12v Basics | OVERLAND BOUND COMMUNITY

12v Basics

  • Hi Guest, you may choose a LIGHT or DARK theme that works best for you with the "Style Chooser" button at the bottom left on this page!
  • HTML tutorial

reaver

Rank V
Expedition
Member

Traveler III

1,674
Caldwell, ID, USA
First Name
Brian
Last Name
McGahuey
Member #

23711

Ham Callsign
GMRS WRMV941
The intent of this post is to help those who aren't very familiar with how 12v systems work, or where to start whe the it comes to building and wiring a 12v system.

12v basics


Automotive electronics work on a 12 volt system. This is different than the 120v system in your house.


12v is what's referred to as DC, or direct current. Dc current is different than the 120v ac current coming into your home.


If you would like to understand the technical differences between 12v and 120v,id recommend the following article:


Electrical Voltage Explained - What Is It and How Does It Work ?


12v is generally simple to work with. Power comes in (positive or +) and goes out to ground (usually the battery negative or -) to complete a circuit.


That is a dc circuit in its simplest form.


There are lots of things to consider with dc circuits.


There are different gauges of wire that is capable of carrying different amounts of current different distances. This is because wires have resistance as the current passes through them. This resistance translates to heat. The wires need to be large enough to handle the heat generated by the resistance of the wire.


Basically remember this one simple rule.


Current (A) + Distance = gauge


The more current you need, and the farther it needs to go, the larger the wire needs to be.


The following chart shows what gauge of wire you need to use based on the amperage draw for a given circuit and how far it needs to run.





One thing to remember about wire gauge. The higher the number, the smaller the wire. 8 gauge is larger wire than 18 gauge.


Let's use a DC-DC charger for example.


I have a 20a DC-DC charger. This charger is fed directly from my battery. The charger will pull as much current as it can, up to 20A. Over a distance of 15ft, 12v circuits have a voltage drop. Based on the chart linked above, I would need to use a minimum gauge wire that is 12 gauge. Using 12 gauge wire though, there would be a 10% drop in amps available at the charger. Because of this, at the battery, the charger would pull 22A. 20A over a 15 foot run of wire would be a lot, so in that instance, it would be better to minimize both heat, and voltage drop by using 10gauge wire. This means that at the battery, the charger would only pull 20.6A instead of 22,and the wire would get as warm, minimizing fire risk.


Could you feed a 20A draw using 18 gauge wire? Sure, but that wire would get rather hot, melt the insulation, and likely start a fire. This is why wire gauge is important.


Fuses.


Fuses are critical for safe operation of 12v circuits. You always want to have a fuse that corresponds to the amperage draw of a given circuit, and place that fuse as close to the power source as possible.


Let's take our previous example using 18gauge wire.


If we placed a 20a fuse close to the battery, as the wire heated up, and the insulation melted, that wire would contact something metal, causing a short. A short would draw as much current as it can. In this case, let's say 60A. 60A would be more than our 20A fuse could handle, causing the fuse to pop. This would break the circuit, and cut the flow of current to the wire, preventing a fire.


Obviously, this is an extreme example, but it illustrates the importance of using fuses. By placing the fuse close to the battery though, the current draw would be considerably more than 20A at the battery. Because we put a 20A fuse there, that fuse would pop before the wire got hot, preventing the wire from getting hot, melting the insulation, and causing a short in the first place.


Relays and switches


Relays are used to turn a high amperage circuit on and off using a low amperage circuit.


A common application to this is turning offroad lights on and off.


Your high amperage draw (lights) draw 20A. You can use a switch that draws 1a to turn the lights on and off. If you only have one light, this might be overkill, but switches have the ability to turn on multiple Relays with a single switch.


For example, I have multiple accessory ports at the back of my drawer system, as well as a voltage gauge. These ports run off two Relays, each powered by a 20A fuse. These are turned on by a single switch. This way, I'm able to run separate wires to each side, minimizing both heat, and voltage drop. A single wire would need both a 40A fuse, and need to be much larger to accommodate the heat generated by a 40A draw.


Most Relays are 5pin,and wired in this fashion (though you can also do switched ground using a relay, but that's a different and less common way of doing it).





Types of switches.


There are three main types of switches you'll find.


Small 3 pin switches like these:





These switches have a positive, negative and accessory circuit, or are 3 position switches power in-acc1-acc2-ground)


The most common type of switch you'll see looks like this:





These switches are available in 3 pin, 5 pin and 7 pin.


3 pin switches are the same as the small 3 pin switches.





5 pin switches have an added illumination circuit with separate ground,





and 7 pin switches work like 3 position small switches, with an added illumination circuit.





With all these switches, the positive out can run to the low voltage input of a relay to switch a higher amperage circuit.


Fuseblocks

Most auxiliary electrical systems use fuseblocks. This makes wiring things simpler, as you can run one large wire to s single location, and add circuits to the fuseblocks as needed.

To give you an example, once you grasp the basics of a 12v system, and how Relays, fuses, switches work, and how to size your wire based on the load you need to pull, it becomes easy to build a system that looks like this from the ground up.



Hope that helps. Feel free to ask questions in this section of the forum, and we'll try and get them answered for you!
 

reaver

Rank V
Expedition
Member

Traveler III

1,674
Caldwell, ID, USA
First Name
Brian
Last Name
McGahuey
Member #

23711

Ham Callsign
GMRS WRMV941
Well done, man! If I could add 1 change and it's minor, on the last diagram a ground coming off the battery.
That last diagram is a work in progress, as I'm constantly tweaking it a bit, as that's the system I've built for the back of my Xterra.

Also, technically, ground doesn't NEED to go to the battery. You know this, but people new to the topic might not, but ground can go to anything conductive. This can be be the frame, body panels, or yes, the negative side of the battery.

But, for examples sake, I do agree with you, and will update the diagram later today. Thanks for the suggestion.
 

old_man

Rank V
Member
Supporter

Member III

2,827
Loveland, Colorado
First Name
Tom
Last Name
Houston
Member #

8300

Ham Callsign
WØNUT
I enjoyed your writeup. Being an Electrical Engineer for over 50 years it never ceases to amaze me the old wive's tales I hear about auto wiring.

First off, everything is not a ground problem. ha ha

Second, if a fuse blows, there is a 99% or greater chance it was not the fuse. Blindly replacing a fuse is looking for trouble.

Third, Backfeeding, while it happens, is not common. Backfeeding is caused by a broken ground connection.

Fourth, most problems can be fixed with a thorough visual inspection.

Fifth, when troubleshooting a problem, don't be cheap, go on ebay or amazon and buy a REAL FACTORY Electrical manual. The cheap Haynes manuals are known to be crappy and inaccurate when it comes to schematics. Most factory manuals will give you not only schematics, but connector diagrams and locations, along with a troubleshooting tree.

Sixth, a long set of test leads, or at least one lead needs to be 16 ft long with a clip if you are going to troubleshoot in a vehicle by yourself.

This is in no meant to demean the OPs info. It is great and well founded. Good Job.
 
Last edited:

reaver

Rank V
Expedition
Member

Traveler III

1,674
Caldwell, ID, USA
First Name
Brian
Last Name
McGahuey
Member #

23711

Ham Callsign
GMRS WRMV941
I enjoyed your writeup. Being an Electrical Engineer for over 50 years it never ceases to amaze me the old wive's tales I hear about auto wiring.

First off, everything is not a ground problem. ha ha

Second, if a fuse blows, there is a 99% or greater chance it was not the fuse. Blindly replacing a fuse is looking for trouble.

Third, Backfeeding, while it happens, is not common. Backfeeding is caused by a broken ground connection.

Fourth, most problems can be fixed with a thorough visual inspection.

Fifth, when troubleshooting a problem, don't be cheap, go on ebay or amazon and buy a REAL FACTORY Electrical manual. The cheap Haynes manuals are known to be crappy and inaccurate when it comes to schematics. Most factory manuals will give you not only schematics, but connector diagrams and locations, along with a troubleshooting tree.

Sixth, a long set of test leads, or at least one lead needs to be 16 ft long with a clip if you are going to troubleshoot in a vehicle by yourself.
Are you saying there's inaccurate information in my post? If so, please, let me know, so I can rectify it. I certainly don't want to give people bad information. I've had to research, learn, and work my way through this information myself, to develop the knowledge that I do have. I will freely admit that I certainly don't know everything, but have some knowledge and experience doing this myself, and designing systems that fit my needs, and as of yet, has not caused me any issues, or caught on fire.
 

genocache

Rank V
Member

Advocate II

1,567
Boulder Creek, CA, USA
First Name
gene
Last Name
L
Member #

24181

That last diagram is a work in progress, as I'm constantly tweaking it a bit, as that's the system I've built for the back of my Xterra.

Also, technically, ground doesn't NEED to go to the battery. You know this, but people new to the topic might not, but ground can go to anything conductive. This can be be the frame, body panels, or yes, the negative side of the battery.

But, for examples sake, I do agree with you, and will update the diagram later today. Thanks for the suggestion.

Yes you do need to have the battery grounded, otherwise there is not a circuit for the electrickery. Yes, the say, rear lights, do not need to be ground wired directly to the battery, the body and frame do that.
 

old_man

Rank V
Member
Supporter

Member III

2,827
Loveland, Colorado
First Name
Tom
Last Name
Houston
Member #

8300

Ham Callsign
WØNUT
Are you saying there's inaccurate information in my post? If so, please, let me know, so I can rectify it. I certainly don't want to give people bad information. I've had to research, learn, and work my way through this information myself, to develop the knowledge that I do have. I will freely admit that I certainly don't know everything, but have some knowledge and experience doing this myself, and designing systems that fit my needs, and as of yet, has not caused me any issues, or caught on fire.
Absolutely no negative connotations meant. Your posts inspired me to add some experience to help others.
 
  • Like
Reactions: TerraRoamer

reaver

Rank V
Expedition
Member

Traveler III

1,674
Caldwell, ID, USA
First Name
Brian
Last Name
McGahuey
Member #

23711

Ham Callsign
GMRS WRMV941
Yes you do need to have the battery grounded, otherwise there is not a circuit for the electrickery. Yes, the say, rear lights, do not need to be ground wired directly to the battery, the body and frame do that.
You're correct in that the battery needs to be grounded. But, we're talking DC circuits here. The big ground wire coming out of the bus bar in my diagram is my primary aux ground. This certainly CAN go to the battery (and in my setup does), but it's not required technically required.

I'm building a new diagram at the moment, as I'm planning on integrating a 50Ah LiFePO4 battery and DC-DC charger into my system.

ButI completely agree that the best way to ground all of your aux electrical equipment is to ground them to your battery (either directly, or through a bus bar).

Absolutely no negative connotations meant. Your posts inspired me to add some experience to help others.
Thanks for the kind words. I was confident that the information I was presenting was accurate, and I appreciate the confirmation! Though, like I said, I'm more than happy to correct any misinformation. I certainly don't want to present things that are not true, and worst case, cause a fire. That would be bad...
 

HYRYSC

Rank I
Member
Adventure

Member I

263
Southwest Missouri
First Name
Robert
Last Name
Conn
Member #

29868

Ham Callsign
KD0YIN
I will be picking some brains here over the coming weeks. I am getting ready to install some rack and rock lights and even though I think I have a good idea about what is going on, it doesn't mean that I do.

My first couple of questions are...

I am planning on running a positive wire (probably 10 gauge) from the battery to the rear of my FJ where I have literally a metric ton of room underneath to put waterproof switch controls etc for the lights (all are LED so draw very little amps). So my questions are, 1. Is 10 gauge sufficient and 2. Do I need to run the negative from the battery as well or could I just ground it on the frame at the rear of the vehicle?


From the schematic that Reaver added in his post above, it appears as if 10 gauge should be sufficient, but just wanted to double check.
 

old_man

Rank V
Member
Supporter

Member III

2,827
Loveland, Colorado
First Name
Tom
Last Name
Houston
Member #

8300

Ham Callsign
WØNUT
As long as you aren't running a huge light bar, 10 gauge should be great.
 

reaver

Rank V
Expedition
Member

Traveler III

1,674
Caldwell, ID, USA
First Name
Brian
Last Name
McGahuey
Member #

23711

Ham Callsign
GMRS WRMV941
As long as you aren't running a huge light bar, 10 gauge should be great.

10Ga wire would do what you want, yes. But..... And I'm playing devils advocate here.... What happens when you want to expand what you're able to run?

I'd run 2 or 4 gauge to a fuse block. Even if you only need 20-30A of current. This would prevent the wire from overheating, barring a short, and give you the option to expand on it in the future.

Just remember to put a fuse or breaker as close to the battery as possible.
 
  • Like
Reactions: HYRYSC and M Rose

HYRYSC

Rank I
Member
Adventure

Member I

263
Southwest Missouri
First Name
Robert
Last Name
Conn
Member #

29868

Ham Callsign
KD0YIN
10Ga wire would do what you want, yes. But..... And I'm playing devils advocate here.... What happens when you want to expand what you're able to run?

I'd run 2 or 4 gauge to a fuse block. Even if you only need 20-30A of current. This would prevent the wire from overheating, barring a short, and give you the option to expand on it in the future.

Just remember to put a fuse or breaker as close to the battery as possible.
Yes, good point for sure! Thanks for the advice.

So fuse or breaker close to the battery, 2 gauge wire to the rear, then ground the fuse block to the frame at the rear and power it with the 2 gauge. Sound about right?
 

reaver

Rank V
Expedition
Member

Traveler III

1,674
Caldwell, ID, USA
First Name
Brian
Last Name
McGahuey
Member #

23711

Ham Callsign
GMRS WRMV941
2 gauge is probably a bit overkill, but yes.

I ran positive and negative 4 gauge from my starter battery, through the firewall to the back of my Xterra. Before it goes through the firewall, it passes through a 100a waterproof breaker.

I use a 100a fuse block in the back, that everything runs off of.

I personally don't like grounding to the chassis if you can avoid it, but there's nothing technically wrong with that.

Use the chart above to figure out what gauge wire you'd need based on how many amps your fuse block is capable of handling.

Remember to account for voltage drop as well.
 

leeloo

Rank V
Member

Member III

1,473
Luxembourg
First Name
Mihai
Last Name
Doros
Member #

19403

grounding to the chassis saves a bunch of wire and making additional or larger holes in your vehicle. There are factory grounds all over the vehicle if you know where to look, used by the manufacturer. I always try to use those if the position and access is relatively easy..
 
  • Like
Reactions: RGILL

Overlanding Lawyer

Rank II
Member
Adventure

Contributor III

357
Fayetteville, GA, USA
First Name
Tom
Last Name
Kirkbride
Member #

30006

Ham Callsign
K1EOD
Service Branch
Army
Outstanding Thread! Great graphics, makes wiring your rig much easier. The key is fuses and wire, aftermarket wiring is the cause of many vehicle fires. If in doubt go bigger, and every rig should have at a minimum, one fire extinguisher, Thank you for your diligence and the research!
 

reaver

Rank V
Expedition
Member

Traveler III

1,674
Caldwell, ID, USA
First Name
Brian
Last Name
McGahuey
Member #

23711

Ham Callsign
GMRS WRMV941
grounding to the chassis saves a bunch of wire and making additional or larger holes in your vehicle. There are factory grounds all over the vehicle if you know where to look, used by the manufacturer. I always try to use those if the position and access is relatively easy..
Like I said, there's absolutely nothing wrong with grounding to the chassis, or using factory grounding points.

I just prefer not to touch the factory electrical system as much as possible.
 
  • Like
Reactions: HYRYSC

RGILL

Rank II
Member
Adventure

Contributor III

357
Ventura, CA, USA
First Name
Robert
Last Name
Gill
Member #

31470

Ham Callsign
KF5OAY
Service Branch
USAF
Good grounds are important no mater how you decide to make them. When troubleshooting electrical issues I look for power first, if that checks good I look for the ground connection. Fixed some pretty funky problems just by cleaning the ground connection. Personally I make sure my ground points are super clean then use a electrical varnish to seal the connection. In the aircraft world grounding points are sealed to prevent moisture from corroding the connection. Just my two cents.
 

HYRYSC

Rank I
Member
Adventure

Member I

263
Southwest Missouri
First Name
Robert
Last Name
Conn
Member #

29868

Ham Callsign
KD0YIN
I decided since I was doing some wiring anyway, why not just upgrade to a dual battery system? So that's what I am going to do.

Since my FJ has the so called "smart" alternators, I will be using the Renogy DC to DC 40 amp controller along with an AGM battery and the Renogy Voyager 10 amp solar charger to trickle charge the battery from my solar panel mounted on my rack when needed.

I don't know a ton about 12 volt systems yet, but was never one to pass up a challenge am I am eager to learn.

Will be taking it slow, doing it right, learning a bushel and will have a good system when I am done (I am giving myself the month of November to get everything up and running including the wiring of the additional lighting that I will be installing.)