There have been a few questions on the Classic Hymers Technical FB group recently about solar panels, and it occurred to me that I haven't done a blog on the subject with specific reference to classic Hymers. This is not intended as a definitive right/wrong instruction manual for solar, just a general guide based on what I have picked up over the years from my own and other solar installs.
Most vans nowadays have some sort of solar installation. Many classics had them from new, or near new, and they were really expensive back in the 90s, but pretty much now most of those will be at least 25% or more down on performance. Technology has moved on - panels nowadays are around twice as efficient and at least 5 times or more cheaper than 90s technology. At the same time, the average need for power has gone up, with most owners nowadays wanting an evening's viewing of TV, a few hours of laptop, plus charging of all their devices - and all in addition to the normal requirement of lighting the van and running the pumps and fans.
The good news is that all this is possible, and more, with a basic single battery system, and a decent modern panel and controller. The aim of any solar system is to put back during the day, what you take out at night. If this is achieved, then a van can stay off-grid - ie no need to plug in or drive. This is of course dependent on the weather, but it is a perfectly reasonable expectation to have - at least in the summer months.
So let's look at what a typical modern system has to offer, in the context of a pre 95 classic Hymer motorhome.
Let's first look at upgrades. My advice generally, if you have an existing system, especially one that you know to be over 20 years old, is to replace everything. The reasons are pretty much as already stated - technology has moved on and prices have fallen. The controller in an old system will be PWM (see below), the panel(s) will be very old and the wiring will also be suspect, and at the very least need checking. Of course, if you do have an old system, and it appears to be working OK, then don't fix it if it isn't broke - the true test of your electrical system is simple - does it ever disappoint you? If so, then it's time to do something about it.
Another good reason to upgrade to modern panels is that they perform so much better in low light. So if you use the van a lot in late winter, early spring and autumn, then you will benefit, as modern panels bring in more power when the sun is lower and weaker.
So let's assume that you have decided to either replace everything, or that you have nothing to begin with. So let's talk about the two main components of a solar system - the panels and the controller. The wiring and layout is quite simple - a solar panel sits on your roof - it has two wiring terminals on it - pos and neg. Two wires go from the panel to a controller, which is basically a box about the size of a normal battery charger, where they connect to two terminals that are clearly marked pos (+) and neg (-) "panels" or a symbol that looks like a panel. The controller is mounted at a convenient place, usually on a wall close to the battery. Then two more wires lead from two similar terminals on the controller marked "battery", and go to the battery. That, for the very non technically minded, is the basic layout. It is very similar to how a battery charger plugs into the mains, and has two wires going to the battery - only in this case, the panel supplies the power, rather than the mains. All a solar controller is, is a sun powered battery charger.
There are other features - such as switches fuses and cable routing and waterproofing - but all these will be covered later.
Panels
So let's talk about panels. Panels come in many shapes and sizes, two different main technology types, and two formats.
The two formats are flexi and rigid. Rigid panels are the traditional type that you see on houses and offices. Flexi types are specifically for leisure vehicles - vans and yachts. Rigid panels are exactly that - rigid, so are difficult to fit on a curved surface. Flexi panels solve this problem nicely. There is quite an online debate about which is best. I have my own opinion, which is that rigid panels should be used where possible - ie if the roof is flat, or near flat. The reason is that rigid panels are cheaper, and they are easier to get off and replace if they go faulty. Flexi panels are more expensive, and if you are unlucky enough to have to replace one, they are really hard to remove. There is also another reason I like to give, and that is that rigid panels are produced by the millions, because they are used in big solar farms. Flexi panels are produced in the thousands, specifically for the leisure industry. Solar farms demand 25 year warranties, so are generally produced to high commercial standards. Generally speaking with regard to flat roofed classic Hymers - rigid are usually used.
While reading up on the latest in flexi panels, I came across this useful info about them -
"One decisive factor in determining the lifespan of Flexible Solar Panels would be the coating material. The two more popular types would be PET and ETFE. PET is a polyester-based plastic and ETFE is a fluorine-based polymer. And, the main differentiating factors between the two would be strength, durability, and resistance to corrosion. While PET-laminated Flexible Solar Panels generally have lifespans of up to 5 years, ETFE – laminated Flexible Solar Panels have lifespans of up to 10 years."
I have also seen ETFE advertised as PTFE, they are the same, or very similar.
Rigid Panels - The two different technologies are mono-crystalline and poly-crystalline. This refers to the type of silicone used in the actual cells. generally speaking mono is better and slightly more expensive, but these days it is less of an argument, especially when you are buying secondhand panels from suppliers like Bimble - see below.
Sizes are nowadays pretty standard - it is not an oversimplification to say that in general, with regard to van installations, there are only 2 basic sizes - big and small! Big panels are those used on solar farms. They are generally about 1750 x 900. Smaller panels are generally around 900 x 500. There were very few large panels around 30 years ago, so original solar installs on Hymers were usually always small panels. In terms of power - the larger panels are in the 300 to 400w range, and the smaller ones are around 100w. More on power below.
Fixing and brackets
Fixing panels to the roof. This is quite a hot topic that always attracts a lot of online discussion. The big debate is basically over screws vs adhesive. Nobody likes to drill holes in the roof of a cherished motorhome, so there is a strong case to be made for sticking them on and many advise this online. Flexible sealer adhesives have come a long way in the last few decades - silicone is a thing of the past - yesteryear's technology - but modern adhesives can literally stick the roof of your van to a crane, and support the weight. But only if chosen and used and prepared correctly. And that is the problem for the DIY installer - no motorhome discussion group or forum is complete without at least one story of panels flying off on the M6, and just one story like this frightens people away from adhesive. Most horror stories are simply down to bad method or workmanship. Even professional installers can have bad days.
I believe that the best course for the amateur installer is screw and glue. There is no real disadvantage to having screws in the roof of a 30 year old van, and a modern panel installation at this age is likely to last the rest of the life of the van. There are also a couple of reasons for screw and glue that might not be apparent until you are up there for the first time. The first is that a screw will compress the adhesive nicely, forming a solid combined bond. The second is that a big panel can be difficult to position with adhesive only - it can slide, and as it does it smears the adhesive. It's fine if you have access ladders and platforms and 2 assistants to help you lower a panel gently, but can be very difficult on your own while crawling on your knees on the roof. But if using screws and Z brackets, you can screw one corner down finger tight, without adhesive, and this will lock the panel in position. You can then go to the next corner and there is enough flex to lift up the corner and bracket, apply adhesive, and then let the panel drop is place in perfect position, then screw, and then onto the next corner, finally arriving back at the first corner where you can remove the screws, lift the corner, apply adhesive, and then drop and screw. This is a tried and tested method for fixing a large panel single handed. If using the long streamlined front brackets, then you do front then back, rather than corner to corner. But please note this method is suggested for big modern panels, which are hard to handle. if you are fitting small panels that you can simply pick up and place, then they are a lot easier.
The absolute number one rule about using adhesive is that the mating surfaces must be squeaky clean and totally free from dirt and grease. This means the section of roof where the bracket is to go, and the bracket itself. This doesn't have to be a complicated process, but you must take into account that the roof on a 30 year old van can have quite a tough layer of dirt and grime, baked on. A good scrub with laundry powder is a good method, followed by a thorough wipe off and then a final deep clean with solvent. All you need is a pack of cheap microfibre cloths and a bottle of solvent from the discount store. Solvent should be at least acetone quality - my friends in the trade use good old fashioned brake cleaning fluid. It works, and is cheap. Avoid solvents that have other chemicals added to them, or are less refined and oily - so turps and white spirit are to be avoided if possible. If you are a beginner, then a good course of action is to do a test. Stick two similar pieces together as a test, leave it overnight, and then try and separate. If you get it right, you will be amazed at just how strong it can be.
As for screws, as well as the problem of water ingress and corrosion, usually 10 years down the line, there is also the problem that motorhome roofs are not exactly the best thing to screw into. There is a thin aluminium skin in the case of B class Hymers, and a reasonable thickness of fibreglass on S class Hymers. Under the skin there is stiff foam insulation. The whole thing is about 32mm thick, so you would never use a screw longer than 25mm. The trick with screws in the roof is not to overtighten. You want the threads of the screw to be fully interlocked with both the aluminium skin and the foam interior. The last thing you want is to apply too much force so that the threads strip and the screw continues to rotate - which it will very easily if you go at it hard with an electric impact driver. You are not screwing into wood or hard metal, where you want the screw torqued up to the max, you just want it to the point where you feel resistance, and the adhesive is just oozing nicely out of the edge.
Personally I use both at the same time - screws and adhesive. Not because I have less faith in one or the other, but simply because (for me) it makes for an easier installation. Screws have to be properly sealed, so I use the adhesive to both stick and seal. The screws I prefer are hex head stainless steel self drilling screws. These drill their own hole, so it is tight, and it is a single operation with a power drill. At the same time it forces the mating surfaces of the bracket to the roof, and seals everything. See photo.
You can make your own brackets from aluminium angle, or you can buy commercial ones like in the photo. You can also get really snazzy curved aluminium extrusion which also acts as an air deflector.
As for screws only - there is no argument for screws only! You need to seal the screws against water ingress anyway, and sealer adhesive is the best way to do this. The only case for screws only is when the panel is attached to metal roof bars.
This is a nice example of plastic brackets glued and screwed (photo Mark Earles)
On the subject of Sikaflex, I have noticed that there is a whole bunch of internet snobbery about Sika. Every time somebody asks any question about sealer adhesive, several answers will come up like "only use Sika 123", or 789. Or XYZ. With no further explanation. This is no more than some blokes just liking to sound expert. It has its roots in the yachting forums, where sealers are much more important, and Sika has traditionally ruled the roost. But Sika is no different to the rest. Except for the price. And all the numbers are confusing for the beginner, so why bother when you have at least 2 cheaper and easier alternatives readily available - Soudal or CT1.
Finally, on the subject of brackets, let's talk briefly about removal. No owner fits a panel knowing it will have to be removed one day, but shit happens - hail damage, collision with trees, drunken rooftop dancing, vandalism, or even just a big upgrade, can all result in the need to remove a panel. Removing panels can be a real pain, especially those fitted by the germans 30 years ago using screws and mastic! But it is always possible. The trick is to use a thin scraper blade, and help from a heat gun or hair dryer if possible. Work the blade under the edge and keep working the gap wider and wider. It needs patience and elbow grease, but it will come off - one bracket at a time. This is another reason why I favour Z brackets screwed and glued - they are the easiest of all to get off because there is less surface area and you can get at them, certainly compared with the plastic brackets. It is only when you actually try and remove a bracket fixed with a modern polymer adhesive that you realise just how bloody strong they are! You have to use the same sort of method as a stonemason splitting a block of marble - you create a small gap, and keep driving tiny wedges in and slowly and carefully separate the two. If you are careful with the scraper you will be just left with 2 screw holes and an undamaged roof. Screw holes can be filled with the same adhesive and smoothed off. Holes filled in this way will never leak. I have holes in my roof from 10 years ago filled with Soudal and I have to get right down and look hard to even find them - the filler has weathered over. I am on my 3rd major panel installation in 12 years, and I consider my roof to be no worse for wear because of panel screws.
Controllers
Now let's talk about the other main component - the controller.
Controllers also come in several shapes and sizes and also in two technologies - PWM and MPPT. It is important that the controller is matched to the panels, in terms of power rating. PWM is now generally regarded as "yesterday's technology", replaced by MPPT.
For those who are technically minded, older controllers were PWM (pulsed width modulation). MPPT stands for maximum power point tracking. Basically a solar panel's voltage goes up and down in direct proportion to the sunlight. The controller takes this varying voltage, and converts it to a constant voltage to charge the battery. A PWM controller just uses simple power transistors to convert the voltage, and it uses a very simple formula to do so. As a result of this simple formula, bits of power get lost. It's a bit like 10 divided by 3 is 3 remainder 1, and the 1 gets discarded. That is a gross oversimplification of course! The bits that get lost are dissipated as heat.
An MPPT controller on the other hand has its power transistors controlled by a computer chip. This enables it to make the most efficient conversion at any point in time, so the "remainders" are collected - ie maximum efficiency - about 10 to 20% more efficient, which adds up nicely over time. the same computer chip that controls the conversion, also controls the battery charging - much in the same way that a modern digital 230v battery charger is more efficient than the original "dumb" charger that was installed in the 80s and 90s.
MPPT chargers used to be so expensive as not to be affordable by motorhome owners, but thanks to those lovely hard working Asians that have brought us all sorts of digital goodies in the last few decades, so it has been for solar controllers and you can now buy a decent quality 20amp MPPT solar controller for less than fifty quid - so it's a no brainer.
See below for more on controllers.
Cabling
So having described panels and controllers, let's talk about another aspect of the system - the cabling. As already stated, cables run from the panel(s) to the controller, and from the controller to the battery. It is important that these cables are thick enough to carry the power from the panels, to the controller and to the battery, efficiently - in other words, they have to be thick enough. Nowadays there is a standard solar panel cable that is used across the industry. It has its own type of connector - the MC4. The standard as far as motorhome use is concerned is 6mm. This is a semi rigid black cable sold simply as "6mm solar panel cable" - and is capable of carrying 70amps. You should use this if at all possible - for the simple reason that most panels nowadays come with short "tails" of cable with MC4 connectors on them, and 6mm solar cable usually also comes in fixed lengths with pairs of MC4 connectors on the end. So all you have to do is run the cable and plug straight into the panel. The MC4 connector is designed for commercial solar farm use and is weatherproof and easy to use. MC4 connectors are either male or female which enables pos and neg wires to be safely separated. Unless you have a very good reason, it is best to use dedicated solar cable and MC4 connectors, between the panel and the controller.You don't have to of course, you may have a good reason not to, but in the absence of a good reason - use the industry standard. The only slight drawback with solar cable is that it is a bit more rigid than normal cable - which means that it doesn't like to go round corners easily - you should bear this in mind when planning your cable routing. But generally speaking most van solar installations can use it easily.
From the solar panel to the controller, at some point the cable must come in through the roof, and of course this must be made weather and water proof. The point at which this occurs is generally called a "cable gland" - see pictures. This is generally a protective plastic (or metal) cover, which is sealed against the ingress of water, and covers the hole you have to drill in the roof for the cables. Where to drill this hole and how big is individual to each van - but it usually comes down to common sense. If in doubt ask online for specific advice. But I will pass on a couple of tips I have picked up from the trade. Pick a convenient cupboard or wardrobe, and drill up from the inside out with a small drill first. You will have measured and estimated - nothing is more stressful than drilling your van roof! Then enlarge the hole from either side, minimising dust and swarf - a vacuum cleaner nozzle is useful to catch this as you drill. Don't be shy on hole size - 12 to 15mm - is best for a twin cable solar run - you should be able to get an MC4 connector through the hole.
From the controller to the battery, you can also use solar cable if you like, but sometimes you will find that solar cable is not as flexible as you would like, so if you run up against this, you can use normal cable. Just make sure that the cable you use is thick enough - if in doubt, ask online. Generally speaking, an average 200w to 400w van solar system will need 10 to 20 mm2 cable. Unless there is a long distance (ie over a metre) between the controller and battery, then for a 400w panel I use 10mm2 flex cable between controller and battery.
If you are wanting to use old cabling for a new installation - ie because it looks ok and is already routed and fixed, then fine. An easy way to check if it is up to the job is to use a multimeter. All you have to do is to measure a fixed voltage at one end of the cable, and again at the other. Measurement must be to within 0.1v - point one of a volt. Then measure the diameter of the copper in the cable. Then either google for the standard loss chart, or just ask online. You don't want any more than half a volt difference - ie 12.5v at the top should be no less than 12.25v at the bottom. Preferably a lot less - less than 2% is always preferable.
Cable routing is a bit of a black art, but generally a common sense route should be obvious. If you can't get a hidden route down from the roof to the floor, then consider using that plastic cable trunking you can buy in BnQ - it comes in white and brown, and it soon blends in.
Classic Hymer Specifics
So that's the basics. Now let's talk about Classic Hymer specifics.
The first discussion is - what size panel? Well the modern answer is - as big as you can fit on your roof. This really is true. The problem is that many owners simply don't understand solar power. They think a 300w panel is a constant 300w - like a charger or a battery - it's not. A 300w panel will only deliver 300w in absolutely perfect conditions - that is a perfectly clear sky and with the sun absolutely directly overhead. This actually never occurs in the UK, because we are north of the tropic line. So the sun is never directly overhead - the highest it gets is on the summer solstice, the longest day, June 21st. The most I have ever seen from a 300w panel in the UK is about 260w, and this only for an hour either side of midday.
At the opposite end of the scale is winter - December 21st, the winter solstice, the shortest day. Even if the weather is clear on this day, you are unlikely to see more than about 100w from a 300w panel, and again, only an hour either side of midday. You literally only get a fraction - 25% if you are lucky, from a solar system, in mid winter, than you do in midsummer.
Another problem is that panels on a vehicle are mounted flat, which reduces the efficiency. Solar farms have their panels tilted south at 45%, which makes a huge difference. It is possible to tilt a van mounted panel, but it is mechanically complex and not relevant to this article.
So you really do literally have to fit the biggest panel you can! With a modern solar system you generally have more power than you need in summer, and never enough in winter. But given that most owners use their vans in summer - then that's OK. But other owners - full timers for example, who want to be off grid as much as possible in all seasons, then these systems really do have to be as big as possible.
The good news for classic Hymer owners however is this. Big panels of 300w and more, are now available at good money from suppliers like Bimble, and there is a perfect place to mount them - on the front roof of the van, over the cab and the drop down bed. Unless you have a rooflight here, it is a perfect place for a big panel. See photo. It is out of the way, and doesn't affect the rest of your roof - if you have a top box or satellite dish etc. It is also a convenient place to route cable - down the wall just behind the passenger or driver seat.
It is the perfect place for a big panel - whether you think you need a big panel or not, with a big panel costing as low as £90 from Bimble, you can have a 300w MPPT solar system on your classic and have change from £200 if you DIY.
For power users - ie those who full time, or just need more power, then the front roof is the place to start - by buying the latest panel you can get 400w on there, and by shopping around you can get 2 small 100w panels that fit either side of the big roof light - see photo.
If your van already had old panels, mounted usually in pairs, in the middle or rear of the roof, then you have several choices, depending on what you want. You can re-purpose that part of the roof - for a top box perhaps. Or you can find modern higher power panels to occupy the same space. However you will also (possibly) find that the original panel mounts are stainless steel and very solidly mounted - and removing them can be a real pain. You can try and find new panels that are an exact size match, but generally I haven't had much luck on this score. Sometimes it's just best to leave them. I have seen serious damage done to roofs trying to remove old panel brackets - so be careful.
That's why I'm such a fan of the front roof large panel setup - it's easy.
Now let's talk about specific controllers, and monitoring and display. Your pre 95 classic Hymer will have the usual twin meters mounted in a panel. One meter gives voltage, and the other (the famous STROM meter!) displays power in amps. The power meter should go into green when the battery is receiving power, and red when you are using power. So far so good. But the problem is that solar systems were never fitted as standard at the factory by Hymer - they were always fitted by dealers or owners. And due to an idiosyncrasy of the electrical design of the Hymer 12v battery system, the circuit that drives the Strom power meter is buried deep behind the fuse panel. The result is that if you connect your solar controller (or anything else for that matter) directly to the battery terminals - any power that flows in and out through that connection, will NOT be displayed on the Strom meter. It is possible to have a solar system that displays on the Strom meter, but to do so you have to connect the cable to a specific terminal which is located behind the fuse box under the dash. It is quite an awkward job, and most owners, even dealers, just never bothered. The main reason for this is that most controllers have their own display (and nowadays maybe an app) so although the solar power going into the battery would not show on the Strom meter, it would show on the display of the controller, so you could at least know that your solar is working and doing its job.
But the downside of this, is that you have to mount the solar controller where you can actually see it. This isn't always convenient, and for this reason, you can get solar controllers with remote displays. But even with a display of solar power on the controller, you then have to look at the Strom meter to see how much power you are using at night, or getting while hooked up.
Sorry if it sounds a bit confusing! But that's the way it has been over the years in most solar installations. But now there is another way, again thanks to modern technology. It is called a battery monitor. A battery monitor comprises of a digital display, and a sensor that mounts directly on the battery. It is quite easy to install. If you have one of these, it displays everything that the two panel meters would, plus it incorporates the solar as well. This has the added extra benefit for some owners that you can mount the controller out of sight - you don't need to see it. Plus many modern controllers come with the option of a display - and the ones without a display are always cheaper!
Once you have a battery monitor - it displays everything you use, and everything you receive - at a glance. And it does it on a nice digital display, often in colour, and much more accurately than the old fashioned panel meters. Now I do understand that the old meters have a certain retro chic, and there is absolutely no reason to do away with them - they will still work as before. And maybe you don't want or need to see the exact state of your electrical system - you just want the lights to stay on and the pumps to run. Every owner is different. But just as panels and controllers have come right down in price, so have digital battery monitors. So if you want to see the status of your power system at a glance, you can - inexpensively. These days you can even have it all displayed on an app on your phone or tablet - if you want - it's up to you. There is a separate article on this blog about battery management and monitor systems.
Suppliers and parts
Panels. In the UK, my one stop shop is Bimble Solar. Bimble is a strange outfit - they started many years ago in installing the very first solar powered stage at Glastonbury, and the business appears to have grown out of that. They stock a wide range of new stuff - mainly for the off grid property market - ie houses with solar. Their speciality appears to be buying secondhand panels in bulk from large companies. These usually have plenty of warranty left on them, and really do represent excellent value for money. They are particularly good for the large panels that fit perfectly on the front of a Hymer. They also stock Epever Tracer and Victron.
If you have decided you want a flexi panel, you should also consider Bimble - they stock a flexi panel that a friend of mine in the motorhome trade now only uses, simply because the failure rate is far lower than any other he has used.
Other well respected UK suppliers (and there are many more) are 12v planet, Sunshine Solar and Photonic Universe - if you google UK solar panel suppliers, you will get plenty of hits, but bear in mind that leisure vehicle solar is a niche within a much bigger industry - house and office solar. So make sure whoever you buy from at least knows something about vehicle solar. The components are basically the same.
Edit 2024 - large solar panels are now appearing in normal electrical wholesalers - in the UK example City Electrical, who have branches everywhere. This is because building roof panels are now pretty much mainstream and roof and solar farm big panels are being churned out in huge numbers. We now have the weird situation where a 400w panel costs under £100 from City Electrical, but if you want a smaller one to fit a space on a leisure vehicle, you may have to pay double the money for half the size.
Controllers. I am a great fan of Epever - a Chinese company that makes the Tracer range of controllers. They are well made and perform well. Top of the pile is Victron - a Dutch company who have their stuff made in the Far East to a very high standard. They are not cheap, but neither are they stupidly expensive. They have the very latest in technology and innovation. If you want to see on your phone what your battery voltage is and how much solar you are getting then both Victron and Epever have a solution. Bluetooth monitoring via app is now all the rage.
Ebay and Amazon are also good sources, as you would expect, but you do have to be careful. There is a mass of either counterfeit or copied or just plain rubbish Chinese stuff out there. You really need to know what you are doing. The worst case is that nearly every controller that is sold as MPPT on sites like Alibaba, Wish, Ebay and Amazon, unless it is from a reputable UK supplier - isn't MPPT at all. It's not even decent PWM - they just shove a big transistor in a fancy box, and write MPPT on it. They get away with it, because it does work after a fashion - ie your battery will get some charge - but only a professional armed with test equipment can tell you whether it is true MPPT or not - so they get away with it. Just search Youtube for "fake mppt" and you will see videos of guys opening them up and exposing them. With a genuine Tracer available for less than £50 - there is no point in taking the risk. My standard advice is just buy Victron or Epever - they are not expensive.
Other names and brands that I have read about or seen recommended are - Renogy - a one stop shop for almost everything solar. Redarc - an Australian company. The ozzies make some really cool solar kit - off grid solar is a big thing over there.
SRNE are another Chinese company who make proper kit, similar to Epever.
Morningstar and Outback are two more top notch manufacturers - mainly for house off-grid systems - very high quality.
Conclusion and more notes.
"Load" terminals. Most controllers have 6 connections - 2 for the panel, 2 for the battery, and another 2 marked "load". This often confuses van owners. Basically the load terminals are not used in a motorhome installation. But they can be used in other installations. A very simple example would be a remote building not connected to the mains. For security lighting it might have a single battery, solar panel, and a light. The light in this case could be connected either directly to the battery, or if more convenient, the load terminals on the controller. That's all they are - an extra set of terminals that are in parallel with the battery. They can be used in a van if you like - you can connect anything to them if it is more convenient to do so, rather than wiring directly to the battery, or to the van's other circuits. If you do so, then the circuit should be fused accordingly. A typical example in a van would be, for example, if a vans solar controller was installed under a bed, or in a garage, or other storage area, and the owner also wanted to install a light in that area, then they could power the light from the load terminals of the solar controller - if it was easiest to do so. I have used them to wire up extra 12v USB sockets occasionally. It all depends on where your controller is located in your van, but generally they are unused.
If you want to read more about digital battery management ...
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