7. Fuses and Connectors:
Warning:
The most overlooked items in vehicle wiring are fuses and connectors.
Most of the time the source for problems is located here.
In our application a cheap ass wiring is simply expressed dangerous.
We are dealing with currents high enough to weld or to set our Jeep onto a nice campsite fire.
- Fuses
Even if the purpose of a fuse is always the same, there are differences out there:
Don't get fooled by shiny fuse holders using gold plated connectors. If you want to spend the money. Ok, just go ahead.
It is not necessary.
Gold is used in applications where contact resistance is important. In a 12 V system resistance is an issue.
I'm not saying gold plated fuses are bad- you just have to look at the type.
There are a couple of different fuse systems out there:
- Automatic fuses
automatic fuse resistance: fluctuating up to 200milliΏ
Lets start with automatic fuses.
They have the advantage that you can open the circuit with a flick of a switch when necessary.
Sadly the contact resistance of these devices is very high. This causes a voltage drop and we have to fight for every tenth of a Volt!
Automatic fuses are a NONO for our application!
The worst fuse of all- convenient, but not for our application:
- glass type fuses
As the name says glass type fuses use a fuse that is covered in a tube of glass. Usually this tube is held by springs bent around the tube. Contact depends on the pressure of the springs. Resistance therfor depends also on the springs.
Touching the contact area with bare fingers also adds grease onto the contacts.
Spring pressure decreases over time. Contact resistance is to high. Avoid glass type fuses. I know the car hifi guys go: "yaddayadda not true..."
It is true - simply do the measurement with a DMM in your complete application and you will see.
glas type fuse resistance: 2.4 milliΏ
A typical glass tube fuse:
- ANL fuses and Mini ANL fuses.
Now we are coming to the real stuff.
Both, ANL and mini ANL fuses use real bolts and nuts to tighten the fuse blade in place.
Contact resistance is the lessest of all available fuse types so this is the way to go.
Just make sure to have a couple of spare fuses at hand since they are not available on the road.
ANL fuses are available in gold plated or tin plated. It depends on your wallet what you want to spend.
ANL-fuse resistance: 0.16 milliΏ
This is what an ANL fuse+ fuse mount looks like:
As you can see the center block is made out of concrete or ceramic- avoid the resin or plastic made ones.
Hint:
Good fuses do have a small window inside the fuse element. This prevents sparks or glowing pieces of metal to fall out in the event of failure. This is a quality indicator!
The proper way to hold a fuse- avoid touching the contact area with the fingers- to much grease!:
Do not think:
" Ha, I'm gonna save on resistance and I will not use any fuse"
Believe me it is no fun to try to rip out or to extinguish a glowing 35mm˛ cable.- You won't make it!
There is a reason for all these fuse thoughts:
You have a battery in the back of your car which is a high potential source of power and you have a source of power- the alternator.
A conductor coming into contact with the grounded frame produces a nice firework of sparks.
You need to fuse the wiring from the alternator and the battery itself. therfor the need of
two fuses when using an aux battery.
The first must be placed directly behind the isolator.
The other must be placed in front of the battery.
It can't be overstressed. Do not skip on two fuses!
It must be mentioned here:
After inserting the two ANL fuses in line the supply battery voltage at the battery itself dropped by 0.1V. It is not much, but even the lowest drop is measureable!
It still has no effect on the battery charge. I just want to mention it.
- Connectors
Soldering or crimping? Maybe both?
Modern applications scream for crimping!
Why?
Easy to explain: Crimping is prefered (also called cold welding); Contact resistance is lowest and contact corrosion is avoided.
In a soldering connection the lead adds resistance and the connection is getting hard. That means the possibility of a fracture at the connection due to vibration is very high.
secondly the flux being used to get rid of corrosion during soldering is aggressive. You can't get rid of it. After a while the flux attacks the wiring and the connector. Corrosions sets in. And with Corrosion resistance does come- we don't want that!
Spend the money and get your connectors crimped. Do not think of buying a crimping tool. In this diameter being used professional crimping pliers are expensive.
Stay away from positap's, T-taps, 3M taps, T-locks or any name they might have and do look like this:
They offer quick connections- with aprice a couple of weeks or months and they start to get loose, or to rust. Enjoy the fun to seek for the cause.
Do yourself a favor- just stay away from them.
- Resistance
While at it we need to talk about restistance:
Copper is the most favoured material in automotive applications.
Since everybody tries to sell cheap stuff watch out for cable marked with CCA.
This is no copper cable - it is:
Copper Cladded Aluminum- it is aluminum with a surface treating of copper. You won't recognize the difference.
BUt- it has 30% more electrical resistivity than copper. So be alerted when you found some super cheap cable.
Tip:
Go into a welding supply shop. You can get your OFC (Oxygen Free Copper) cable for a fraction of the price than in a camper or boat supply shop.
How to calculate resistance of a cable:
We need a formula for this:
- R is the electrical resistance of a uniform specimen of the material (measured in ohms, Ω)
- l is the length of the piece of material (measured in metres, m)
- A is the cross-sectional area of the specimen (measured in square mm˛).
- As you can see the thicker the diameter of the cable, the shorter the cable, the better the material of the cable the less loss we have.
- Depending on the application minimum cable diameter is given to achieve a low loss.
- There are two categories:
- - Sensitive equipment demanding for a voltage drop less than 0.3V
- - Standard equipment demanding for a voltage drop less than 1.0V
To charge a battery we must keep the voltage drop as low as possible.
- The higher the flowing current the higher the loss.
The following chart gives you a comparision between materials. That P (roh) is the electrical resistivity - the less the number the better (lesser) the electrical resistivity :
Silver p = 0,015
Copper p= 0.01786
Aluminum p= 0.0278
Connectors can be had in all sizes and shapes. Gold plated or simply tinned.
It depends on your choice what to spend.
Just make sure the base material is copper or brass.
During my beginning I loved the gold plated style. After a couple of years I found out it only helps the dealer to justify the high price tag...
Hints:
Make sure you use ring eyelets- don't use open spades.
A loose connection will stay in place with a ring eyelet. An open spade will slip out of the stud and can cause serious damage.
Use the correct size of eyelets. A 10mm eyelet is not made for an 8mm stud.
Use heat shrinking tubes to isolate the eyelet.
It does look professional and gives you additional safety against unwanted shorts.
Always have in mind Murphy and his law is always there.
Choosing the proper cable diameter for your application:
Depending on your application you need to invest some thoughts about cable diameter.
In a dual battery setup the maximum current is the current delivered from the alternator.
But this is only true if you use a real diode isolator!
With a relay the batteries can always draw a significant higher current draw from the other battery!
The cables should be appropriate to the alternator and the length of the connection.
You find the correct diameter according to the length down in chapter 11 in the Comparison charts.