Introduction: Small 12V Battery Solar Power Charging Rig for a Caravan or Camper

About: I like to make stuff for the garden, house, shed & out and about. Occasionally some of it works :)

If you’re reading this, then for reasons of what can only be curiosity or stupidity, you’ve decided you want to have a go at building your own DIY solar rig to complement your caravan, motorhome, van, or camper setup

Hopefully, what you’re reading will be a semi-coherent walk-through guide on how to build a bog-standard unit that will help to charge up your leisure battery, and also give you a handy 12V charge point to allow you to top up a mobile phone, run a light etc. But I’ll also run through a whole bunch of other side-issues along the way as some food for thought.

Ultimately the size & scope of the rig you build is only limited by the size of the devious thoughts in your head and your own ambition – you can make it as simple as the example here, or build something that powers a sound system, lights, chargers and much more – and it’s incredibly easy to customise your rig to suit your own ends & purposes….

Why Bother???

The aim of the game is simple: How to keep your camper or caravan battery topped up while you’re away camping / traveling / at a festival. Over the time you’re away, your use of lights, water pumps etc drains the battery; so to ensure you can keep the lights on you need to have science on your side and MacGyver-like cunning

Step 1: A Small Disclaimer

The Numpty Instructs...

The disclaimer: - I’m not a pro, or even qualified in this sort of thing, the terms & techniques used in some of this would probably make a pro double up laughing. But hey – if you want to follow my advice then great, and if you want to scoff then that's fine too. Also read the final words at the end of this about trusting anything you read on the internet....

….But – on with the guide.

Step 2: Tools & Safety

It’s Tool-Time: The tools you’ll need.

You don’t need a large array of tools to build a simple rig, and you can probably get away with 3 or 4 tools of you’re skint, tight or both, but I’d recommend having

  • Screwdrivers – a cross-head and a flat-head are a must
  • Wire cutters
  • Wire strippers
  • Multi-meter – this will be your best friend when it comes to testing the rig and you can get them for as little as 3 quid online
  • Soldering iron (& solder) – again they are dirt cheap online
  • Drill & drill bits – handy for making holes in things - naturally

Elf n’Safety

Work safely – Electricity. Can. Kill. – even a voltage as low as 12V can kill if it’s applied across the right part of your body, and even if it doesn’t kill, it can hurt or injure. Exercise extreme care when working “live” with kit that’s plugged into a battery and ensure you design your rig with safety in mind.

If you are unsure of anything consult a qualified sparky.

Be aware that if you short across positive & negative on a 12V battery there will be sparks, a bang, melted tools, soiled underwear or worse (personally speaking I’ve achieved 3 of these and almost the 4th). Also be aware that charging certain types of 12V batteries in a confined space may produce all sorts of nasty fumes so exercise caution.

Step 3: ​ the Fundamentals of a Basic Rig.

It’s not rocket science – essentially a solar panel is connected to a charge controller, which in turn is connected to a battery (which in an existing caravan or camper is itself usually connected to your lights, water pump etc. as per a normal setup). When there is daylight available the solar panel converts this into voltage which charges the battery. A change controller will usually have the functionality to connect a “load” to the controller as well, so you can draw some charge from the battery for other things (phone chargers etc)

Step 4: But First – Balancing Your Load - Watts, Volts & Amps: Lets’ Do Some Science! :o)

This is actually really simple as long as you can count:

  • Solar panels are rated in Watts
  • Batteries are rated in Amp Hours (or Amps)
  • By using a very simple formula
  • Amps = Watts / Volts
  • It’s possible to work out how many amps a solar panel can provide and thus gauge how quickly a panel will charge a battery
  • So with a 10 watt panel:
  • 10/12 = 0.83 amps.
  • i.e the solar panel will provide 0.83 amps per hour of charging.

So lets be practical about that:

  • It’s then possible to work out for a typical day, how many amp hours of charge will be pumped back into your battery
  • (For summer) – assume useable light from 6am to 7pm = 13 hours of light. 13 x 0.83 = 10.79 Amp Hours
  • However – this assumes direct sunlight for 13 hours, so assume 2/3 of this for a more realistic figure (7.1 amp hours)

Next – you need to try and work out what your luxury vehicle of choice will use in terms of amps per day.

  • For example: Try and work out how many lights you have in your van, and what wattage they are; add them all up and reverse the above calculation to work out how many amps (amp hours) you use per hour
    • E.G – 2 x lights at 10 watts each, plus 5 minutes of running a water pump per hour (pump runs at 60 watts):
    • (2 x 10) + (60/12) = 25 watts
    • Amps = Watts / Voltage – so 25 / 12 = 2.08 amp hours
    • Assuming needing this load for 6 hours per day this equals (6 x 2.08) 12.5 amp hours per day

So - In an ideal world, you should make the rig big enough to totally balance your consumption against your charge capacity

  • Assuming your load is 12.5 Ah per day, you need about 15 watts of solar panels
  • However – if you’re only a way for a long weekend, don’t worry too much about drawing a little bit more current than you’re putting back – the aim of the game is to avoid being blacked out by the end of a trip away, not being miserly about balancing your charge & load

Step 5: ​ Choosing a Solar Panel

Assuming you’ve read the above section about balancing consumption, you should have a rough idea of how much power you will consume. You therefore have an idea of how much you need to put back into your battery per day. Unless your battery is old and knackered, don’t be too worried about completely balancing charge in vs. discharge; and for a long weekend you can probably get away with something that gives you around 75% return on your use.

So – if (using maths!) you work out you use 10 amp hours per day, you need to be looking for a solar panel that will put in about 7.5 amp hours per day (i.e a 10 watt panel). If you use double that, a 20 watt panel would be needed.

Panels are usually sold in wattage outputs starting from 5 and doubling as they go: 5, 10, 20, 40, 80 etc. Choose your panel based on your number crunching.

The Boring price stuff - The main consideration when buying them is quality – as the saying goes “you pay your money and takes yer chance” – the vast majority of them are made in China and that neck of the woods (as they’re less bothered about the environmental issues associated with mining some of the precious metals that go into a solar panel), but the quality can vary immensely – though of late it’s got as lot better. Personally speaking I’ve had 30 watts of cheap solar panels up on my shed roof for 4 years 365 days a year and they still work fine (and keep my batteries topped up and the shed-PA running fine). Plus over the years as the technology has matured the quality has got better; and more importantly the price has dropped.

A few years ago a 10 watt panel cost around £30-£35 on ebay, now you can get them for around £25 quid or less if you shop about. Similarly a 100 watt panel would cost £3-400 in 2005, now you can get the same size panel for around half that. Part of it is just the technology maturing, but recently the price has dropped again because the demand bottomed out for solar panels due to the economy going south, the manufacturers have excess stock to shift and so the price drops.

So now is a great time to invest in a solar panel or 2....

Step 6: Your Battery

Choosing & using your battery is probably more important than the solar panel – at the end of the day it’s a lot easier to utterly goose your battery than it is to stuff up a panel

The fundamentals of it all: Car vs. leisure – there are 2 types of battery available to you, make sure you get the right one as the wrong choice will cost you.

  • Car batteries are designed to give short bursts of high current (amps) for short periods to turn over car starter motors. They kick start the engine and are then usually quickly recharged by the cars alternator. As such they can’t cope with a traditional “discharge and recharge” cycle (similar to what you’d do with a mobile phone) where you run the battery down a lot then recharge – this will kill them in no time at all!
  • Leisure batteries (often known as “deep cycle” batteries) are designed to deliver low(ish) current (amps) over extended discharge periods, and as such are much better suited to use in caravans, campers etc.

Unless your camper or caravan was fitted out by a fool, you should have a leisure battery for powering your lights – if anyone tells you that you can get by with a normal car battery then they are a charlatan: don’t listen to them.

Capacity / Size: A typical leisure battery will be around 100 amp hours (i.e, it will deliver 100 hours of power at a load of 1 amp per hour - say for arguments sake a 12 watt light bulb / strip-light at 12 volts). Some batteries are significantly larger capacity, and as such will cost you more. Generally more amp-hours is better, but this will cost you. The size you choose will depend on your ability to do maths based on the formula of :

(amps = watts/voltage) x number of hours you think you’ll be using power for

Step 7: Quirks of 12 Volt – 12 Volt Ain’t Actually 12 Volt!

OK – just when you thought you were getting this 12 volt thing, here comes a curve ball – 12 volt batteries are very rarely 12 volt, solar panels & wind turbines don’t charge at 12 volt, and by now you’re confused.

To explain it a bit - A basic principle of most modern electronics is that they can run at a variety of voltages within a certain tolerance – so for example while in the UK we tend to talk about mains voltage being 240 volts, all of our modern technology can run at anything between 240 & 220 volts

The same is true for 12 volt. Most 12 volt batteries are charged at anything between 14 volts & 18 volts (a solar panel may push up to 18V in bright sunlight) and a battery itself at optimum charge is around 13.8 volts. So what does this mean to me?? – In reality, it doesn’t make much difference to your maths for working out load, it doesn’t make much difference if you’re planning on using any sort of bespoke kit like a PA (you still use “12 volt” kit) – the only reason I actually mention it is because the actual voltage of your battery is a great way of telling how healthy the battery is.

By using a bog standard 3 quid multimeter you can easily check the health of your battery

14+ Volts - Your battery is being charged – happy days

13.8 volts - Your battery is (as near as possible) fully charged and ready to rock

13.7-12.5 Volts - Not quite fully charged but pretty healthy

12.4 – 11 Volts - It’ll do, not ideal the lower you go, but not time to panic just yet

10.9 – 9 Volts - Time to start worrying – you’ll not get much out of this

Below 9.8 volts - find a charger

And on top of all that... You need to factor in some other things

A battery may be 100 amp hours, but you’ll not get 100 amp hours out of it – figure on getting maybe 80-90% efficiency from a brand new battery before the voltage drops off too much if you’re really drawing power.

Older batteries don’t grow old gracefully – expect to loose another 20% on an older battery. It’ll drop charge faster and take longer to reach full charge again

Basically think of your 12V battery as a beefed up version of your phone battery – the longer you use it the crappier it’ll get.

Words of warning

Never leave a battery discharged for more than a day or two or you will completely goose it and cost you money, once a battery is goosed it’s worthless. As soon as you can, recharge the battery if you discharge it. It’s also worth considering disconnecting a battery from its circuit if it’s not going to be used for a long period, on top of that consider trickle charging it over the winter to keep it topped up where possible (or use that solar panel!) as a battery will discharge even with no load attached to it

Step 8: ​ Charge Controllers – What?

In brief – a charge controller sits between the battery and the solar panel – it acts as a go-between to ensure your battery isn’t overcharged, as well as providing a connection to give you an outlet for 12V power for chargers, PA’s etc if you feel that way inclined. A charge controller will also stop the battery discharging via the solar panel at night (though to be honest any decent panel should have a blocking diode to prevent this as well). You could get away without one if you’re handy with resistors, diodes and such – but if you know that sort of stuff why are you reading this?

Basically use one or suffer the consequences!

PS - Charge controllers are rated in amps, so make sure the controller can cope with the size panel you’ve got

Step 9: Before You Start

Plan out what you’re doing thoroughly:

  • How big does your rig need to be in terms of solar output? Do you need a small 10 watt panel, or something more meaty (see above)?
  • Do you want to power anything or only provide charge to the battery?
  • Do you want a portable rig or hard-wired into your caravan / camper?
  • Do you want to bolt the panel to the roof or just stick it on the roof on-site?
  • Think about the connectors you want to have for the various inputs and outputs.
    • You can get away with hard-wiring everything together so a solar panels cable runs directly into the enclosure and to the controller, and the battery connectors run straight out to the battery. But I’d recommend putting plugs on both the battery and solar cables so you can fully disconnect the battery from the panel and avoid accidental shorts (imagine if your battery connections are crocodile clips, and you carry the rig outside in the sun – if the solar panel starts generating voltage and the +ve & -ve crocodile clips touch, that can make nice pretty sparks and possibly screw up something.
    • 4-pin XLR plugs are considered a standard for 12V power in some circles, so for the solar panel I’ve used them, which enables you to disconnect the panel and the battery. You can use whatever connectors you have lying about or can get for cheap (in another rig I built I use 3-pin XLR’s for the solar side of things and powercon plugs & sockets for the battery side of things. That way they can’t be mixed up)
    • I’d always recommend that for any plugs and sockets that connect batteries, use connectors that are more difficult to accidentally short across – Anderson connectors are good for 12V, as are powercons
  • Work out what size cables you need for both solar connectors & batteries – especially on the battery side, the rule is “the thicker the cable the better” – many fires are caused by using cables that are too thin running heavy loads (the cable overheats > the sheath around the cable melts > the bare cables touch > sparks set fire to things), and while that’s less likely in 12V power it’s still a consideration. For even a very basic rig I’d still recommend battery cables of 3-4mm diameter.

Step 10: What to Buy for a Basic Rig

Of course it’s up to you exactly how you build your rig – your choice of materials is influenced by your budget, but to give an idea of cost for the rig built here (Spring 16)

  • 10 Watt Solar Panel - £18 (ebay)
  • Charge controller - £8 (ebay)
  • Enclosure - £5 (maplin)
  • Connectors & glands - £9 (maplin)
  • 12V outlet, switch & fuse - £6 (maplin)
  • Solar Panel Cable – free (scavenged from a skip!)
  • 4mm Battery cable & connectors - £6 (ebay)

Step 11: Getting Started

First thing you need to do is to get an enclosure for the wiring. In theory it’s possible to just run the cables into the charge controller, but it looks crap and won’t last. So buy or make an enclosure slightly bigger than the charge controller so you can screw / bolt the controller onto one side. The one used here was from maplin and cost about a fiver

You also need to plan out how many holes you need in the enclosure for switches, sockets, fuses, outlets etc. In the case of this the enclosure has 5 holes:

  • For the 4-pin XLR socket for accepting the solar panel cable connector
  • Switch to control on/off for the 12V outlet
  • Fuse for the 12V outlet
  • The 12V outlet
  • The gland for the cable to run to the battery

The enclosure face also has 6 x 4mm holes for the cables to run from inside the enclosure to the charge controller (solar + & -, battery + & -, “load” + & -)

Mark out then drill all the for your connectors and such before you start work, you’ll only make more work for yourself if you try drilling holes later on and risk damaging something

Fix the controller to the enclosure – I used small M4 bolts, but depending on what the enclosure is made of, screws or pop rivets will do.

From there you need to start connecting the different elements together and hooking them up to the charge controller

Step 12: Solar Connections

Depending on the solar panel you’ve bought, you may need to wire on a cable to the panel.

Crack out the soldering iron and a decent length of 2 core cable – for a small panel (anything up to 30-40 watts) 0.75mm cable will be fine and solder it onto the +ve and –ve terminals on the panel.

Where possible try to follow standard wiring conventions and use red or brown as positive and black or blue as negative.

Once you’ve soldered up one end you need to solder the plug onto the other end. For this rig I’ve used a 4 pin XLR make plug.Then solder on a pair of wires to the 4-pin XLR socket that will attach to the enclosure. Stating the obvious here but ensure the pins match on both plug & socket. I’ve used pin 1 as +ve and pin 4 as –ve

Then attach the socket to the enclosure using bolts, screws or rivets (I used pop-rivets as I had some lying about).

Feed the 2 cables up through the body of the enclosure and attach to the terminals on the charge controller (on the one I used, the +ve & -ve for the solar input is marked with a little solar panel icon).

Pull any excess cable back into the body of the enclosure so it looks neat, and make sure the terminals are good and tight

Also worth mentioning that in circumstances where the +ve & -ve are close together, it never hurts to put a twist of electrical tape over one of the connectors – hence the red bit of tape on the photo

Step 13: Battery Connections

This will depend on whether you’ve opted to go with a plug & socket arrangement for the battery cables:

If you’ve got a plug and socket, solder / attach the main cable to the plug, do the same for the internal cable from the socket that will run through the enclosure to the controller. Attach the socket to the enclosure, route the cable up through the body to the controller and attach in the same way

In the case of this rig, I’ve used a fixed wired outlet onto an Anderson connector, the other side of the connector will then be hard-wired onto the leisure battery with a short length of wire. With that in mind I’ve just attached a cable gland to the enclosure, routed the cable through the gland and clamped tight, and then run the cable through the enclosure and attached to the controller.

The Anderson connector will allow isolation from the battery when required

Step 14: Additional 12V Output

I’ve decided to include a 12V cigarette lighter output on the enclosure for the hell of it rather than because it’s necessary (though should stress that if you tried to use an actual cigarette lighter on it it’ll probably blow the fuse!). For safety I’ve included an isolator switch and a fuse in the circuit, and the whole thing should be wired as in the diagram

Note that the wiring runs in a single chain: +ve out on the controller to +ve in on the switch > -ve out on the switch to +ve in on the fuse > -ve out on the fuse to +ve in on the outlet > -ve out on the outlet to –ve on the charge controller

As with the solar & battery connectors, connect / solder it all up and clamp onto the +ve & -ve on the controller used for the “load” (in the case of the controller used here, it’s indicated by a little lightbulb symbol).

Because the controller has a maximum capacity of 10 amps, I’ve fused the outlet at 10 amps so nothing can go horribly wrong & create fires.

You don’t have to use a 12V outlet – you could add a small 12V amplifier to power a couple of speakers to give you some music, or maybe an outlet to run to some 12V striplights – at the end of the day as long as it’s 12V you could connect anything to it

Step 15: The Final Wiring Inside

So – the finished product looks like the photo

Now it’s time to attach the back plate to the enclosure and move on to testing….

PS – don’t do what I did and lose the screws!

Step 16:

This is important as you don’t want to assume it all works, then get out into a field and find you’ve arsed something up – and at this point your Multi-meter comes in handy:

Every Multi-meter is different, but in general, a nice cheap one will have 3 main functions that you can easily use without too much difficulty

  • Testing for AC voltage
  • Testing For DC voltage
  • Checking continuity

In this case, you’ll be wanting to use it for option 3.

Rather than me go on at length about how to test for continuity, I’ll just say that there are many great guides on testing continuity out there – find one, follow it and you’ll be fine.

But try and test as you assemble / solder rather than finish it all and then test: If you know and have tested individual elements it makes fault finding so much easier.

Make sure you test continuity from each element through to the charge controller – so you’ll need to test

  • Solar +ve from the panel terminal to the charge controller +ve clamp
  • Solar -ve from the panel terminal to the charge controller -ve clamp
  • Battery +ve from the cable connectors to the charge controller +ve clamp
  • Battery -ve from the cable connectors to the charge controller -ve clamp
  • Any outlets you’ve added in for load (don’t forget to switch on any switches you’ve included!)

Set the multimeter to check continuity, put one probe at one end and connect, put the other probe at the charge controller clamp and if all is well the reading on the meter should drop to read 0, indicating a good circuit.

Step 17: That's All Folks

Final Testing

This the best bit, the moment you get to mutter to yourself “it works!” and reward yourself with beer. And it couldn’t be easier:

  • Step 1: Lug your kit outside
  • Step 2: Connect the battery to the charge controller
  • Step 3: Connect the solar panel to the charge controller
  • Step 4: Put the panel in the sunlight
  • Step 5: Watch the lights come on on the controller that will / should show that the panel is generating voltage and charging the battery
  • Step 6: Test any outlets
  • Step 7: Do a little dance of joy and drink beer

And you’re done.

So – that’s about it...

I hope this wasn't too dull, and hopefully this will equip you to go forth and bodge, solder, clamp, screw, (charge)/discharge, have fun and mess about.

If you get really stuck then check on the web since every problem ever experienced is documented there alongside solutions, And if you manage to hurt yourself then you really should have consulted a qualified sparky and I should make it clear that it’s not my fault – if you’re daft enough to listen to advice you got on the web then you deserve everything you get :o)

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