The Components – 12V Electronics I

Setting up the 12v electrics makes the campervan from a shell, to a liveable space, and they need careful planning not only because electricity is dangerous, but because you’ll want to have your wiring run around the van before you start cladding it.

Needless to say, I am no expert here, and you follow any information here at your own risk!

Below outlines the basics of what your system will need, and then how and why I chose these products.

Product Cost Source
Numax CXV31MF 110Ah leisure batteries (x2) £171.98 Tanya Batteries
Split Charge Relay Kit (30A) £25.95 Ebay – switchedkwb
12 Way Blade Fuse Box £29.99 Amazon
18 AWG/1mm/16.5A 2 Core Wire (65m) £42.25 Ebay – recyclestar
5 AWG/4.6mm/110A Battery Terminal Cables (3 Black, 1 Red, 12”) £13.24 Ebay – pc3105-ukbatteryleads
Battery Isolator Switch £6.89 Amazon
Assorted Insulated Crimp Terminals Kit £12.99 Amazon
In-line Fuse Blade Holder (x5) £5.69 Amazon
Assorted Blade Fuses (2-40A 5-each) £2.99 Amazon
6 x 12V 2W LED Recessed Down Lighters £35.99 Amazon
Cigarette Lighter Socket (240W) £4.99 Amazon
100A DC Multifunctional Meter £11.48 Banggood

Numax CXV31MF 110Ah Leisure Batteries

Researching batteries kept me up for several nights in a row, scrolling through pages and pages of on the Self Build Campervans Group on Facebook, articles and forums.

Initially I wanted to copy the VanDog Traveller, using the 4 x 6v Trojan T105 batteries, which have magnificent reviews. But that was cut in its tracks when the price he bought them (£397) had increased to £560 (Brexit perhaps?). At this stage, I don’t know for how many years I will have Frank, and didn’t want some lucky bugger in 2 years time getting a right bargain because I had over-spec’d Frank (plus monthly maintenance? No thanks!) – so the game of quality vs cost began.

From my calculations in the solar post, I wanted to get around 312Ah long-term, but since a fridge and heater where a while off, I figured 220Ah would suffice for now – so two batteries, on the reasonably large size of 110Ah seemed doable.

To Halfords or Not to Halfords?

I figured expensive batteries could be ruined if the other electronics are crappy, so the best batteries mean getting the best mains charger, best MPPT charge controller, all adding up.

Similarly, cheap batteries can be made to last longer with some decent surrounding electronics, and then if I want/have to get more expensive batteries at a later date, once I’ve spent some time living in the van, I’ve got the good gear that good batteries deserve.

Cruise, Use, Repeat

On top of this, much advice is based around when the only means for a campervan to charge its batteries was with a split charge relay, or mains hook-up, aka ‘Cruise and Use’ – the batteries are charged, used and depleted, and you have to do something to power them back up.

With how common and easy solar power has become, this method is dated, and many battery banks are continually trickle charged during the day. As long as the camper is in the sun, many people are adequately able to generate more power than they use each day and throughout the evening. This trickle effect therefore blurs the lines of ‘X Cycle Lifetime at xx% D.o.D (Depth of Discharge), as it’s perfectly possible to only skim the DoD before getting topped up again, then thrown in the split charge relay AND a mains charger (plus the maintenance that conducts).

I still wanted to get dedicated Leisure batteries, and maintenance-free sounds pretty good to me!

I nearly bought the…

Photonic Universe 100Ah Gel Deep Cycle Battery

At £179 EACH, this would have still been quite the investment! But gel offers working temperatures down to -10C, as well as promising a 5 year service life, 10 if well treated.

In the end, I couldn’t justify this expense, I don’t plan on running TVs, microwaves or anything too energy intensive in the future. Add on the fact that getting ONE of these was the same price as 2 of the Numax CXV31MF, if I get a couple of years out of them, I’d be happy!

Numax CXV31MF

So after coming across a few brand names and opinions on the Self Build Campervan Group, I did my own research for the names I came across, and found only minor grumbles of the Numax CXV31MF. They offered the amount of Ah I was after, at a price that, having looked at all these expensive options, felt pretty cheap, with decent reviews and no maintenance.

Time will tell!

Service with Tanya was excellent, batteries came real fast, well packaged, and if you ask, you may event get a discount!

Split Charge Relay Kit (30A)

Another VanDog Traveller recommendation, the kit was reasonably price, and I though pretty powerful.

However, when I uploaded my schematic to Mick Bs Self-Build Electrical Solutions group, there were concerns that:

  1. 30A was too low
  2. A Mk6 Ford Transit might have a smart alternator, which would counteract the volt sensing relay and not charge the batteries.

1) I think this may be true, and was also expressed by VanDog Traveller, with him recommending a 100+A alternative. But as he also points out, the SCR is a secondary power source to solar. Higher Amps would charge the batteries much faster.

Again, this is a throw back to traditional motorhomes, where if your batteries dropped, you’d run the engine for 10 minutes and pump up the juice (and explains why so many motorhomes where idling at the motorhome park in one of Canterbury’s park and rides that time…)

Since the plans is 1) to fit solar and 2) the early phase of living is just to run 6 x lights (1A/12W) and 1 USB (2.1A/24W), I went with the budget option – so hate me!

2) This was more of a pressing concern, with no obvious answer to be found in the FB group, the internet, forums or the Haynes manual.

I think as more modern vans came out, a smart alternator was part of compliance, something to do with Euro4/5 I believe. This split charge relay kit is super simple – one wire to the starter battery negative, one to the starter battery positive, and then a live wire you connect to the leisure battery – voila, charge!

When the engine starts, the starter battery charges, the volt sensing relay notices the increase in voltage across the starter battery and turns on a relay, which then chargers the leisure battery.

Turn the engine off, and as the starter batteries voltage drops (back to normal), the relay opens, which in turn stops charging the leisure batteries – simples!

You can never drain the starter battery dry, even if you drain the leisure batteries, as the relay prevents this from happening.

However, were there a smart alternator fitted to the van, the engine starts, charges the starter battery, the voltage sensing relay notices the increased voltage, closes the relay, starts charging the leisure battery, the smart alternator notices somethings up, drops the charging voltage on the starter motor, the relays opens, the leisure batteries stop charging, the smart alternator is happy, the voltage rises across the starter battery, the voltage sensing relay closes… and so on.

Thankfully though, so far, so good!

Today (2nd february 2018), I checked the voltage on the batteries (12.69V) with the 100A DC Multifunctional Meter, turned off the 12v system and turned the engine on for 10 minutes, and waited to hear the tell-tale switch of the relay on, then off.

I didn’t hear it.

I turned on the 12v system, looked at the voltmeter and… 13.8V!

Now calm down dears, the voltage doesn’t stay this high. Around 12.8v is considered 100% full, but in order to charge the batteries, the voltage needs to sit higher than this, around 14v. So over a decent period of seconds, this slowly dropped…

When I check tomorrow, after around 24 hours, I’ll have a good idea of if/how much this 10 minute run charged the batteries. I’ll also give Frank an hour’s run around this weekend to see what kind of charging that provides, but so far, so good!

12 Way Blade Fuse Box

This sits between the batteries and the appliances, fed with our thick, red 5 AWG wire from the battery (well, actually through the battery isolator switch – for added safety).

Blade fuses are used to prevent damage to all the appliances, designed to be the weak point that breaks if there is some kind of electrical problem, and then the red wire of each appliance is connected to the fuse box through the fuse.

The power then passes along the red wire, through the appliance, and back along the black, negative cable. What I particularly like about this fuse box is that it has both the positive, fused connection, and also has contact points for all of the negatives built into the same case. Other fuse boxes only connect to the positives, and you have to sort out your own plan for the negatives (bus bar, or connect them to the van chassis(?) – I don’t actually know).

So the power comes back to the fuse box, and our big, thick black 5 AWG cables connects back to the battery negatives (and one such cable is connected to the battery negatives and then bolted to the van chassis to earth the system).

So not only are you positive and negative contacts all in one convenient spot, with 12 fused connections, but it has a lockable plastic cover over all the connection – safety, safety, safety!

18 AWG/ 1mm /16.5A 2 Core Wire (65m)

Let me say first, there are smarter ways to wire the van, that use less wire, for example if you are clever around how you connect all the negatives, and join them up together.

I’m not that smart, and so to be cautious I ran this 2 core (red and black wires) from where the batteries were going, around the van, to the point where the appliance was going to be, knowing I could therefore definitely get positive and negative connected.

You could also be more picky in choosing different thicknesses of wire, the thinner it is, the cheaper and more flexible it is, but it can carry fewer amps.

You need to make sure that the wire can carry more amps than the appliance will need, otherwise your appliance will burn out your wire (could be straight away, or after a little while depending on current draw and the wire thickness) and you don’t want to damage all that wire that’s now behind the wall (or start a fire!).

Next, you wire needs to be able to carry more amps than the fuse. This wire is rated around 16.5A, so any fuse connected to this wire should be no higher than 15A (10A to be safer). This is because we want the fuse to be the part that burns out, if there is an electrical problem. The fuse protects both the wiring and the appliance, and can be easily changed if it blows.

In the end, I chose 18AWG/16.5A for nearly all the appliance wiring. When I created a list of the power requirements of each appliance, no appliance required more than 15A (the cigarette lighter socket at maximum consumption, ie 5 x 2-slot USB adaptors in use, aka charging 10 phones from 1 socket), with the lights being the lowest at 0.9A, if all 6 LEDs were using the same fuse and wire.

Later, when fitting the inverter, this will use 30A wiring, with a 20A fuse and will all be hidden in the cupboard, so isn’t trapped behind the walls.

5 AWG/ 4.6mm / 110A Battery Terminal Cables

You could buy cable and crimp-ends to make these, but CBA, it’s only pennies more to get them from ebay, so I did!

The fuse box itself can only handle 100A, and the sum of all the appliances, if on at the same time is less than 40A. On top of this, each battery has its own fuse of 25A, so can’t supply more than 50A in total.

That said, some electrical components do have very large instantaneous draws, drawing hundred of amps for fractions of a second, which is what a starter battery is specifically designed to provide, and does when you start your car! With the appliances I have chosen, this isn’t really an issue, but it’s nice to know you have a lot of head room, especially when choosing a grounding cable, as I’d rather this were thick and safe, than I become the path of least resistance in an electrical problem!

Battery Isolator Switch

Probably the most important switch, with which you can completely disconnect the batteries from the fuse box and appliances, making it safe to unwire and wire the cables without the risk of them accidentally touching the wrong thing – either you or another wire – and causing a short circuit, a big flash and dead you or appliance.

However, doesn’t stop you accidentally short circuiting between the positive and negative of the batteries and blowing a chunk off the terminal…

Assorted Insulated Crimp Terminals Kit

This is a great set. Strip the end of a wire, stick the metal wire into a crimp, and then crush it with some special pliers (I like these ones as the cut, strip different wire thicknesses and have colour-coded crimp teeth) and now you can connect your wire to something else!

Turn out each coloured insulating cover on a crimp is designed for a specific wire thickness – ingenious! Just be sure if you’re joining two wire together, that one end has a female connector, and the other has the corresponding male connector!

In-line Fuse Blade Holder (x5)

I wanted each battery to have its own fuse, so that if one has a problem, it doesn’t damage the other. I threw on a crimp to each end of the fuse holder wire, connected one end to the battery positive, the other to the isolator switch, and repeated on the second battery.

By connecting the ends of both fuses to the same leg of the isolator switch, this is the same as running a wire from one battery’s positive, to the other battery’s positive. Then when we connect the negative of one battery to the negative of the other battery, our batteries will be running in parallel, which keeps the combined voltage of the batteries at 12v, whilst doubling the amount of current (amps) that can feed into the fuse box.

Assorted Blade Fuses (2-40A 5-each)

In general, each appliance has its own positive wire running from the fuse box, to the appliance. Each appliance then has its own fuse, which if there is a problem, will break, cut the power to the appliance and thus protect the appliance from breaking.

Looking at the specification of the appliance, it will state how much current it will need to operate. The fuse will need to be higher than this amount (otherwise it’ll just break everytime you turn the appliance on), normally I would chose the next available fuse size above this number.

You will also need to ensure that the wire is able to carry more current than both the device needs, and more than the fuse (we want the fuse to be the bit that breaks, not the wire!)

6 x 12V 2W LED Recessed Down Lighters

You can’t got wrong with LEDs in terms of power! You may have used LEDs with main electricity and remember that they come with a little transformer block that you plug the strips of lights into? Well that transformer is just converting 240v AC mains into 12v DC – so 9 times out of 10, we can connect the LEDs straight into the fuse box.

Since we know the LEDs need 12v and the power consumption is 2W, we can work backwards from,

Power (W) = Voltage (V) x Current (A)

To learn that each LED is using 0.17A – a tiny amount!

The main thing to watch out for when picking down lighter ceiling lights is:

  1. Don’t get halogens! They take way more energy – just feel all that heat from them!
  2. Watch out for how thick the LED unit is – you have limited space between ceiling and the van’s metal roof!

These LEDs are only 1.54cm thick, which gives plenty of room to not press against the roof metal.

Cigarette Lighter Socket (240W)

Not all cigarette lighters are created equal!

Look out for the maximum operating current, at 12v (some of them can run at 24v too). A fast USB charger uses 2.4A, often coupled with a 2nd USB slot of 1.0A or 2.4A, so a total of around 5A.

This cigarette lighter can supply up to 15A, so with an adaptor that’s up to 6 phones at 2.4A, or 10 phones with a fast and slow USB slots.

There are also options to get a 3 way set, such as this one, which comes with a cool plate.

100A DC Multifunctional Meter

I like this unit as it shows both the voltage of the batteries and how much current all of your appliances are using – allowing you to double check everything is off.

It also has an alarm you can set for when your voltage drops too low (around 12.4V is 50% charged/discharged and you don’t want to go lower for fear of damaging the batteries).

Originally I bought the 20A version, before I had done all my maths – d’oh! I replaced it with the 100A version, which requires a shunt (I don’t know either, something to do with the additional current?).

In the future, I think it’ll become redundant when I have the MPPT charge controller running the show, but at this bare bones stage, being able to keep an idea on the Voltage and Current is reassuring.

Check out the Part 2 of the 12v Electrics series in which I work through the electrical schematic, discussing why and how my electrics are the way they are.


By J.Molkenthin

James Molkenthin is an enthusiastic and energetic British Designer, with a background in Graphics, Website and Product Design.

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