The Basics - what it does.
A solar charge controller - SCC for short - converts the power from a solar panel into power suitable to charge a battery - usually, but not always - 12v.
A solar panel converts light into power. As the light increases and decreases, the power also increases and decreases, as you would expect. As the light increases and decreases, the voltage from the panel does the same. But in order to charge a 12v battery efficiently, you need a constant voltage, and this is what the SCC does - it converts a varying voltage to a constant one.
So in a basic setup you connect a panel to the SCC and the SCC to the battery.
Solar panels are specified in watts, but SCCs are usually specified in amps, which can be confusing if you don't understand the relationship between volts, amps and watts. There are technical reasons why this is so, but I want to try and keep this simple. In order to match an SCC to a panel, you take the amps of the SCC and multiply it by 12 - volts X amps = watts - So if you see a 12v SCC advertised as 10 amps, then 10 x 12 = 120. So the max panel that SCC will cope with is 120 watts.
Or you can work it back the other way. If you already have a panel and you want to choose a 12v SCC, then you divide the watts of the panel by 12 and the resulting number is the amps the SCC has to deal with. So for example a 200w panel needs a (200 / 12) 16.6 amp SCC. As with most things electrical you add a bit on for safety, so in this case you would probably choose a 20 amp SCC.
If you have multiple panels, then you just add up the watts and perform the same calculation.
Most vans will usually have just 1 or 2 panels, but sometimes more on a big van. If you are looking at a multi panel setup, then you also have to decide whether to connect them in series - ie daisy chained, or in parallel, this decision has an effect on which SCC you choose. However, for now, I want to keep it simple, so we will just talk about a 1 panel setup.
The final part of the calculation is to do with your battery setup, and what you can expect from your solar system.
A 12v battery likes to be charged at roughly 10 to 20% of it's total capacity. I prefer to use the higher figure. So a common 100ah battery - that's 100amps for 1 hour, or 10 amps for 10 hours ... etc, will benefit from a charge of around 20amps, maximum. If that battery is flat in the morning, then in theory, on a sunny day, a 200w panel should charge it back up in about 4 or 5 hours.
A reasonable rule of thumb is that you should have roughly 1 or 2 watts of solar for every 1ah of battery.
The next choice you have to make is what type of SCC you want. You will probably have seen that there are 2 types - MPPT and PWM. These refer to the design of the circuits inside the SCC.
PWM stands for pulse width modulation. How this works in very simple terms is that if you have a higher voltage and need to convert it to a lower voltage, you can do this by "pulsing" the higher voltage - if there are gaps between the pulses, then for all intents and purposes the pulsed higher voltage appears to be lower because it averages out. That is an extremely over simplified analogy, so tech folk please don't criticise! The pulsing is done by chips - integrated circuits - and these chips are really simple because they only have to count and time the pulses and gaps. So PWM controllers are the cheapest. And they work fine.
MPPT stands for maximum power point tracking. How this works is that electronics in the SCC "tracks" the variable voltage from the panel, and calculates the most efficient way to convert it to the required lower voltage to charge the battery. It does this in real time, so it basically needs more complicated chips - tiny computers, and the voltage conversion is done by powerful transistors. This means that they are more expensive than PWM, but they are also more efficient - about 25% better.
MPPT controllers used to be so much more expensive than PWM that they were out of range for most users, but as with most things, prices have come right down in the last 10 years.
MPPT controllers used to be so much more expensive than PWM that they were out of range for most users, but as with most things, prices have come right down in the last 10 years.
PWM controllers still have their uses, but nowadays most van owners will go for MPPT.
However there is a problem with crime and counterfeiting. Many cheap Chinese SCCs on Ebay and Amazon are labelled as MPPT, but are really PWM inside. Unfortunately there is no way to check in advance, and the only way to check the difference between the two is with test equipment. So it's a bit of a lottery, and most folk just never know, and of course the police and the government don't care. However I am not saying that all "noname" SCCs on Ebay are fake - just that the likelihood of ending up with one is higher. The "nonames" are also usually over specified - like broadband speeds - so a cheap SCC advertised as 20amp, will likely be a lot less.
So the best advice is to buy a recognised brand and spend a bit more. It's not like the difference is hundreds - it's tens. In my experience the best budget range of genuine MPPT controllers are made by Epever and are called the Tracer range. They are very popular, and readily available. I have never seen or heard of a counterfeit Tracer.
The next step up is Victron. Victron are the BMW or AUDI of the industry. Their controllers are about twice the price of anything else, but they are the best you can get, but are still affordable for reasonably small van installations. Additionally, Victron are at the cutting edge of design, and nowadays their stuff comes with Bluetooth functionality so you can always see what is going on on an app in your phone.
But for most folk, a Tracer will do the job nicely. It's what I have been using for years.
Another way to protect yourself from dodgy gear is to use a reputable UK company. There are several in the UK, but I can recommend 2 off the top of my head - Bimble Solar and Photonic Universe - both easily found online. Anything these guys sell will have been checked for quality and specification, and you can buy with confidence.
Finally - I just want to say a word about a situation that I have found many times causing confusion online. Most controllers - SCCs - come with 6 terminals. 2 for the solar panel in, 2 for the battery out, and 2 usually marked "load". It is these extra 2 load terminals that cause confusion. In a van installation, these terminals are not used, or if they are, their use needs to be understood.
The load terminals are for other applications. In a very simple installation - for example a solar powered outdoor light or whatever, then the light, which is the load, could be connected directly to the load terminals of the SCC, rather than all the way to the battery, which could save wiring. But in a van, where you are adding solar, the battery is already connected to all the stuff in the van, so there is simply no need to use the load terminals on the controller and they are left unused. The confusion arises because of the way it is labelled - people think that just because there are 2 terminals, they should be connected to something. But in the majority of van installations - they are not needed, and not used.
But you can use them if you want to in certain situations - if you do happen to have something new that needs 12v in the van, and it is closer to the SCC than the battery, so will save wiring, then you can connect it to the load terminals for convenience - it is just another connection to the battery. If you do this, and your SCC has a display, bear in mind that the display's "load" section will only display the values for whatever you have connected - not the whole van.
Above is a basic Epever Tracer 20 amp MPPT - less than £50 (2019)
An example of a Victron MPPT - generally recognised as the best available.
This is advertised on Ebay as MPPT - for less than £10 - I wouldn't, and neither should you!
