How do I ensure a good cable connection?

A loose or poorly connected cable in your DC electrical system can have a huge impact on the charging and discharging of your battery. Poor connections pose a significant safety hazard in your electrical system and can lead to a wide range of performance related issues. The symptoms of a poor electrical connection can vary greatly, but they typically include issues such as:

 

Poorly made electrical connections can result in voltage drop, which can affect devices that require good constant voltage for optimal performance. This includes devices such as fridges, inverters, and battery chargers. Even the best made connections have some resistance and cause a small amount of voltage drop. This is why the number of electrical connections made should always be kept to a minimum. More information on voltage drop can be found here.

 

The resistance caused by poorly made connections will also generate heat, which has a negative impact on performance and safety. Loose, damaged, or corroded connections are the most common cause of electrical fires. Many electrical devices also perform poorly when they overheat and will either reduce output or shutdown entirely as a result. Luckily, the heat generated by poor connections also makes them easy to identify with the use of a thermal measuring gun or thermal camera. Even without these tools, feeling your hand over connections can also help find heat build up.

 

If you are experiencing intermittent faults in your electrical system, then there is a good chance that a loose connection is to blame. This is because the quality of contact at the connection will also vary as it physically shifts back and forth. The random nature of intermittent faults also makes the difficult to resolve because they can be difficult to replicate or find when your vehicle is stationary.

Battery Not Charging

A battery charger will often mistake the resistance from a bad connection for the resistance from a full battery. This causes the battery charger to stop charging and go into the float stage early. Your battery charger display will show that the battery is full, while your battery monitor will say otherwise.

Best Practices

To ensure that your electrical system is safe and performs properly, you can take the following steps to ensure that your system has good connections:

Layer terminals correctly

When you have multiple connections at a single point, the highest current draw connection should be at the bottom and the lowest current draw connection should be at the top. Your highest current draw connection should also be the largest terminal at the connection point. This means that your stack of connections will be shaped like a Christmas tree or pyramid, with the largest connection at the base and he smallest at the top. All connections should make direct contact with each other, with no washers, spacers, or nuts in between.

The connection seen in the bottom left image is poor because there is a large stack of washers that prevents the large cable from making direct contact with the battery terminal. These washers are a poor conductor of electricity and may eventually heat up to the point where they melt and cause an electrical fire. This installation also features a large cable sitting on top of a much small cable. This reduces the amount of contact the large cable has with battery terminal and forces the current to flow through the small lug as it passes through the circuit. The bottom right shows lugs stacked correctly, with the largest lug making good direct contact with the inverter terminal and the smaller ground wire sitting on top.

 

Use correct terminals

Using the correct size of terminal can also help to ensure a good quality connection. The surface area of a lug is designed based on cable size and terminal size. Lugs that are meant to be used with large cables will have large surface areas, and if they are meant to fit over a large stud then the footprint of the lug will be made larger to compensate for the larger hole in the centre. Because the relationship between the cable, terminal, and lug size is designed to ensure a good connection, altering any of these components can affect performance.

For example, trimming excess cable strands so that a cable can fit into a smaller lug will reduce the quality of the connection and increase resistance. Fitting an oversized eye terminal or drilling out an undersized eye terminal will have a similar affect by reducing the amount of contact area between the lug and the terminal it is attached to.

Limiting the number of connections per terminal

Every lug that is added to a terminal increases the amount of resistance, heat, and voltage drop. Stacking too many lugs can also reduce the depth that your bolt threads into a terminal, which increase the risk of a loose connection or stripped thread. To prevent poor connections, it is recommended that no more than 4 connections be made at a single terminal stud. Even with 4 lugs, a poor connection is possible if they are too large to be seated flush against each other. If it is necessary to make more connections, it is recommended that you change to a bus bar.

The connection shown on the bottom left is poor because the lugs are too large to be seated flush against each other, which has caused there to be gaps between them. The height of the combined lugs will also have reduced the depth that battery terminal bolt will reach, increasing the potential for a loose connection or threaded bolt. The largest lug is also at the top of the stack, which increases the resistance of the connection.  The image on the bottom right shows similar problems, with too many terminals and the use of washers in between them. It also appears that the positive and negative terminals are too close together, placing the system at risk of a short circuit.

The third image shows how a bus bar can be used too reduce the amount of connections made at a single point and to improve connection quality

  

Crimp Connections

Ensuring that your terminals are well crimped will also improve the quality of connections.  It is recommended that you crimp instead of solder connections wherever possible. This is because crimps will handle the movement and vibration of a mobile application better than solder. You should never use solder with a compression fitting, as the solder will cause the joint to become rigid and prevent the compression fitting from properly clamping down to make a good connection.

When checking the quality of crimps or soldered connections, it is best to give the wire a slight tug to see if it holds. A connector can pass visual inspection but still have a poor quality crimp. A poor quality crimp will leave air pockets between the wire and the connector. This air pockets will allow moisture to collect, which will then lead to corrosion and increased electrical resistance.

Connections should be made using high quality lugs or connectors. Dual wall lugs are preferable to single wall lugs, as s there is a significantly lower chance of the lug snapping, especially in applications like starter motors or alternators where there is a lot of movement. The crimps should be made with a proper crimping tool, not a hammer and hole punch. With the smaller insulated (red, blue, yellow) terminals, ensure you are using a crimper that crimps the conductor and insulation, not just a single crimp. The wire should protrude just slightly through the crimp, however not enough to interfere with the mating surfaces.

All other crimps such as Deutsch, QK, & Ferrules should be made using dedicated crimpers. Consider the use of glue filled crimps where water or moisture may enter the cables, the green wire cable can be one of the harder faults to find!

Heat shrink connections

Heat shrink serves as an insulator, can be used to identify the polarity of a circuit, and shows a high level of workmanship when used correctly. As an insulator, it is important to ensure that the heat shrink is not overhanging onto the conducing surface of the lug. In marine applications, especially in wet areas, you might consider glue filled heat shrink to stop moisture from entering the lug.

 

Regular maintenance

In a mobile application, electrical connections will loosen over time as a result of vibration. You should check your connections as a part of your regular maintenance routine. You can make this easier by adding a visual indicator to your connections to confirm that connections have been tightened correctly. This can be as simple as using a paint pen to mark where a nut or bolt sat when it was tightened. Doing this will also make fault finding quicker and easier. You should not use Loctite, as you still need to be able to undo connections for servicing and maintenance.

Summary

It may seem like there is a lot to remember when making cable connections, but much of it just comes down to common sense. Keeping things tidy and using the correct equipment will go a long way toward keeping your system safe and functional. The following points sum up most of what is required for good cable connections.

• Use good quality automotive or marine cable, as well as good quality lugs of the correct size
• Ensure connections are tight and make good contact without gaps or washers in between
• If you are stacking lugs, have the largest at the base and the smallest on top (like a pyramid)
• Do not stack more than 4 connections at a single point and use a busbar when necessary
• Avoid using solder for mobile applications
• Use heat shrink to insulate lugs and identify circuits