Wiring Leisure Batteries and Monitor

DISCLAIMER: Working with electricity can be dangerous. I am not a professional electrician, nor do I have an extensive history with electrics. All the information provided has been gathered through advice and my own research and is specific to my electrical system.


TOTAL: £656.00

To power my electrical system, I wired two 12V leisure batteries to a positive and a negative busbar. All the other other circuits in my system can then be wired directly to these two busbars.

To protect my system, I attached a circuit breaker on my positive wire between my batteries and positive busbar, then before my negative busbar, I added a Victron BMV-712 'Smart' Battery Monitor.

Click here to see my full wiring diagram and here to see the section being described in this post.

What size batteries?

The first step for any electrical system is to set up a battery bank in order to power the whole system. To work out what size battery you’ll need, you’ll have to work out the total energy consumption of all the appliances in your system.

The Solar Calculator by Parked in Paradise, which I used to work out how big a solar panel to buy, allows you to enter all the appliances that you’ll be using in your circuit, such as a fridge, lights, laptop, etc, by adding the appliance’s wattages and then the amount of hours per day you’ll be using that appliance.

From this, the calculator will give the appliance a total amount of Watt Hours (Wh) and then add the Wh of all your appliances together to give you a total Wh figure for the day. For my van, it calculated a total of 1346Wh a day.

If you then assume that you’ll only be discharging your batteries to 50% of their overall charge, that means you’ll need to double your Wh figure, which brings my usage up to around 2700Wh.

Battery capacities are often measured in Amp Hours (Ah). If you remember back to science class in school, you might remember the below equation:

Watts = Amps × Volts

As the electrical system in my van is 12V (volts), dividing 2700Wh by 12 gives me 225Ah, so I bought two 125Ah batteries, giving me a total of 250Ah - More than enough for my electrical system.

What type of battery?

There are a variety of different battery types that all differ in price quite dramatically:

Flooded Lead Acid

Whilst the cheapest option, these batteries contain a liquid electrolyte so they need to be mounted upright. With the 'standard' variety, you actually have to keep the battery topped up with this electrolyte. They can also produce fumes and so need to be well ventilated.


More expensive than lead acid but maintenance free and don’t require to be as well ventilated. More effective for deep cycle applications but they are sensitive to overcharging.


Absorbent Glass Mat batteries are also maintenance free and have a lower internal resistance that allows for much faster charging and better performance under heavy load conditions.

Lithium Ion

Considerably the most expensive option but are much more lightweight and effective. They can also be deeply discharged so in theory you don’t need as big a battery bank.

I opted to go for AGM batteries for my system as they are great for repetitive use and for colder climates.

Cable sizing

One of the main things to consider when working with 12V systems is voltage drop. Voltage drop occurs from resistance in the connectors between battery and appliance, so in this case in the wires themselves. 12V appliances are relying on this voltage in order to be able to function, so if the voltage has dropped considerably between the battery and the appliance, then the appliance may not be able to work effectively.

Resistance is increased by the narrowness of the wire and also the distance that the wire has to run. Generally, the thicker the wire, the higher the current and so less resistance. In an ideal world, we would just use the thickest wires possible to power our appliances, however it’s not always possible to feed 70mm cables throughout your van, plus it will be way more costly.

All wires have an Amp (A) rating - The high the amp rating, the higher the current. I decided to use 35mm wiring to wire my batteries, which have a rating of 240A.

To cut wires to their appropriate lengths, I used some heavy duty wire cutters, then I used some wire strippers to strip around a centimetre and a half of rubber casing off each end.

I used a mixture of 8mm and 10mm cable lugs which I attached to the ends of the wire using a crimping tool. I then sealed the ends with some heat shrink, using a heat gun to melt them in place.

Wiring up your batteries

If, like me, you have more than one battery, you’ll want to connect them together in parallel, as opposed to in series.

Wiring in parallel means you connect the positive terminals to each other and then separately connect the negative terminals together. Doing this means you’re combining the capacity of the batteries, whilst maintaining the same voltage.

Wiring the rest of the circuit

Wiring my circuit breaker to my leisure batteries

Before wiring up the rest of my circuit, I screwed down my two busbars, battery monitor and circuit breaker to my housing. As I’m very limited with space in my van, I decided to mount everything horizontally below one of my benches. I stole a really nice design from the Climbing Van peeps as it optimises the space perfectly.

As well as using 35mm cables to connect my batteries together, I also used 35mm cables to connect to my busbars.

I first connected some red cable from one of the positive terminals of the battery to my 200A circuit breaker. I chose a 200A circuit breaker as the wires themselves have a 240A rating. This means that the circuit breaker will trip before the amp limit is exceeded on the cable.

I then connected the other end of the circuit breaker to one of the terminals on my positive busbar with more 35mm cable.

Similarly on the other side, I connected the negative terminal of the other battery to my battery monitor, then my battery monitor to the negative busbar.

The last thing to do was to ground the circuit. Because the vehicle chassis is made from metal, you can ground directly to the chassis.

I drilled an hole for an 8mm bolt in the metal frame of the van and filed off the paint to make a good connection. I then fixed some 35mm cable to the bolt and connected the other end to my negative busbar.

Now the negative busbar itself is grounded, I can simply use the busbar to ground other circuits and appliances, rather than individually connecting them to the chassis.

If you have any questions at all, feel free to ping me an email or drop me a message on Instagram!