Van solar panel installation and wiring

December 23, 2013
20 Comments

I was lucky to inherit two 100w solar panels with the van – they just need to be wired in.

I used an external waterproof junction box mounted on the roof of the to take the panel cables into the van via compression glands and then into the MPPT solar controller charger (MPPT30 solar charger – bad choice in hindsight, see last section of this post).

For more about the electronics see the 12V electrics and wiring

A look at my solar panels

Like most things of this nature, I like to poke around for a while with a screwdriver and multimeter to see what’s what before I do anything else.

The panels came with no labels on them; no indication of min/max voltage or power ratings. I googled ‘mono-crystalline solar panel’ and the ones with 4 by 9 solar cells are 100W so its probably safe to say my panels are 100W each.

Back in November I checked that each panel was giving the same voltage – if they disagreed then it would probably indicate a dead cell somewhere but all seemed well. Despite some cells having visible hotspot damage (scroll down to photo), both panels gave the same voltage of 18V and a closed current of 4.33A so around 80W of power. Not bad for November, although it was a clear sunny day and the panels were directly pointing to the sun.

100w-solar-panel — I positioned the panels to get the maximum output – you can tell it is winter by the angle

solar-diodes — Each panel has a diode across the cell string (bypass diode) and a blocking diode in series with the full panel circuit.

Installation and wiring

I wired the panels in parallel just because there is no point wiring in series (higher voltage) unless you have an MPPT controller with decent DC-DC converter. Maybe I will experiment with other configurations at some point in the future

The 2 100W panels will be wired in parallel using MC4 t connectors

The panels are bolted onto two length of wood which are then fixed to the roof rack.

junction-gland-box — At first I was going to gland straight into the roof but the junction box method allows for easier expansion plus I can also take the mains hook-up cable out through the roof.

sikaflex-junc-box-roof — I used sikaflex to glue the junction box onto the roof. I then drilled a 20mm hole down through the bottom of the box and into the roof making sure I had a decent seal round the hole with extra sikaflex. It worked really well.

junction-box-on-van-roof — Junction box on roof with room for 5 more glands if needed.

solar-cables-van-roof — The 2 black cables are solar. I also put the mains hookup cable in whilst I was there

solar-panels-on-van — Plenty of more space on the roof for either more panels or a garden.

Solar panel damage- hot spots?

It took me a while of searching and asking on forums what these marks were on the solar cells. I’ve decided it must be hot spot damage from prolonged shadowing of a cell. The loss in power due to the shadowed cell will be dissipated across that cell causing hot spots and eventually burning the cell out.

hot-spotting — I hope I can just ignore this and it isn’t as bad as it looks. It does appear to work fine

Solar charge controller

I found out there are 2 types of solar controller; normal PWM (pulse wave modulation- pulses of varying duration) and MPPT (maximum power point tracking – efficiently down-converts a higher DC voltage from the panels to the lower voltage used to charge the batteries)

Without much further research, I went straight onto Ebay and bought an MPPT controller for £42.

ebay-mppt30-solar-controller — Ebay Chinese MPPT controller. Looks too good to be true for £42? … I HAD to see inside

inside-mppt30-solar-charger-controller — The main thing that will distinguish an MPPT unit from a regular controller is the presence of a DC-DC Buck converter and considering this is 30A, I’d expect a decent sized inductor somewhere. So where is it? Look at the very top right of the PCB, that is the the full Buck converter complete with tiny little coil. I have a feeling this Buck converter is being used for just the 500mA supply for the USB charger output. I only noticed this when I went back and looked at the photograph I took but i’ll probably investigate more one day.

I asked for a part refund and just forgot about the chances of getting super efficient MPPT charging for now. This unit does have some nice features such as load shut-off setting so the batteries will not discharge less than, say, 11.8V and also overload protection.