How to stop backfeeding on a portable solar panel.

[Foxinthehouse]

Hi everyone,
You were so helpful with my last enquiry, I am going to ask a related question.
I bought a portable solar panel to keep my caravan battery topped up. The one I bought was made in the UK from a firm Low Energy Supermarket, 18volts with a 10 year warranty. Good specs, or so I thought, but I have noticed that there is no circuit protection to stop power being fed in reverse from the battery. The power to the battery is fed from the Solar Panel by two crocodile clips and I also wonder what would happen if they touch while setting it up each time the caravan goes into storage as the cable has to run out of the caravan window to the battery box.
So after my previous post regarding a back up battery on my water timer, I am wondering whether I could place diodes along in cable to prevent the feedback, and would thus prevent any short circuit, if that would happen with a solar panel.
I now have some spare 1N5404 if these are suitable.
Pics attached.
Thanks for any advice.
Mike

 

[brianmoooore]

What exactly are you worried about? The red and black wires getting damaged, shorting together, and burning from the full battery current??

 

[Foxinthehouse]

I am concerned on two points. First, I read that these portable solar panels should have a diode to prevent backfeed in weather where no current is generated, resulting in the battery discharging. The second point is, with no protection that I can see, what would happen if the bare parts of the two crocodile clips touched while the panel was generating electricity. Would it short circuit the panel? This second point is a concern as, when setting it up each time, there is a good chance this could inadvertently happen, due to the panel being set up inside the caravan window and the battery box only accessible from outside. This results in the cable dangling out the window while I am setting it up. I was hoping to set it up outside, but the manufacturer has said it must be protected from Rain.

 

[Avo Mk8]

Wouldn't you use a charge controller? Lots on fleabay starting from £8.
The Co. you purchased the panel from sells them, and they are included in the kits they sell.
However I know nothing about solar

 

[Foxinthehouse]

Hi Avo Mk8, I also know very little about Solar Panels, but the sales pitch from Low Energy didn't mention the need for a controller. Being supplied with crocodile clips suggests a connection direct to the battery. In any case, the instructions state a direct connection, see attachment. I always disconnect the caravan from the battery anyway, so it is just a trickle charge at a max of .56 amps, direct from the Solar Panel, which would hardly risk overcharging the battery, which is 110 Amp hour.

 

[DPG]

Charge controller sounds the best bet to me. Although I also don't have any experience of these, other than what I've read.

 

[Avo Mk8]

Hi Avo Mk8, I also know very little about Solar Panels, but the sales pitch from Low Energy didn't mention the need for a controller. Being supplied with crocodile clips suggests a connection direct to the battery. In any case, the instructions state a direct connection, see attachment. I always disconnect the caravan from the battery anyway, so it is just a trickle charge at a max of .56 amps, direct from the Solar Panel, which would hardly risk overcharging the battery, which is 110 Amp hour.

Thats useful info.
So for my own education I was just having a look
I don't think you need to worry about momentarily shorting the leads together, even when illuminated. I suspect the panel will withstand the short circuit current without damage. But that's my 'estimate'
You are correct about needing a diode in series. Suggest IN5408 as in example below
Lots of examples of more elegant ways of connecting here:

9 Simple Solar Battery Charger Circuits - Homemade Circuit Projects

Simple solar charger are small devices which allow you to charge a battery quickly and cheaply, through solar energy. A simple solar charger must have 3 basic features built-in: The […]

www.homemade-circuits.com

but your suggested v basic method will indeed work :
(with the caveat that you could conceivably 'overcharge' the battery at times. Is it OK to keep bunging in 1/2A when it's fully charged? Maybe? but some sort of controller would alleviate that issue)

 

[brianmoooore]

As the OP says, he's using the panel for the job it's intended for, so I would expect, possibly optimistically, that it's suitable for use without modification.
If the wires are going to be trailing outside the caravan, my main worry would be damage to and short circuiting of the wire, and hence the large caravan battery, which won't end well. A small line fuse as close as possible to one of the croc clips would be my recommendation. Include the diode as well, if you like.

 

[Avo Mk8]

Brianmoooore, that's a fair point. Should work properly as supplied!
The 'blocking diode' could well be incorporated in the panel.
I couldn't identify the exact panel the OP has, but the spec. should make it clear.

 

[Foxinthehouse]

Well that is good advice from this forum again. Thanks for all those tips. I think what I will do is add the fuse and diode just for my own peace of mind. I am not concerned with overcharging as this small panel is behind a window and it is doubtful whether it would ever reach anywhere near maximum output, and it is charging a 110 amp hour leisure battery which is a bit of a brute. The caravan in store in the UK faces east, so it only gets morning sun and most of the day relies on just natural daylight. However, I will monitor it and maybe let you all know in a couple of months. Out here in Portugal it faces west and very sunny, so a bit more vigilance may be required.
My initial concerns were brought about by the fact that the first panel failed while out here in Portugal, and I wondered about a short circuit with the crocodile clips. However, Low Energy Supermarket have replaced it without question and it is waiting for me at home in Wiltshire when I get there in a few weeks.
Many many thanks to you experts - Avo Mk8, DPG and brianmoooore.

 

[Foxinthehouse]

Just in case anyone is interested, this is the solar panel I bought. There does not appear to be any diodes or circuit protection, nor does it mention any in the description- see my photos of the rear connector in initial post. The reviews also complain of no diode, it is a problem easily fixed thanks to you all.