Battery and Motor Cables

The high voltage cables carrying current from the battery pack to the motors will be discussed here.

• Flexibility

  • These cables will likely be moved around, plugged in, unplugged, etc. many times during debugging and testing. This means we need a high strand count in the cables.

• Gauge

  • The wires must have sufficiently low resistance in order to minimize the resistive power losses and increase the efficiency of the drivetrain. We have selected a minimum of 4AWG (25mm2) for DC battery and 6AWG (16mm2) for DC motor controller wires. Motor phase output wires should be no larger than 16mm2 (10mm outer diameter) to fit in the motor controllers)

• Color

  • We will keep all high voltage cable ORANGE, as the recommended industry standard (recommended by SAE, though I can’t find the document). Black or red heatshrink will be used on the ends of the wires to denote positive and negative high voltage wires.

• Shielding

  • MSXII did not have shielded HV cables, and no significant issues were observed (after switching to differential communications). I would love to do more research into this, but for now we will stick with non shielded wire.

• Insulation

  • Concerns with PVC insulation - it will slowly pull back from the connector over time, as it does when soldering

  • Crosslinking and irradiation explained here: https://chargedevs.com/features/cable-sizing-and-selection-for-ev-systems/

  • We will go for crosslinked, irradiated insulation, per the recommendations from Champlain Cable:

For reference, I believe that most welding cable is EPDM insulation.

Tritium Cable Recommendation:

Open

This is essentially double insulated welding cable.

IEWC Part Numbers:

32102862

Huber + Scuhner 16mm2 1kV XLPO (Crosslinked polyolefin), double insulation

Open

Champlain Cable:

EXRAD--HVFXI-16 (or 20)

EXRAD-XLE6-6X (or 4X)

EXRAD-FXIF-16 (or 20)

EXRAD-FX-6XX (or 4XX)

We would love for IEWC to provide recommendations or a call on speccing wires, and see what they have available.

Good discussion on cable lugs and why speccing the proper one for the type of cable you have is important - finer strands will lead to a physically smaller cable, so a proper flanged or bellmouth lug is required.
https://www.solarpaneltalk.com/forum/off-grid-solar/off-grid-solar-panel-systems/351593-is-welding-cable-suitable-for-battery-systems

Also be sure to use silicone grease on the copper cables to prevent oxidation.

Constraints

Cables

Given by Motor Controllers:

Open

Open

This applies to BOTH the DC Bus and Motor Phase cables.

The cables MUST be sized to support the maximum bus and phase current for our setup. Max bus current is 122A, and max phase current is 100A per the controller spec, but can be limited in software.

The chosen fuses for the motors are rated at 80A. They are sized to protect the wires. Josh’s fuse calculations (Fuses selection for MSXIV) require a minimum cable ampacity of 78.4/0.75 = 105A. Any cable we choose must have a higher ampacity than this at ambient temp (40C in car). The fuses were selected as the next highest size after determining max motor current of 55A at max load (5kW / 90V). Because these cables are protected by the fuse, the 16mm2 (or 6AWG) cables were selected for the DC Bus to the motor controllers.

We are making the assumption that pretty much any cable we use will have similar ampacity to the ones given in this datasheet from Champlain Cables for their EXRAD-XLX series.

Color must be ORANGE to designate HIGH VOLTAGE.

Cable Options (Shielded):
1 - IEWC
2 - IEWC
3 - IEWC
4 - IEWC

Cable Options (Unshielded):
1 - IEWC
2 - IEWC

The motor phase wires are protected by the motor controller, and have a different amount of current flowing through them than the DC Bus cables. Our motors are rated for 5kW maximum. At min voltage, that gives 55A as mentioned above, so we don’t need quite as much of a safety factor for the phase cables here. Also to note is that the only heating of the cables is due to resistive losses (no switching losses as with MOSFETs). A more simplified argument → Since the motor phases have 3 wires and the DC bus cables only have 2, they will heat up on average less than the DC bus cables.

Average Resistive Losses in Cables:

For rough calculations, 6AWG and 16mm2 / 8AWG and 10mm2 were assumed to be identical. In reality, the mm2 counterparts are slightly larger in cross section and will have less power loss and higher ampacity.

We have chosen 10mm2 cables for the motor phase cables as there is minimal power loss and it allows the use of cheaper, easier to obtain connectors (the Amphenol ePower Lite series). Note that they also have an ampacity of > 105A (the minimum required for fuse protection on the DC motor outputs from the battery pack), but is just at the limit.

Cable Lugs

Bellmouth crimps are recommended to prevent stress concentrations at the end of the crimp, though can likely be avoided since both ends of the cable will be secured and not move around. An example of a crimp with a bellmouth is shown here:

Digikey has suitable lugs made by Wurth Elektronik. The search query can be found here: https://www.digikey.ca/en/products/filter/terminals-ring-connectors/394?s=N4IgjCBcoEwJwA4qgMZQGYEMA2BnApgDQgD2UA2iAMxUAMArAyALrEAOALlCAMocBOASwB2AcxABfYvDjIQaSFjxFSFagHZaANgAsOluy6ReAkeIkWgA

We will need to choose one appropriate to the wire gauge that we will use. They are available in sizes: 10mm², 16mm², 25mm², 35mm².

Heat Shrink

Black and Red heat shrink should be used to protect the crimp and identify the positive and negative terminals as shown:

Similar color coding would likely be a good idea on the motor phase wires.

How much cable do we need?

This table show the per-side lengths

Total Requirement (for 1 car)
16mm² = (1+3+0.6) * 2 = 9.2m → order 30m?
10mm² = (0.9+1.8) * 2 = 5.4m → order 20m?

Purchase List

Add 3x the amount of cable required to have lots for test setups and for repairs. 1.5 or 2x of other items should be fine.