Custom Connectors

The goal of this project is to make a ‘universal’ high-current connector that fits common car jump-start batteries.

Introduction The jump starters this project uses come in a lot of different sizes and shapes. I currently have two types, and both seem to use a connector that looks similar to an EC5 connector, only with minor changes. Other types I have come across are usually fitted with a similar but, again, slightly different connector. After searching the web for many hours, I realized these exact connectors are hard to find. Additionally, if I were to find them, it would potentially mean that I would need conversion cables if I wanted to be able to change between the two types of packs that I currently have.

EC5 connectors are frequently used in RC cars, planes, and drones as they are designed for high-current operations. Diagrams can be found relatively easily, and the gold-plated lugs or a complete set can be acquired from the usual sources.

The Packs The first pack I used comes with a connector that is compatible with off-the-shelf EC5 connectors with one terrible disadvantage: the polarity is reversed from what the connector states. Besides this, the stock connector works reasonably well. It definitely takes more force to plug and unplug than the connector that is part of the booster set, but that’s it.

The second pack has a slightly different connector shape, but the size and distance of the pins seem to be identical. Due to the different shape, in this case, the stock EC5 connector does not fit, and instead of sanding off the edges, my project to design a universal connector started.

Custom EC Connectors Based on the specs of the original connector, I have recreated a connector that would fit into the battery packs, first by creating a simplified, symmetrical version, without reverse polarity protection.

This made it quite clear that more changes were needed since the fit of this connector was not as expected.

After determining the optimal dimensions, I’ve added flanges to prevent plugging the connector the wrong way around, added a label, and a snap-fit for the gold-plated lugs.

After testing the connector at various stages, I noticed that the electrical connection was quite poor, something I never experienced with the stock EC5 cable.

No Connection Testing with bare female bullet-connectors concluded that the connector on the battery pack is not compatible with the 5mm EC5 connector, but is slightly smaller, yet still considerably bigger than EC3.

Since the standard banana-plug is 4mm, a second guess was that this connector would use 4mm banana plug parts, but this was still too tight.

With more details on what the size needs to be, it turned out that the - also very common - XT90 connector has design basics that are more similar to the connector than the EC5 connector:

The distance between the centers of the circles aligns better with the connector on the starter pack. The diameter of the bullet connector is slightly smaller and has the same 4-way split as found on the starter packs. Final XT90-Based Connector The pins and terminals from the XT90 connector are shaped slightly differently, and exact drawings are hard to find. It was time for another parameter sweep to find the best fit.