UKC

Hoping someone can help me out here - 

I'm thinking of buying a couple of PV panels for a van, which will be running mainly on a 12V system.

Their open circuit voltage (VOC) is 39.25V and their max power voltage (VMP) is 31.85V.
They're 285 Watts each giving a total of 570W - which is quite a lot! (though we do have many grey days in cumbria)

I understand that I need a MPPT controller in the middle somewhere, but the ones rated to higher voltage & wattage seem to be £200-£500.
Will some of the cheaper ones or PWMs do the job? Or do I simply need to get lower voltage panels to avoid spending that sort of money?

I've done some reading online but it's all very technical... Any help much appreciated.

Jon

You’ll probably only see half that power in realistic conditions given the angle the panels make to the sky and weather etc.

You need to get a controller rated to the maximum voltage but it doesn’t need to be rated to the maximum wattage.  They automatically de-rate the current to their safe current limit.  (Their power limit depends on the battery voltage * safe current limit and many will go to 24 or 48 V batteries giving increased wattage over 12V).

I’d probably only bother getting a controller rated to half the max theoretical power.  A Victron MPPT 100/20 would exceed the voltage of your panels in series and would manage up to 280 W (assuming 12V charging at 14 V, 20A)  of charging, costs £120.  Or you could use a seperate MPPT 75/15 for each panel which will give you a max of 210 W per panel and allow separate power point tracking per panel.  They’re £80 each.

Personally I’d go for two controllers as a way of keeping the system voltage below 50 V DC.  Two panels in series is ~80 V which is veering into hazardeous territory.

Edit: 285 W - is that a standard “house roof” panel?  These are pretty big and heavy and not good for vehicle mounting. Have a look at some of the semi-flexible ones out there; 2 mm thick and 100 W or so @ 30V.  You’d probably get 4 on a van roof.  More expensive than house roof mounting ones but massively lighter and simpler to vehicle mount.

Thanks, that's some really good info. I hadn't thought about having two separate controllers.

If for example I used an MPPT 75/15 on each panel, on the odd summer days where they were generating up to 285W the max I could gather would be 210W through the MPPT?

The Victron MPPT 100/20 seems to say just 12V and 24V, how do I marry this up with the 32(?)V panels?

Yep, they are just standard 'roof panels' - 18kg each. Although a fair bit heavier they are a lot cheaper, and my thinking with rigid panels was to be able to set them at an angle when stationary. 
I like how the semi-flexible ones sit flat on the roof though - better mpg!

Have you looked on eBay for a 100-120W motor home solar panel kit, they seem to be around the £120 Mark and include the controller, panel, mounts and cable gland. You haven't stated what your battery sizes are or what your maximum load is but you are best to stick to a 12v solar system to charge a 12v system. As soon as you step down or up the voltage you are losing efficiency.

You may have done the calculations already, but if not it's worth thinking about how much power you're actually going to need. I've got a 210w panel and ~200Ah of leisure batteries and it's been plenty to power lights, charge phones, laptops etc. I had a split charger for a while, then I realised I'd wired it up wrongly and it wasn't actually doing anything and the solar was easily enough to keep it topped up. I've used my van a fair bit in the UK so plenty of cloudy days.

I went for a second hand house roof style panel. Very cheap for what you get (I paid £90 for a 210w panel) and I didn't find it too much hassle to mount. Need to be happy with an angle grinder to make up some brackets and drilling a few holes in the top of your van, though. Personally I wouldn't bother making the panels variable angle. Unless you're going to be in the same spot for a long time it's probably going to be more hassle than it's worth.

Exactly.  It’s not uncommon to have panels at higher wattage then the controller, as that higher wattage is rarely reached.  I’ve got 220 W of panels on an MPPT 75/15, and they reached that maximum value twice in one year.  Briefly.  For about half an hour during a day.   Over the course of the day conditions will almost never be right - angle or weather - for peak power so very little is lost underspecifying the charge controller current rating.  Under specify the voltage rating and it’ll either shut down or slag itself.

Two such panels each with an MPPT 75/15 could then do 420 W if your battery can take it.  You don’t really want to go charging a single 120 Ah leisure battery that rapidly however; you’d cause a lot of hydrogen generation and gassing.   The bubbling sound distresses me as well.  It’s not right.

The “100” is the maximum solar panel voltage input, and “20” is the maximum battery charge current output.  Confusing.  So the MPPT 100/20 can take a solar panel voltage of up to 100 V and and will convert this to the appropriate voltage to charge a 12 V or 24 V battery at up to 20 A.  So that’ll just work.

It sounds like wintertree knows what he's talking about and has covered most of it already. I'd just add that if you're on a budget the ePever Tracer MPPT controllers are quite a bit cheaper than Victron. Victron have the best reputation, but a lot of van users have ePever and I haven't heard of any problems. I've got a 145W panel and a 10A Tracer. In theory the controller could limit the panel output, but I've never seen it get near 145W in real life. You could probably get away with a 30A Tracer. Only 360W ish but it's rare that your panels will be producing more than that. If you do find it limiting, you can flog it on eBay and upgrade.

With 31V panels on a 12V system I would put them in parallel, not series. Then if one gets shaded you still get some power.

Just about to press the button on a Tracer MPPT controller from photonic universe.  £59 and does upto 130w at 10a.  Will be fitting a 100w flexible panel to charge a 100ah leisure battery alongside a split charge and hookup system.  

Not a large electric user but feel this will add more flexibility and keep the battery in better condition.

Those panels the op is using are serious output.  Are people really using that amount of electricity?  A couple of phones, a blue tooth speaker, water and lights is all I seem to run when parked.  2l bottle of water frozen in coolbox is enough to keep it chilled for 2-3days if you run it when driving.  

Some people certainly use that much. On the van forums some people have 2kW inverters to run a microwave and espresso​ machine!

My main draw is the fridge and Webasto heater, which will barely do two days on battery hence the solar panel.

Bear in mind that if the OP is parked up for several days in winter he will only get a fraction of the rated output and for only a few hours a day, so the panels need to be rated far higher than your actual consumption.

Wow, I haven’t even had time to read a book in mine yet, never mind have need for an espresso machine.

I have a night heater waiting to fit but tbh decided I wasn’t too bothered.  Do they use much leccy?  My van is comfortable mid Feb to end October without heating so not sure I will bother

A fridge was the main draw in mine. If you're in the van long term a coolbox doesn't cut it. 

Also charging my girlfriend's laptop. She was remote working from the van so some days had it on for eight hours. 

Thanks for all the info guys. Some really useful stuff there, I understand a bit better now! The Epever MPPTs look good, I'll go for one of those.

There will be two of us living in the van full time, nature of work at the moment means we are forever travelling about the country, and it's just uneconomical to be renting somewhere and paying for accommodation.
We're planning to run a fridge (sparingly), lights, charge laptops & phones, fan, diesel heater, wifi hotspot, water pump.
The americans seem to run blenders and coffee machines too, but that's a step too far...

Two panels of that size might be overkill when drawing full, but Hooo has it right that we will be parked up for multiple days at a time, come rain or sun. My thinking was the more cells, the more power even if that's still pretty low most days.

If you're living in it, then definitely go for more cells. You don't want to find yourself on a cold wet night having to start the engine to run the fridge and heater! 

It's worth making the panels hinged so that they can face the sun too. In winter with what sun there is low on the horizon you need to get them pointed towards it.

As someone who believes that you get what you pay for, this was a surprise. Chinese cheapo panels beats expensive version.

youtube.com/watch?v=3-sc4rlV93g&

Ah yea, I've seen that video, was equally surprised by the result. I've bought panels already, they were a good price and good spec so fingers crossed.

Hooo, I've been thinking the same. Just need to work out a hinge setup that will sit flush under the panels.

I've a 200w panel on mine with a tracer. It would not be sufficient in the UK in autumn or winter - main load is a compressor fridge, propex, and phones. The sun being low on the sky means it is hitting the panel at an angle so you will get maybe 10% of the panels rating in the middle third of the day. 

I would recommend getting as many panels as possible unless you're going to some where with good weather. 

Avoid the flexible panels - no air gap underneath is less efficient and are less reliable / harder to replace (based on experience of people on the self build motor homes forums I'm on) .