Contactor and Control Boxes

I have just finished building the boxes that will house all of my contactors and control boards. These will make the components more resistant to the elements and vibration in addition to keeping high voltage away from my hands! Cable glands are used to secure the cables to the boxes, seal them, and prevent undue strain on the cables/insulation as they pass through the box. A lid with a gasket screws down to the top of the box, completely sealing it.


The contactor box contains all three contactors: positive, negative, and precharge. When starting the vehicle, the negative contactor and the precharge contactor are both closed. The precharge contactor connects the large resistor across the positive contactor, allowing it to slowly charge the capacitor in the inverter. When connecting the power to the inverter, the capacitor appears as a dead short. Simply closing both contactors could result in catastrophic damage. After about 5 seconds, the control board verifies that the precharging has occured, and it then closes the positive contactor and opens the precharge contactor. At this point, everything is connected!

  • Contactor box and connectors top view. Here you can see the diodes across the contactor coil terminals. The diodes will protect the control board from the voltage spike which occurs when disconnecting the coil from power. The diodes conduct the spike through themselves and back into the coil until the energy is dissipated.

Control Boards

The control boards include the BMS master from Tom de Bree and two fan controllers. Tom’s board will manage the BMS slaves in each battery pack, enable the charger via CAN bus, manage the contactors, and read the state of charge of the battery pack with a current sensor. If you’re curious about using it for your own conversion, the wiring diagram for Tom’s SIMP-BMS can be found here: SIMP-BMS Wiring.

The current sensor is a DHAB S/18. I salvaged it from the i-Miev battery pack that I disassembled for this build. It is good for +-350 amps, and you can read more about it’s specs here: DHAB S/18 Datasheet. I used the original plug from the i-Miev, so I didn’t have to buy anything new for that.

The fan controllers are just cheap boards that will trigger a relay at a temperature. I bought 10 of them on eBay (here is an auction for reference) for another project, and used a couple for this build. The board reads the temperature with a thermistor, which I will put in the radiator fins. They’re supposedly rated for 20 amps, but I will have them just complete the circuit to trigger a larger automotive relay to handle the inrush current for the fan to be on the safe side. Since there are two fans on the Nissan Leaf radiator, I have two boards. This will allow me to turn on a single fan at a midpoint temperature, and a second fan at a higher temperature. My goal is to reduce the power drawn by the fans, which will translate into more range. This could be done with some kind of PWM fan controller, which is how the fans and pumps are normally controlled in the Leaf, but this was cheaper and simpler for my application.

  • This is the SIMP-BMS board, which is connected to a Teensy Arduino. It can be interfaced with over the micro-USB port. The empty connector on the board is used for controlling Tesla battery modules. The wire with the gray sheath goes to the current sensor, and the rest go to the contactor outputs and various other inputs. My contactors do not have economizers built in, so they must be wired to 12v. the SIMP-BMS then completes the circuit to ground.

4 thoughts on “Contactor and Control Boxes”

  1. The box/housing. Can you give us the manufacturer? Looks like a decent way to waterproof.

    1. It’s actually a basic electrical junction box from Ace Hardware. I’ve seen them at Lowes, as well. I’m using cable glands from eBay to pass the wires through the box and keep it waterproof.

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