Power cables and connectors: Choosing the right ones for your electric vehicle is crucial

Careful selection of the battery, motor, and controller is crucial, followed by connecting them using available components. A common mistake is overlooking the cables and connectors, which carry all the current from the machine. These components often overheat, melt, or break. This guide explains how to properly size them, using a simple calculation.

Section, AWG (American Wire Gauge), and square millimeters

The size of a cable is determined by the amount of copper in its cross-section, measured in square millimeters, or by the American Wire Gauge (AWG). Be careful, the AWG scale is inverted: the smaller the number, the thicker the cable.

For a given section, a cable made of many fine strands, with silicone insulation, is more flexible and better withstands heat than a rigid cable with standard insulation. This type of cable should be used on vehicles that vibrate.

The calculation that matters: voltage drop and heat

Copper has a resistivity of approximately 0.0172 ohms per meter for a cross-sectional area of one square millimeter. Therefore, the resistance of a cable is:

`Resistance (Ω) = 0.0172 × Length (m) ÷ Cross-sectional Area (mm²)`

For example, a 1-meter battery-to-controller connection, which requires 2 meters of cable (out and return), with a cross-sectional area of 8.4 mm², has a resistance of approximately 0.0172 × 2 ÷ 8.4 ≈ 0.004 Ω. At 100 A, this results in a voltage drop of 0.4 V and dissipates approximately 40 W of heat. At 200 A, the voltage drop doubles, and the heat dissipation is multiplied by four: 160 W.

Two consequences:

• Voltage drop reduces the voltage available to the motor, which in turn affects the speed and peak power;
• Heat softens the insulation, oxidizes the contacts, and accelerates the wear of the connectors.

The allowable current for a cable also depends on its environment: a cable exposed to the open air cools down much better than a bundle of cables tightly packed within a closed deck. Manufacturers' current charts assume specific conditions, so it's important to allow for a safety margin.

Phase cables

Between the controller and the motor, the current is that of the phases. During startup and when climbing hills, it can be significantly higher than the current drawn from the battery: this is explained in our controller guide. These cables should therefore be at least as thick as those of the battery, and as short as possible. In a hub motor, they pass through the hollow axle of the motor, which limits their cross-section: this is a point to check before increasing the currents.

Connectors

A connector adds a small amount of contact resistance. When new and properly installed, this is negligible. However, when worn, oxidized, or poorly crimped, it can become the hottest point in the entire circuit. Three families of vehicles are commonly affected:

• `Cylindrical pin connectors (bullets), sold in pairs, are often used on motor phases, such as our 8mm bullet connectors;`
• Polarized battery connectors, which prevent reversed polarity connections;
• Spark-resistant versions: These versions incorporate a pre-charge resistor, which prevents sparks from occurring when connecting, as the controller's capacitors charge up suddenly.

Crimp or solder

A properly crimped connection, using the correct crimping tool and the right size terminal, provides a solid and vibration-resistant contact. A well-executed solder joint conducts electricity effectively, but it stiffens the cable at its exit point, where it is more likely to break due to vibrations. To protect it, a heat-shrink sleeve is used to support the cable.

Best Practices

• Keep the power cables as short as possible, and position the positive and negative cables next to each other to minimize interference.
• Protected areas against sharp edges, and fasteners that prevent friction.
• A fuse or circuit breaker, appropriately sized for the battery, should be installed to protect the wiring in the event of a short circuit.
• After the initial rides, perform a manual inspection of the connectors: a connector that feels warm while the others are cold indicates a potential problem.