Ultimate Guide – How To Plan A 12 Volt Setup

G’day guys and girls.

WARNING – Learning how to plan a 12 volt setup turned out to be a massive topic to cover, so this post is very long.

In this post I wanted to go through how to plan a 12 volt setup, using my own circumstances as a case study into the matter so that you can see a practical example of real numbers and scenarios so you know how to plan a 12 volt setup for yourself when required.

This post will contain some mathematics, how could it not?

That’s no reason to be scared, the maths is all very simple and will be explained numerous times, with real examples used to put some actual relevance to the numbers.

I assume you’re reading this because you want to know how to plan a 12 volt setup, so hopefully you were well aware that there was some basic maths involved.

Some of the things this post will cover include:

• Why you would want to know how to plan a 12 volt setup?
• How to calculate the battery size you will need.
• How to calculate the cable sizes you will need.
• How to allow for volt drop.
• How to calculate how large an inverter you need.
• Why using Amp Hours is not the best way to determine power draw.

These are a few of the key points in knowing how to plan a 12 volt setup, we will be starting with a few basic principles.

This post attempts to strike a balance between not leaving information out that is relevant while also not trying to get bogged down in detail, but if you feel something is missing feel free to drop a comment.

*Affiliates Disclosure

Affiliate links are present on this page. Through partnerships with, but not limited to: Amazon, eBay and Commission Factory, I will make a small commission through qualifying purchases. This comes at no extra cost to you and is just a way for me to try and support myself and the blog.  Thank you.

Why do I need to know how to plan a 12 volt setup?

There are a few reasons that you want to know how to plan a 12 volt setup, let’s go through the most obvious ones here.

Safety

If you don’t know about 12 volt, you might not know that it can be dangerous. Just because the voltage is low it does not mean that it is safe, batteries are capable of discharging very high currents!

12 volt systems are inherently dangerous due to the perception of them being low risk and the view that anyone can do it. Without doing this properly you run the risk of burning down your car.

Save costs

If you don’t know how to plan a 12 volt setup and you try and learn by trial and error, you’ll be back and forth to the shop more than you need to be and spending more and more money.

By planning your 12 volt setup properly you will have the right sized fuses, the right sized cable, the right sized lugs and the right sized heat shrink straight off the bat. No costly revisits to the shop.

Save time

As above, but imagine the time wasted if you spend a day on the car on the assumption you have the right size cable because the packaging on the front says “capable of carrying 50 Amps” but in reality your accessory won’t work due to volt drop.

By planning the work out and knowing about the things in this post you will hopefully only have to visit each job once.

Correct sizing

This sort of ties into the point about saving money, but imagine making a purchase of thousands of dollars on a new lithium battery and finding out three months down the track it can’t actually handle your desired setup.

You will then lose money on the resale, have to purchase again, have to refit a new battery of larger capacity or you will need to just suck it up and go without accessories you really wanted.

Why I am writing about how to plan a 12 volt setup

As I am in the process of building a 4WD myself, I am approaching the stage where I begin to mount 12 volt gear into my canopy.

I have done hours and hours of research comparing things like:

• Lithium batteries.
• Inverters.
• Solar blankets.
• Canopies.
• Portable solar panels.
• DCDC chargers.
• Fixed solar panels.
• DCDC charger, AC charger and inverter combinations.

Now it was time to bring it all together, to do this I needed to know what the end game was. What I mean by this is, you need to know roughly what you plan on owning and using regularly.

So that is when I started learning how to plan a 12 volt setup.

No point buying a lithium battery without knowing whether it is big enough, or finding out it’s too big for your needs and you’ve wasted money as well as carrying  unnecessary extra weight!

I already knew about volt drop, amps, watts and resistance (being a sparky) but there is a lot of differences between 12 volt electrical and 230 volt electrical. However the same basic principles apply.

So with all that in mind, let’s get into the basics of how to plan a 12 volt setup, starting from the start, with the maths.

The Maths Involved

Unfortunately, part of knowing how to plan a 12 volt setup is maths related, in fact a good portion of it is, but most of it is the simple addition of accessories once you get the basics sorted.

Just before we go into the maths I want to go through the terminology we will encounter when working with 12 volt systems, or all electrical gear for that matter.

12 Volt terminology explained

When we are talking about electricity, it is always handy for people who aren’t electrically minded to think of cables as pipes and electricity as water.

This helps the majority of people understand the forces at play, here’s a quick description of the terms we will be using in the formulas that are to come.

Volts

Volts is easy, volts are a pushing force. Volts are represented in formulas by the letter V and also the letter U.

12 volts is considered extra-low voltage, you can grab a hold of the positive and negative terminals of a battery and not get a shock because 12 volts is too low of a pushing force to overcome the resistance of your skin.

Voltage can also be referred to as a potential difference. This means an imbalance, where an electrical force tries to equal itself (discharge), which is sort of what’s happening all of the time.

Amps

Amps are often considered to be capacity, but that’s not quite right. An amp is an electrical current, it is the rate of electron flow.

Low resistances can cause high amps to flow, this is why 12 volt batteries are still dangerous despite their low voltage. If there is a fault or short circuit then high currents will flow and can start fires.

Amps are represented in formulas with the letters I and A.

Ohms

Ohms are a measure of resistance, it is these three things that make up ohm’s law and form the basis of what we’re about to learn.

In electrical cable it is desirable to have low resistance, conversely in the outer sheath of that cable you want a very high resistance to protect everything that the cable touches in between its termination points.

It takes one volt of pressure to push one amp of current through one ohm of resistance.

Ohms are represented by the greek letter Ω for omega or with the letter R for resistance.

Watts

Watts are a measurement of power, it is a multiplication of amps and voltage that gives us this power rating.

1 Watt is equal to 1 joule of energy over the period of 1 second. Outside of Ohm’s law that we are about to look at, Joule’s law (or a variant of it) is another of the three that will be mentioned today, with the last being the voltage drop calculation.

Watts are usually represented by the letter P in formulas.

Ohm’s law

Ohm’s law is the basis of all electrical work, it is something you learn in your first year as an electrical apprentice and are expected to never forget.

If it’s simple enough for first year apprentices, it’s simple enough for you.

Look at the following image as an example. If you have any two pieces of information for the triangle you can figure out the third.

Place your thumb over the bit of information you don’t have and the remaining arrangement is the mathematical formula required to reveal the bit of information you don’t have.

For example, if we had a 12 volt accessory and it stated that it drew 6 amps, we could put our thumb over “R” and we would see:

V
–
I

In other words, 12 over 6 (12÷6) = 2 ohms resistance.

Ohm’s law affect on planning a 12 volt setup

Here is a bit on each facet of ohm’s law in relation to the planning of a 4WD build.

Ohms / Resistance

Resistance increases as the following things take place:

• Conductor size gets smaller (choosing too small a cable).
• Circuit path increases (running cables too long).
• Heat increases (engine bays and the Australian outback).
• Connections become loose (solder cracking over vibrations).

Voltage / Volt drop

Voltage is a constant output from your battery, but thanks to voltage drop that voltage might not be present at your accessory.

Voltage gets diminished as resistance builds and that 12 volt from your battery might only present as 11 volts by the time it gets to the accessory, we will discuss this in the volt drop section.

Amps / Heat

Amps are electrical flow. You want the bare minimum flowing to power your accessories, because more amps means more heat which is wasted electricity from your battery.

Amps are often used as a unit of power consumption, 12 volt accessories will list their power usage in Amp hours (Ah) which is how many amps are used over the course of one hour.

The reason this works is because those 12 volt accessories are made to run on the voltage that is being supplied by your battery.

Further down this post we will discuss what happens when you’re charging another battery (laptop, phone, drone etc) that uses a different voltage from your 12 volt setup and why we don’t use Ah for that.

Joule’s law

In a similar vein to ohm’s law we have a variant of joule’s law. This equation works the same as the above in regards to it having three pieces of information to create the formula.

This equation gives us power ratings. Therefore it goes hand in hand with ohm’s law when looking into how to plan a 12 volt setup because now we have all of the information that we need: power, voltage, current and resistance.

These pieces of information are usually provided by the manufacturers of the 12 volt accessories we purchase, with any two pieces of the information we can work out the rest.

Joule’s law affect on planning a 12 volt setup

Joule’s law only introduces one new measurement to the above mentioned and that is power, which is measured in watts.

Watts

Watts are a measurement of power. This is the correct way to determine the power draw of an accessory, by determining the Watt hours (Wh) that a device will use.

You will often see 12 volt calculations done using Amp hours, this is technically correct as well if two things are of the same voltage.

We will look into Wh and their importance further down the post as we look at the practical example of my own power requirements.

Voltage Drop

The final calculation we will encounter in this post is the formula for voltage drop.

Before we get to the calculation, let’s look at what it is and why it occurs.

What is voltage drop?

Voltage drop can get complicated, but for this we will keep it simple.

It is as stated in its name, the voltage drops (reduces) across a circuit. A circuit being from the power source, through the fuse, to the accessory and back to ground.

The voltage from your battery will not be the same as the voltage that is seen if you were to measure the cables where they connect to your 12 volt accessory.

You will experience more voltage drop as resistance increases.

As described above, resistance increases as temperatures rise, as cable lengths rise, as cable size reduces and as poor connections are introduced into a circuit.

Basic voltage drop example

Let’s look at the most basic of examples with easily divisible numbers.

You own a fridge that draws 1 amp per hour = 1Ah

It is a 12 volt fridge, therefore it draws 12 watts per hour (P = IxV which is P = 1×12).

The fridge uses 12 Wh, your battery provides 12 Ah to cover this perfectly as there is no discernible volt drop with the fridge wired right next to the battery.

Now the resistance of the circuit increases as you move the fridge to a new location with a longer cable run.

Let’s say that after volt drop the fridge connection is only getting 10 volts, it still needs 12 watts of power to run.

The equation now becomes:

Amps   =   Watts      =   12             = 1.2 A
                     Volts            10

So now your battery is losing 1.2 amps to power that same fridge, an increase of 20%.

The fridge is still only using the 1 amp, the other 200mA is lost in heat dissipation which in turn makes the cable hotter again increasing the resistance even more than before.

Voltage drop formula

Here is the bit that matters for knowing how to plan a 12 volt setup, using the below formula you will be able to correctly select cable size.

The formula you will probably see everywhere is this one below:

CURRENT (amps) x LENGTH (metres) x RESISTIVITY (0.0172 for copper)
                                                                                                                                                           
                                               AREA OF WIRE (mm²)

So let’s dive into this formula and clear some things up about each component of the formula.

Where do you get the current reading from?

The current reading will be the reading taken from the manufacturer’s data about their accessory or whatever the item is that you are connecting. What is the maximum current draw that the item can reach?

Where do you get the length reading from?

Before having done the install, this will be an estimation. Cars are generally not too hard to figure out within a few hundred millimetres of how much cable you will be using.

This length must include the cable run back to negative/ground. Basically just double your active conductor length.

Where does the resistivity reading come from?

This reading is just a scientific fact, the next section will go into more details on resistance, resistivity and the different things to look out for when learning how to plan a 12 volt setup.

Resistance

The number that you see in the formula above is great, but it doesn’t tell the full story. I’ll be the first to admit that the next little bit of information isn’t key to knowing how to plan a 12 volt setup but I think it helps to know how we arrive where we do.

Resistivity changes depending on the conductor/material. Since automotive cables are copper, that is why you will see the number “0.0172” appear in voltage drop formulas across the internet.

Below is an excerpt of a much larger table that lists a lot of common metals and their resistance at 20°C.

Common materials resistivity at 20°C table

Why does the volt drop formula use 0.0172Ω ?

What that number means in the table above when converted is that the resistance of copper is 0.0000000172 Ω, not quite the same as the 0.0172 that appears in the volt drop formula.

Copper can’t just have a resistance, it needs to be a resistance of something.

That something is a standardised measurement so that everything is measured equally, that standardised measurement is 1 cubic metre of that material.

The reason that the volt drop formula uses a smaller number is because of the fact that we are dividing it by cable size in mm² rather than in metres².

This makes the formula much easier to understand, because cables are sold in mm² and our brains can then input that data without needing to make changes to convert the cable size, instead the resistivity is already converted to compensate for this fact.

How temperature affects copper resistance

You can see from the table above that the resistivity for copper is given but only for 20°C.

Remember that resistance increases as temperature increases.

Copper increases at a rate of ~0.393 % per °C increase above 20°C.

With that in mind you can work out the resistivity of copper at any temperature, let’s use 50°C as an example.

0.0172 (resistivity @ 20°C) x 0.00393 (0.393%) x 30 (the amount of °C between 20 and 50)

= 0.00202788 

That is the increase, now we add that to the original =  0.0172 + 0.00202788

COPPER RESISTIVITY at 50°C = 0.01923 Ω

Copper resistivity in 10°C increments

Here is a little cheat sheet for you:

20°C = 0.0172 Ω

30°C = 0.0179 Ω

40°C = 0.0185 Ω

50°C = 0.0192 Ω

60°C = 0.0199 Ω

Why this increase matters?

That is the resistance of the copper, as it is working and sending current around. Think about the temperature in Australia, or if you live elsewhere think about the temperature in your engine bay if you have cables originating from there.

For those who live in Australia I strongly suggest using the reading at 40°C as your base level when calculating volt drop and then going up from there if you want to travel through the north of the country.

It’s ultimately up to you, but remember the copper will be warmer than the ambient temperature due to the fact it warms itself up as it is in use. Heat is an unavoidable byproduct of electricity.

The things that affect resistance

There are things you can do to decrease the level of resistance in your circuits.

Heat

Resistance will increase with heat, to avoid this select an appropriately sized cable for the job to keep things cool and run the cables in a protected manner out of harsh environments where possible, avoid any sharp bends in the cable that will limit current flow.

Cable Size

Much like a pipe, the larger it is the easier the flow. The larger the cable size the easier it will be for those 12 volts to push the current through the circuit.

Connections

Connections must be made off to a high standard and then tightened. It’s worth checking your connections every now and then as they might not be tight forever.

A bad connection is akin to having a really small section of pipe or a partial blockage that restricts water flow. Due to the poor surface area connection the amps will climb, it will get hot, the battery will be less efficient and worse case scenario something will break or start a fire.

Cable length

Keep this to a minimum. Doubling the length of a cable doubles the resistance. Make your way to the accessory in a nice fashion with sharp bends in the cable but don’t go the long way around the car, try be direct.

How much volt drop can we have?

This is open to debate, the less you have the better for so many reasons. It will be cheaper for you, your accessories will last longer, they will remain cooler, your cables will not deteriorate as fast and you will not risk blown fuses and fires.

The general target is a maximum of 3% volt drop.

That is THREE PERCENT, not 3 volts, 3 percent.

3% of 12 volts is 12×0.03 = 0.36 volts.

The less you have the better but this needs to stay within the realms of possibility, you don’t want nor need massive cables running all across your vehicle, you want to find the perfect size for each different scenario.

Which brings me to the next point, using myself as a practical example.

Now I will go through the real life application that I have gone through in my own purchasing decisions so you can see how to select a correct battery size, how to select cable sizes and how to select an appropriate inverter size as well.

*Affiliates Disclosure

Affiliate links are present on this page. Through partnerships with, but not limited to: Amazon, eBay and Commission Factory, I will make a small commission through qualifying purchases. This comes at no extra cost to you and is just a way for me to try and support myself and the blog.  Thank you.

I went with an Enerdrive 125Ah B-Tec. Now let’s see how I came to make that decision.

How to select the correct battery size

How to plan a 12 volt setup? Start with what battery size you will need.

Through this section I will use myself as an example.

The battery is everything, it is the beginning. Only after we figure this out do we realise what we will use to charge it, how much current that will draw and all the other fun bits that go with it.

Without knowing the battery size we don’t know how large a DCDC charger to buy, which will affect how large a cable we run. This is also a great time to list all accessories and their expected power draw.

With it all written out in a list, you now have all of the power draw written out in a list, you only need to figure out cable lengths in the next step for assessing voltage drop.

Questions to help you determine how big a battery you need

Ask yourself the following questions to help determine how large a battery system you might need. Then head over and read my lithium battery comparison post to see how the big names compare to each other.

What is your reason for wanting 12 volt power?

If you want to know how to plan a 12 volt setup, get a piece of paper out or open up a word document and start listing. List everything that you think you will be using, whether you own it yet or not.

Obviously only list things that will come off your auxiliary battery, so if you wire your UHF or a compressor off a lithium (not a suggestion just an example) then include it, if it’s off the starter it’s not relevant here.

Ask yourself why you want the 12 volt setup, is it just for lights and a fridge? Great, you won’t need too much then and can travel light.

Want an induction cooker and a travel buddy oven? Then you’re in for a hefty bill for a larger battery but there’s nothing wrong with that!

My use case

For me I wanted a fridge, some basic camp lights, a small inverter for charging larger electronics and then I needed to figure out how much storage I needed to charge all of my camera gear.

To do this I wrote it all out in a table which we will go through in a second.

How long will you be off-grid?

If you will be moving from powered camp sites, then doing 1 night off-grid and then back to a powered camp site, you can probably get away with a smaller battery than someone going bush for a week.

Battery capacity isn’t just a matter of adding up all your accessories and making sure it is large enough to cover them all once, it’s a multiplication of the amount of times it will be required to charge each item between guaranteed charges.

My use case

I tried to plan for a scenario where everything would be fully charged before leaving, then I tried to estimate how often I would use each item per day and find the total power consumed.

The thing is, some things won’t even need to be recharged at all over the course of 2 days, whereas others would need to be charged twice a day.

My results are in the table below but needless to say I allowed for a lot of head room and as a lot of the things are unessentials I have potential to increase my time off-grid where needed.

The other consideration I had to make was the next question…

How will you recharge and how often?

There are three ways you might fully or partially top up your battery mid trip.

1. DC charging – Driving your car around will charge the battery as you go.
2. AC charging – Pull into a powered camp site and plug her in for a full charge.
3. Solar charging – As stated, solar panels will replenish those losses.

So how often will you be driving? If you’re driving every day then you won’t need the capacity to power your accessories or charge batteries five times over.

If you stop at camp sites every now and then and plug the battery in you can have it fully charged before you even go off-grid, then use it all up for a day or two with small drives replenishing it.

If you have solar you could potentially stay off-grid forever depending on conditions, usage and the size of your solar system.

The point is that this is such a fluid thing that it is hard to pin point exact numbers so all we’re looking at is ball park figures here to help plan our 12 volt setups.

My use case

I don’t plan on being off-grid for any more than a day or two to begin with, but amongst that will be the inevitable driving around and I also plan on having solar fixed to the roof. Check out my fixed solar panel comparison here for details.

What this does is allow me to calculate for a worse case scenario, but with basically no chance of ever reaching it thanks to the generally sunny conditions in Australia. The safety factor is massive.

Will I run an inverter?

Inverters can draw a lot of power, so this doesn’t just mean you need the capacity to run it, but there are limitations in what amps a battery can output consistently.

The larger the battery, the higher constant output it can maintain (generally speaking).

So think about whether you will want an inverter, if you want a large 2000W inverter then you will automatically be looking at a battery up around 200Ah or more regardless of if you need that capacity, you’ll have to go large for the output factor.

My use case

I plan on running an inverter, I calculated the usage though and found that I only needed quite a small inverter so the issue of maximum current output was less of an issue for me.

How much room do I have? How much weight can I carry?

Lithium batteries are much smaller than their AGM counterparts but knowing how much room you have is a key part in knowing how to plan a 12 volt setup.

Think about your future plans and how things can change, for example if you find out you only need 100Ah, will you have room to change your mind down the track when you want to add another 100Ah in parallel or change it out for a 200Ah?

My use case

I have plenty of room around my battery to allow for the eventual increase in off-road comforts that I might be tempted to indulge in down the track.

I have left it open for a change out but also the option to parallel the batteries is there as well.

How to calculate how large a battery you will need

The following part of how to plan a 12 volt setup will be simple, followed by a slightly more challenging portion regarding the difference of watt hours and amp hours.

Let’s start with some basic things to remember when choosing a battery.

Usable capacity of a battery

A lithium battery should not be fully discharged, you should only discharge the battery to 80% DoD (Depth of Discharge).

With an AGM battery you should only discharge to 50% DoD.

The reason for this is to prevent damage from occurring. When you discharge too much you will seriously shorten the lifespan of your battery and you’ll be replacing it more frequently.

Therefore:

• A lithium of 100Ah has 80Ah usable capacity.
• An AGM of 100Ah has 50Ah usable capacity.

It is important that this is taken into consideration when choosing your battery’s chemistry type as well as its overall capacity. You always want your intended use case to fall within the usable capacity, not the total capacity.

Figuring out how much power your accessories use

When you’re looking at 12 volt accessories whether it be a fridge, camp lights, compressor, induction cooktop, inverter on standby or whatever, they will all list their amp hours usage.

So let’s pretend that you buy an upright fridge that uses 1.5 Ah on average.

Therefore over the course of 1 day, it uses 1.5 (amps) x 24 (hours) = 36 Ah.

Let’s say you have a 200Ah lithium (160Ah usable), you have been parked in camp with no solar and are not driving around to recharge through the alternator.

Over the course of 1 day, 36Ah disappears, this is 22.5% (36÷160×100) of your usable capacity gone, so you could run this fridge for a long time.

Without any other accessories, you can run this fridge for 4.5 days.

This is basically as simple as it gets, now you need to write out a list of all the accessories you plan on having and estimate how long you will use them for each day. This will give you your estimated daily power usage.

Depending on how often and the manner in which you plan to recharge you should be able to figure out your ideal battery size.

Why using Watt Hours is better than using Amp Hours to measure capacity

When you are looking at 12 volt accessories their power draw will be listed in Ah.

This all appears to make sense because your batterie’s capacity is listed in Ah, eg: 200Ah.

There is no issue with this when you’re looking at something that is just consuming power, but where this method falls down is when you’re trying to do determine how many times a battery will charge another battery.

If these batteries work on different voltages, the whole calculation above about using amp hours falls apart!

The reason for this is that Ah is dependant on voltage

The smartphone example

When your phone says that its battery capacity is 3000mAh (which is 3 Ah), that is only its capacity at its operating voltage of 3.8 volts..

Now remember that P = IxV  therefore this battery in your smart phone has 3 (amps) x 3.8 (volts) = 11.4 Wh of power.

What it is saying is that at 3.8 volts it will use 3 amps of current to provide 11.4 watts of power.

But as voltage increases, there is less current required to provide the same amount of power.

To achieve this same amount of power with 12 volts, we go back to the formula triangle and see that:

I = P÷V (11.4 watts ÷ 12 volts)

Therefore at 12 volts:

I = 0.95 Amps.

It’s a pretty stark difference, your phone uses up 3Ah from your 12 volt battery with the original (wrong) way of thinking, but when you figure it out in Wh you can see it actually only uses 0.95Ah from your 12 volt battery.

If you had 100Ah usable capacity, the phone charge from dead to fully charged would use 3% of your battery when using the wrong formula.

When correctly converted the phone charge now uses 0.95% of that 100Ah usable capacity.

**The above does not take into account losses in conversion, heat, etc.

How does this affect the fridge example?

It doesn’t.

All that is changing are the numbers, but we will go through that fridge example that draws 1.5 amps per hour here to show that it takes the same percentage of your battery in Wh or Ah.

For reference sake, we are using a 200Ah lithium which gives us a usable 160Ah. The fridge was an upright fridge that operated on 12 volts and used 1.5Ah.

1.5 (amps) x 24 (hours) = 36Ah / day.

As a percentage of the 160Ah usable capacity this equates to 22.5% (36÷160×100).

Now let’s do the same maths for the fridge in watts per hour (Wh).

Fridge Power = I x V

Therefore:

P = 1.5 (amps) x 12 (it’s a 12 volt fridge)

P = 18Wh

18 (watts) x 24 (hours) = 432 Wh / day.

Battery Power = I x V

Therefore:

P = 160 (usable Ah) x 12 (volts)

P = 1920Wh

As a percentage of the 1920Wh usable capacity this equates to 22.5% (432÷1920×100).

Why I recommend you use watt hours

I think it’s important to note due to the ever increasing amount of electronics we travel with and the amount of batteries we need to keep charged in our day to day living, that using Wh as a capacity measurement makes more and more sense.

As you can see above, the Ah calculation falls apart when we start looking at charging other batteries. This is why for the tables that I am about to show for my real world examples I have used watt hours.

Does it really matter if I use Ah?

No not at all, it is perfectly accurate for things that are purely consuming power, but if you’re looking at charging another device then you need to change to Wh to get proper calculations and estimates.

How do I know what voltage my electronics operate on?

Check the batteries, not only is it listed there but you’ll see the watt hours are usually listed there anyway so you don’t actually need to figure out the maths side of it at all.

*Affiliates Disclosure

Affiliate links are present on this page. Through partnerships with, but not limited to: Amazon, eBay and Commission Factory, I will make a small commission through qualifying purchases. This comes at no extra cost to you and is just a way for me to try and support myself and the blog.  Thank you.

Battery sizing table

Here are the results of my own little experiment.

Clicking the title will take you to Amazon, eBay or similar to see latest pricing or view the product, these are affiliate links.

¹ This is the Ah rating as stated on the battery, not converted to Ah at 12 volts.

² These numbers are just a guess, keeping in mind that when you go off-grid everything already holds 1 charge (as it should be fully charged) so these estimates are on top of the existing charge.

³ Upright fridges will give an average Ah rating, of course it will use more in hot environments but less at night as the temperature drops.

⁴ Camp lights can often be dimmed and will use a lot less power than this rating of 0.5Ah.

⁵ This number came from my rough estimates of sizing an inverter which is covered further down in this post. It is massive overkill as some of the batteries listed above are accounted for despite appearing again in the inverter section.

Determining your battery size

With the above information we have all that we need to determine the battery size.

Convert a battery capacity to Wh and use the total estimated daily Wh to figure out the size you need, if you plan on being off-grid for extended periods you can multiply this number by the amount of days you plan on being off-grid.

So if I took a 100Ah lithium for example we would have the following.

Usable capacity and Efficiency

With 100Ah usable capacity for a lithium with 80% DoD we get 80 usable amp hours.

But the battery is not 100% efficient at charging, I like to take 10% off the top for losses in heat or voltage conversion. This means we’re left with 72Ah (80 x 0.9).

That equates to 864 Wh of usable energy, which is too small to cover the 1047.84 Wh estimated daily use.

So now I go up to 125Ah = 100Ah usable = 90Ah after losses = 1080 Wh.

So there you have it, that is my ideal size battery.

Where’s your headroom?

Although it looks like I have just enough power to get me by for 1 day completely off-grid, the head room is built into a lot of the numbers I put down since I was using a worst case scenario.

A key part of knowing how to plan a 12 volt setup is erring on the side of caution, which I have done in the following ways:

• A lithium battery can discharge further than 80%, it just isn’t recommended that you do this.
• You won’t be draining batteries to exactly 0%, there is always about 10-15% available when you decide to charge.
• The camp lights are dimmable and can use less power than is listed.
• I will rarely if ever go through EIGHT of the Sony batteries. Here I listed over 64 Wh for charging these, keeping in mind they will be charged before leaving a power source.
• Will I need to use FOUR Mavic 3 batteries a day? Not unless I am extremely bad at getting shots.
• Will I need to charge headphones every second day? Unlikely.
• The inverter is listed here despite some of the batteries it will be used to charge already being listed above, because they can also be charged via USB-C, essentially a bit of a double up.

There’s plenty of other factors as well that more than make up for the “lack of headroom”, the most important thing being the following.

Recharging

I will undoubtedly be driving around recharging the batteries and I also plan on having fixed solar on the roof. Although solar is not guaranteed in all conditions, it will boost the battery significantly.

The massive overkill I built into the estimated camera battery charge requirements and the fact that the car will be driven and have solar lead me to believe that 125Ah battery capacity is way more than I need.

In reality I am more likely to get 2 or 3 days worth of power with no recharge. The real reason I added in the headroom was so that I had capacity to add things I have undoubtedly forgotten to include.

Outside of this, I have the option to parallel another 125Ah battery to the system if I start adding more power draw, like heated water, more lighting, travel buddies or anything else that might come along.

I went with an Enerdrive 600 W inverter. Now let’s see how I came to make that decision.

How to calculate how large an inverter you will need

Next step in how to plan a 12 volt setup is sizing up an inverter.

This will be a lot quicker than the battery sizing section as all of the maths has basically been covered already.

I suggest using the same method as before, write it all down, whether you own it yet or not if it is in the plan then just allow for it now.

Once you figure this out, head over to my 12 volt inverter comparison post to see which inverter is the best you can get for your particular needs.

How do I know how much wattage something will draw?

If you plan on using something like a microwave, coffee machine, sandwich toaster or whatever else you can think of, the power consumption in watts will be written on it somewhere.

If your stickers or markings have fallen off then search the product online and the manufacturer’s website should have all of the technical data required.

Here are two examples:

This sandwich press would require a massive inverter as it draws up to 2000W when operating.

This battery charger does not specify its watts, but using the maths we’ve learnt so far we know that the watts will be 8.4 x 1.6 = 13.44 W.

Inverter calculations example

The following is all that I came up with, I haven’t included a table this time as it is all about the inverter handling the power draw, not about the accumulative capacity so it is a lot more simple (clickable titles are affiliate links).

Sony BCQZ1 Charger: 14 W
M1 Max Macbook Pro Charger: 140W
DJI Mavic 3 Charging Hub: 100W
Samsung Galaxy Note 10+ Fast Charge: 25W
Hahnel Pro Cube 2 Sony: 18W
Dewalt Battery Charger: 88W

TOTAL = 385 W

This is a lot shorter list than most people will come up with, because I don’t really plan on using any house hold appliances, just a bunch of battery charges.

The reason I want to allow for these is because they provide me with fast charging that outperforms what I can get from USB-C charging.

Allowing for efficiency and losses

An inverter is not 100% efficient. This is why I did not choose a 400W inverter.

There are a few ways to allow for these inefficiencies, we will just be using general guidelines here.

Multiply your total expected load by 1.2 to allow for some losses and head room.

385 x 1.2 = 462 W

Alternatively you can multiply an inverter’s rated output by 0.8 which will give you 80% of its rated output.

This is just a general rule to help to provide a starting point and prevent people buying something too small, the nitty gritty details are a lot more involved and aren’t worth going into here.

Heat is the killer of efficiency, if your inverter is in a very hot area with poor air flow, it will use a lot more power than it would otherwise need to power the same accessories.

Staggered loads

Let’s be honest, you won’t be running ALL of your appliances at once.

You can limit yourself from even having that temptation by having less power-points available than you do appliances, limiting yourself to only plug a few items in at any one time.

Because of this, if you don’t go crazy with power-boards, you have a natural buffer here between what your inverter can handle and what all of your accessories will draw all at once.

Power factor

I am not going to go into this in detail, but I want to quickly acknowledge it for those who are unaware.

If you are running an AC load off the inverter, there is a chance that this AC load will not have a power factor of 1. A power factor of 1 represents a 1:1 ratio of apparent power and real power.

The most basic example possible:

1 amp AC load with power factor of 1 = 1 amp of power used.

1 amp AC load with power factor of 0.8 = 1.25 amps of power used.

It is basically a built in inefficiency of the load that the inverter will compensate for.

For me, this could apply to the battery chargers that I use for 230 volt charging. However I won’t actually know the true affect of these until I use them as all chargers have different technologies inside and aren’t all exactly the same.

Other examples might be if you were to plug in a 230 volt LED work light, use a corded drill for work, run a 230 volt fridge (or something with a motor) or maybe a 230 volt vacuum?

Overall I don’t think you really need to worry about it, but you may experience this extra inefficiency depending on what sort of 230 volt appliances and tools you use.

How To Choose The Correct Cable Size

The last portion of learning how to plan a 12 volt setup is learning hot to select the correct cable size. Once we learn this, you basically know how to plan a 12 volt setup from start to finish, you’ll just need to make your purchases.

Of course you’ll also need to learn the actual skills required to terminate etc, but that’s not the point of this post.

In this section we will look at three examples:

1. Starter battery to DCDC charger.
2. DCDC charger to auxiliary battery.
3. Auxiliary battery to accessory.

Within these examples will be the information you need to determine how many amps the cable will be required to carry and then calculate how large the cable needs to be so we have no more than 3% voltage drop.

Choosing the correct cable size means choosing a cable that will be able to handle the current, but it must also meet the requirements for delivering the lowest possible voltage drop over the length of the circuit.

Current carrying capacity > Volt drop

The cable must have the ability to carry the current of the load, this is non-negotiable regardless of the volt drop equation. The cable sizing must meet the current carrying capacity before taking volt drop into account.

As an example, let’s say you hard wired a 50 amp compressor into the ute canopy, to avoid volt drop you mounted in right next to the battery using 2.5mm² cable.

Because it is so close there is 0.5 metres of active and 0.5 metres on the ground/negative totalling 1 metre of cable.

Volt drop:

50 x 1 x 0.0172
                                 
          2.5

= 0.344 volts

0.344 ÷ 12 x 100 = 2.86 %

The above example meets the volt drop requirements, but a 2.5mm² cable can not withstand 50 amps of current and it will blow up or catch fire.

This is why you must ensure that the cable can handle the amps before worrying about volt drop!

Cable sizing charts

For some unknown reason the naming of cable sizes in the automotive industry is absolutely illogical. Despite the completely logical and superior measurement of mm² being available they continue to use idiotic things like AWG and B&S.

AWG = American Wire Gauge = B&S

Here is a handy conversion chart that simplifies everything.

Current carrying capacity of cables

This information is one that is difficult to come by, as cables get derated depending on how they are bunched together, what is surrounding them, ambient temperature and so on.

The following tables will be good enough for a rough guide, however I would recommend never approaching the maximum numbers and you have to allow for the cable to be derated.

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Practical example 1: Starter battery to DCDC charger

If you need help choosing a DCDC charger, head over to my DCDC charger comparison post and read up on the different options available.

In that post it will describe why I chose the Redarc BMS1230S3R (eBay) which will be the DCDC charger that we are about to use in the practical examples.

How many amps will I need to account for?

To find out this information, for whatever you choose to charge your battery, read the instructions.

You might have a smaller battery than me, or a larger battery, so you might have a smaller charger than me, or a larger charger to suit your setup.

The point is, all the information is in the manual, you don’t even need to buy something as almost all manuals are available in PDF format on the manufacturer’s website.

For example with my BMS30 the manual states:

The Manager30 is capable of drawing up to 50A from the Vehicle Battery (which may be several metres from its installation location) and is limited to 30A output to the House Battery.

Pretty self explanatory. If for some reason I missed that piece of information, it also states that it operates at a 560W (560÷12=46.66Amps).

DCDC cable size calculation

Figure out your cable run now, the total length of the positive and negative cable, then add whatever temperature rating you think is most appropriate for your intended vehicle use.

I am going to use 4 metres of cable each way, with a copper temperature rating of 40°C (0.0185).

I will start with 16mm² cable and see what the result is.

50 x 8 x 0.0185 ÷ 16 = 0.463 volts.

0.463 ÷ 12 x 100 = 3.85%

That’s pretty close to our target of 3% volt drop, but for this particular example there is a bit more to it.

The instructions of the BMS30 actually have a section for cable sizing, with the following snippet.

As our cable is >3m long we could go ahead and just use 6 B&S (13.3mm²).

This is in part because the DCDC charger will operate by taking whatever voltage is supplied (within limits) and boost it before sending it into your auxiliary battery. It can handle lower voltage, but you still can’t have too low a voltage or it will shut down.

So although I could probably use 6 B&S cable as per the instructions, I will go up a size or two.

The next part of things is the practicality side, what can you get your hands on in your area the day that you want it?

For me, that size is 4 gauge wire (21mm²).

I don’t need to calculate the voltage drop for this as even 16mm²  was an acceptable 3.8% and this sized cable can easily withstand 50 amps of current.

Practical example 2: DCDC charger to auxiliary battery

The next thing down the line is the output from your DCDC charger to your auxiliary battery, in my case a 125Ah lithium.

To provide the best charge for your battery you want it to be as close as possible to the DCDC charger, the good news though is that this power draw is limited by the DCDC charger.

In my case, the maximum that the BMS30 will output as stated in the instructions is 32 amps.

Due to the fact that these two will be very close together and the amperage is limited at 32 amps, this is a pretty safe selection to make, but for arguments sake we will do a calculation regardless using 8 B&S (8.36mm²) cable.

32 x 1.5 x 0.0185 ÷ 8.36 = 0.016 volts.

0.016 ÷ 12 x 100 = 0.88 %.

No issues whatsoever here if you have a short run, you can adjust this depending on your length and DCDC charger size.

Practical example 3: Auxiliary battery to distribution hub / inverter

Here we will be applying less maths as I am sure you get the idea by now, but there are just these last few points to cover in learning how to plan a 12 volt setup. Here we will try be practical.

The final example in this how to plan a 12 volt setup mega post is the cable that leaves your lithium battery to feed all of your loads.

There a few ways this could run, it could go to a distribution hub which feeds all of your loads including your inverter, or the inverter will be connected in parallel coming off the battery terminals.

With the cables running separately: one going to the inverter, the other to the distribution hub, we have two separate things to calculate. 

I personally use the Bluesea safety hub 150

Cable to the inverter

Again, it is best to plan your 12 volt layout well and keep the inverter as close to the battery as possible. Even my relatively small 600W inverter at full load can draw 50 amps of power (600÷12)!

To cover this with ease, since I am buying 8 AWG cable for the DCDC charger to the lithium battery, it would be practical to use some more of this size for this run to the inverter as well.

It will easily account for the volt drop and 8 gauge cable can carry about 70 amps in short runs which we have here.

Cable to the distribution hub

Let’s pretend that instead of running the cable from the battery to the inverter in parallel with the cable to our fuse block or distribution hub, we ran one larger cable to the distribution hub which would then feed everything.

What we need to do here is use some simple addition to see what our worst case power draw could be for each appliance, then add them together to get a total.

If the appliances will be run off the inverter though you don’t need to add them all up, just take the inverter at its highest load of 600W which we worked out above was 50A and add the 12 volt accessories on top of this.

If an accessory has a variable load such as a camp light with power settings or a variable compressor in the fridge then we go for the highest loads possible to account for all scenarios.

With the fridge example, this means the average 1.5Ah rating is useless, that same fridge might have a power draw of 2-4.5 amps when the compressor is running, so we use the 4.5 amps.

Given the size of this post I will spare you the addition here and just tell you that my end result when working all of this out was a possible power draw of ~65 amps with a few amps head room.

If I just order a tiny bit more of that 4 gauge wire that I used for the starter battery to DCDC charger run then I will be covered. It is capable of handling 120 amps. This is a good option if I want to allow for growth in the future without needing to run more cable.

To save money though, you can go as low as an 8 B&S cable here which should handle 74 amps in a very short run. As luck would have it we are already ordering 8 B&S cable as well for the DCDC to lithium cable run.

Conclusion

Honestly, my brain is fried.

That is the end, hopefully you understand how to plan a 12 volt setup. You can teach others how to plan a 12 volt setup and share what you’ve learnt.

I guess what I am trying to say is that as long as the safety side of everything is accounted for, you can use your judgement to select a cable size that suits you.

You might upsize to allow for more DC loads in the future, or you may want to go as small as is possible whilst accounting for volt drop to ensure you have the least weight and cost.

I like to go one size larger than the bare minimum if I can find it nearby but sometimes you just go two sizes larger because you can only find certain sizes of cable in your area.

Although it may seem convoluted at first the maths is so simple, just use what is in this post and apply your own numbers to it. Writing everything down also helps you grasp what possible budget you need to set aside for upcoming works.

As I said before, my brain is fried. It has been a long 4 days trying to write this out and if you notice any errors please let me know and I will attempt to fix them as soon as is possible.

Is knowing how to plan a 12 volt setup worth it?

It is, because if you go through all of this yourself you will be able to fault find when something goes wrong.

If you don’t want to do it all yourself, ask your autosparky to run you through the setup at least so you have a rough idea of how different components interact with each other.

Ok, that’s it.