clubman SE fitted with a Phantom tracker. The van is kept at a storage facility when not in use and a 4. 8W solar panel inside the roof skylight always kept the Swift battery in good condition. I wired the solar panel into the Lunar and soon got a call from Phantom to say the battery was running low. It got down to a reading of 6. 6v.

Based on a calculation on one of the caravan forums, I bought and fitted a 20W solar panel feeding the battery through a controller and it shows a current going to the battery. The solar panel is placed in the front curved skylight (it was originally in the roof skylight) facing east. It was fitted on 8 October and the battery fully charged, but is now down to 10. 8v with a lot of recent sunny days. I decided to fit the 4. 8W solar panel as well and checked online to see if it should be connected in-line or in series. Two websites explained that connecting more than one solar panel of differing outputs to a controller would result in the power output reverting to that of the lowest one. The websites were dealing with large arrays feeding into a residential property, but I’m assuming the same rule applies to any setup.

My next idea was to wire the 4. 8W solar panel direct to the battery rather than through the controller and this is where I need advice please. As the 4. 8W panel can’t keep up with the tracker, then I assume the controller should shut down the 20W panel when the battery is full (if it ever gets there).

[Paul_B]

The 4.8w is ok to the battery and the 20w through a controller. However when a panel is behind glass or plastic expect a loss due to reflection.

[OWOMW]

10. 8 volts is a completely flat battery. If left in that state for any length of time it will be goosed.

[JTQ]

Given the 20Watt panel fails in what has been quite exceptionally sunny weather, then there is no chance at all your arrangement will work when the worst of the winter sets in. Plus in such good weather it is inadequate, then adding just another tiny 4. 8 Watts is not going to help much at all come that duller weather.

I am sure the biggest issue here is placing solar panels behind plastic windows, if that is not bad enough in degrading their performance, then the bigger unit is certain to have some shadows thrown on it by the window's frame and moulding features in the window itself. Shadows and shading devastate the output of our technology solar panels, way more than any proportionally to the amount shaded.

Here with the battery having been so low it has probably suffered some lost of capacity.

Grossly oversizing the panels used is not really a solution with them inside as these are compromised by the bigger the panel the more it is likely to suffer shading from the window structure.

The only thing I can suggest is to externally locate the 20 Watt panel facing south in a location where shading is minimised, and prop it up at at least 45 degrees to the horizontal, 50 is better so more vertical than horizontal. Plus, ensure it is well off the ground so any vegetation growth does not inhibit its yield. Adding the 4. 8 Watt panel is virtually a waste of time and could well be a parasitic load on the system, I would definitely not bother.

 

Even a 20 Watt panel optimally placed could not be enough, all depends on how placed, how good the controller is [and its own parasitic loses] and the van's own drains. In the past I recommended a friend use a 65 Watt, he actually purchased a 100Watt as it was only a little dearer,  it has been successful over several winters with a "drainy" Coachman.

[DaveMiller]

I agree with JTQ's appraisal above. I installed a roof mounted panel specifically to maintain my battery through the winter, which it does. Excess power will be blocked by the controller, but if you aren't generating enough then nothing will help; my point being that bigger is better in this case.

[Townie]

4 hours ago, JTQ said:

 

 

Even a 20 Watt panel optimally placed could not be enough, all depends on how placed, how good the controller is [and its own parasitic loses] and the van's own drains. In the past I recommended a friend use a 65 Watt, he actually purchased a 100Watt as it was only a little dearer,  it has been successful over several winters with a "drainy" Coachman.

JTQ is right. Our caravan is in storage and our previous caravan had a factory fitted 20watt roof panel. This would struggle to keep the battery charged, and we had to bring it home for recharging at least twice over the winter months. Our current van is stored in the same place but it is fitted with a 100watt roof panel. That kept the battery charged over last winter, even with the several weeks of snow covering it. The battery had enough charge to power the motor mover on our first trip of the year. Bigger is most definitely better.

[CommanderDave]

I think you'll struggle during the winter months with a 20w panel as I would guess most of the days the controller draw is taking out more than the panel is putting in with overcast .

 

I use 2x25w panels just to keep my 72ah battery topped up .

 

 

Dave

[segapod]

There is little point in fitting anything less than a 100w panel IMO.

With small, cheaper panels, make sure a diode is fitted across the panel output to prevent the battery discharging through the panel during hours of darkness.

[Guest]

The diode has to be in series, not across the output.

 

And as most know a 20w panel shoved behind a window is as good as useless but slightly less useless than a 4. 8watt one

Edited October 28, 2018 by matelodave

[segapod]

7 minutes ago, matelodave said:

The diode has to be in series, not across the output.

 

And as most know a 20w panel shoved behind a window is as good as useless but slightly less useless than a 4. 8watt one

Sorry, poor wording, the diode should of course be in series.

Most panels will already have one fitted but I have seen cheapies supplied without one in the past.

As you say, putting any panel in a window can dramatically reduce output.

[Paul_B]

46 minutes ago, matelodave said:

The diode has to be in series, not across the output.

 

And as most know a 20w panel shoved behind a window is as good as useless but slightly less useless than a 4. 8watt one

 

I tried 2x20w behind the front windows and they were as good as useless, at the moment the 90watts on the roof are just coping and I fully expect to put the van on mains charge when it gets dull and damp.

[SteveMilner]

I use a 60w suitcase type panel, propped up behind the window at the front of the van.

 

The van faces North.

 

I've never had the battery drop below 12v, although I do use my van during the colder months so it will get a top-up.

[Guest]

TO start with, I'm a newbie with a caravan, but have over 10 years experience with solar panels on my boat.   As the boat is kept afloat on a swinging mooring, there is no way of charging other than running the engine.   My domestic battery bank is 3x110AH, and until last year I had 2x45W semi flexible panels through a PWM controller .   These have kept the domestic batteries fully charged, when the boat is only used at weekends.   When we used it (again no EHU) for about a week, with a Waeco compressor coolbox, the batteries just about coped.  

 

Last winter I upgraded and changed the panels for 2x 100W and an MPPT controller, and basically we're pretty much independent with that setup.

 

What I'm basically saying is that a 100w should EASILY maintain a 110 AH battery.   One of the downsides to caravan roof mounted panels is that they are horizontal.   The boat wheelhouse roof is cambered, and the panels are about  10-15 degrees tilted, which undoubtedly adds effectiveness.   Today, a bright sunny day, there was 39v (series panels) output @1A, with the controller floating the batteries at 13. 8V with 0. 2A current

[Heepy]

Thanks for all your comments.

 

I have since had the power output from the 20w solar panel checked by an electrician pal. It’s only producing 9 watts outside in full bright sunlight without the controller wired in so was obviously faulty and has gone back to the dubious merchant at the bay of E who sold me it.

 

I’ve reinstalled the 4. 8w panel directly to the battery until I can source another one and it’s not doing any worse than the 20w one.

 

As I only need the battery kept topped up while it’s in storage, I’m not keen on installing a roof-mounted one and having to drill holes in the van. I’d still like to mount it inside the roof skylight and be removable. If anyone can recommend an honest solar panel supplier I would appreciate it.

[Guest]

How do you know it's only producing 9 Watts? You can measure amps and volts easily from a panel, but not watts. ..a 20W panel will produce a maximum of about one amp when at right angles to the sun in the height of summer. .. I guess you're saying it's producing 9 volts. Panels on open circuit will show over 20V in the same conditions (midsummer etc). 

Best people I've used for panels etc is Photonic Universe, just Google them.

[Les Medes]

We had the same problem with Phantom calling us in the middle of the night to say our battery was down, despite it being fully charged 14 days earlier. I took this up with BCA at the NEC a few years ago and fortunately the Tech Director was on the stand who confirmed that 14 days was about right before the Tracker would flatten the battery, it also depended he said what sort of Mobile Signal strength you have, at the time Phantom were using Vodafone which is totally useless where we live resulting in the unit continually seeking a signal and  running the battery down, The result was that I disconnected the Phantom completely, I also called our Insurers and told them that I had disconnected the tracker. and yes I had to pay a small premium.

The result  a peaceful nights sleep with no Phantom waking us up in the middle of the night Scary Phantom !!

[Heepy]

Thanks again for the comments.

 

To answer Les Medes, the tracker came with a free year’s subscription, but I’ll be binning it next year when it runs out in April. That’s why I don’t want to spend lots on an expensive piece of kit.

 

To answer Guzzilazz, my knowledgeable lecky pal told me to multiply the solar panel voltage by the amps to arrive at the wattage produced. I didn’t know that prior to testing it. From memory, the voltage reading was 22 and the amps 0. 41 = 9. 02w.

[Stevan]

The rated output of panels is only a theoretical figure based on the panel being at 90 degrees to the sun with a clear sky, close to the equator into a resistive load.

Very little relationship to the real world in the UK, well away from the equator, seldom a clear sky and not an optimal angle to the sun.

Actual output will always be significantly less than the rated figure.

[Guest]

3 hours ago, Heepy said:

Thanks again for the comments.

 

To answer Les Medes, the tracker came with a free year’s subscription, but I’ll be binning it next year when it runs out in April. That’s why I don’t want to spend lots on an expensive piece of kit.

 

To answer Guzzilazz, my knowledgeable lecky pal told me to multiply the solar panel voltage by the amps to arrive at the wattage produced. I didn’t know that prior to testing it. From memory, the voltage reading was 22 and the amps 0. 41 = 9. 02w.

Good answer! so yes your 9w is correct it looks like your panel is OK, BUT. .. if these measurements were taken with the panel connected to the controller, the voltage would normally match the controller output voltage. ..  

 

DOn't worry about the 9W, because the sun would certainly not be at right angles to the panel, and it's not under ideal lab conditions.

 

My pair of 100W panels on the boat (using a more efficient MPPT controller) the maximum I have seen output is 5. 5A, that's about 1. 5 A out of the 2 panels in series (thus producing 39-41V) so about 60W