First, start by preparing the outer chassis. Remove and set aside the lid. Select one of the two short sides to be the top. This side will receive the button for the user interface. The opposite short side will receive either the cable gland for the AC supply cable (for the EVSE variant) or the J1772 inlet (for the splitter variant). The two long sides will each have a cable gland for a J1772 cable for each car.

The button hole is 5/8" and should be simply centered on the top side. For the two J1772 cables, you want each to be centered vertically between the back and the front, and as close to the bottom "rib" on the inside of the chassis as you can. The easy way to do this is to take the outside diameter of the cable gland, divide it by two, add a quarter inch and measure that distance away from the rib and mark a vertical line from the back to the top edge of the chassis. Drill the appropriate size hole for the cable gland centered on that spot.

If you're building the EVSE variant, then center a hole for the AC cable gland in the bottom side of the chassis. If you're building the splitter, then you want to mount the inlet as close to the top edge of the chassis as possible, but without extending past it. Even if you're careful to do to, you may find that the mounting hardware hits or gets in the way of the internal mounting plate. If that happens, you will need to cut relief notches in the mounting plate.

Prepare the button by connecting three wires to a 3 pin .1" female SIP header. The center pin should be red, the outside pins green and black. The button will have 5 terminals on it, marked +, -, C (or COM), NO and NC. Connect the red wire to the + terminal, the black wire to the NO terminal and the green wire to both the - and the C (or COM) terminals. Set the button aside for later.

Prepare the internal mounting plate. You will drill holes to mount the two contactors, the power distribution bus, the ground bar and the standoffs for the boards.

The plate can be divided logically into the top and bottom half along the center, which is defined by the two half-moon cuts that accommodate the center ribs on the long sides. The top half (where the button will go in the chassis) will take the logic and HV boards, one above the other. The bottom half will get the ground bus, the power distribution module and the two contactors.

The power distribution module is mounted centered horizontally, flanked by the two contactors. The two contactors should be mounted as close to the center as possible to allow room for the J1772 cables to enter via their glands below the contactors. The distribution module should be mounted towards the center as well to allow room for either the inlet and its wires or for the AC cable gland and the GFI coil. Mount the contactors first and the distribution module second. This makes it easier to get to the screw holes at the base of the contactor since the distribution module will be in the way if it's mounted first.

The ground bus should be mounted in an out-of-the-way spot below one of the contactors, making sure that it will be easy to route the three main ground wires (each J1772 cable and the inlet or AC ground) to it, but that it won't be anywhere near any of the contactor, inlet or HV bus terminals.

Using either of the two PCBs as a template, mark the four corner holes in the top half. The boards should be centered in the available space, but at least an inch and a half away from the top edge (to leave room for the button). Drill 1/8" holes to accept \#4 hardware. Attach a 1/4" \#4 M-F hex standoff in each hole, securing it on the bottom with a lock washer and a nut. Place the HV board on top of the standoffs, with the 5 screw terminals towards the bottom. Secure the HV board in place using the longer \#4 M-F hex standoffs.

Using some spare red and black wire cut from either the J1772 cables or the AC cable, make four jumpers to carry the two AC hot lines from the power distribution module to each of the two contactors. Crimp ferrules onto each end of each jumper. Pass one hot line for each contactor through one of the ammeter CT coils before securing both ends tightly. The coil wires will be secured to the logic board later.

Cut 5 pairs of 22 gauge red and black wires. One pair will go from the line side of the car "A" contactor (on the left) to the AC In terminals, one pair will go from each contactor on the load side to the relay test terminals for each car, and one pair will go from each contactor's coil to the contactor terminals for each car. Cut them to the appropriate length and twist them loosely together for neatness. Attach a .25" QD female terminal to one end of each wire and trim 1/8" of the insulation off the other end. Attach the QD terminals to the appropriate QD terminals on the contactors, and screw the other ends into the terminals on the HV board. Check your wiring carefully to insure that one of each of the two hot lines on the line side goes to the AC IN terminal, one of each of the two load terminals for each contactor goes to the appropriate RELAY TEST terminal for the correct car, and the coil for the same contactor goes to the correct CONTACTOR terminal.

Install the FFC into the FFC terminal on the HV board. To do so, pull the clip outwards until it swings upwards. Insert the FFC with the pins facing away from the swing direction of the latch until it bottoms out in the connector. Make sure the FFC is straight, then press the latch back in until it clicks. The FFC should be snug, able to resist moderate tension without coming out. Connect the other end of the FFC to the logic board in the same way. Place the logic board on top of the four standoffs and attach it with four 1/4" \#4 bolts.

Install the mounting plate in the chassis with its supplied screws.

Install the pushbutton in the chassis and route its cable under the logic board, wrapping around the bottom up to its header. The black wire should go towards the side that says "SW" and the green wire should go to the "GROUND" pin.

Connect the two ammeter coils up to the respective car ammeter terminals on the logic board.

Crimp ferrules on the two hot lines and ground line coming from either the J1772 inlet (for the splitter variant) or AC input cable (for the EVSE variant), then connect the ground line to the ground bus and the two hot lines to the input side of the power distribution bus.

For the splitter variant, route the pilot and proximity wires to the connector on the logic board.

For the EVSE variant only, construct the GFI coil assembly next. Skip this step for the Splitter variant.

Place each GFI coil on the table, so that the coil wires are both at the top and both wires are exiting straight up. Take the left wire of each coil and twist it together. Do the same with the right wire. Cut three long pieces of 22 gauge wire, one each red, black and green. Connect both the red and green wire to the left coil wires. Solder all four wires together. Shrink-wrap the four wires such that the green wire comes out one end of the wrap, and the other three wires come out the other end.

Pass the red wire through the center of the left GFI coil going from the top side through to the bottom. Bring the wire around to the top and pass through a second time (one wrap). Bring the red wire back up to the nexus of wires and then back down towards the right coil (the goal of this is to insure that there's plenty of slack in the red wire). Similarly, pass the red wire through the right coil the same way you did the left.

Solder the black wire onto the two right coil wires. Shrink wrap those together with the black wire coming out of one end and the two coil wires out of the other.

Pass the red, black and green wires through a final piece of shrink wrap insuring that there's enough slack in the red wire that it won't prevent the coils from being separated as widely as necessary.

Connect the three wires to the GFI terminal on the logic board. The green wire is ground, the red wire is the GFI test wire and the black wire is the GFI output.

Set each GFI coil next to the load side of each contactor for now.

For each car, prepare its J1772 cable by stripping away enough of the outer jacket so that the pilot and proximity (for the splitter variant only) lines can reach the logic board. Trim the hot lines short enough so that they can reach the contactor load side terminals and the ground wire so that it can reach the ground bus. Install the cable in its gland and the gland in the chassis. Crimp ferrules on each hot and ground line. For the EVSE variant, pass the two hot lines for each car through one of the two GFI coils. Connect the hot lines to the contactor, the ground to the ground bus and the pilot and proximity (splitter variant only) wires up to the connector on the logic board. For the EVSE variant, the proximity wires (if present) are not used and they can be trimmed short to keep them out of the way.

Set the lid loosely on top of the chassis and conduct an initial test. Plug in the AC power cable or connect a J1772 plug from the host EVSE to the inlet. The LCD should have a white backlight and should briefly show the firmware version and then the backlight should turn green and show that neither car is attached. For the EVSE variant, use the button to enter the menu system and set the total current available as appropriate for your installation, then return back to the idle mode.

Connect and charge a vehicle first with one cable, then the other for 15 minutes as a test. Continuously observe the wiring during this period for any signs of undue heating, smoke, or other catastrophic processes. After the 15 minute test charge, quickly remove power from the system and feel the contactors and wiring for any signs of elevated temperature. It is ok for them to be warm to the touch, but if they're hotter than hot water from the tap (around 50 °C, or 20 °C more than ambient air temperature), then that's too hot.

The button works with either "long" pushes or "short" pushes. The dividing line is 1/4 second. When not in the menu system, a short push toggles the Hydra between operational and pause modes. A long push will enter the menu system, but only when both car plugs are disconnected. The menu system works in a similar manner to OpenEVSE: short pushes will select between multiple options and a long push will accept the current option and move to the next menu item.

You can set up to 4 different timer events. Timer events can be configured to function on any combination of days-of-the-week and will transition the Hydra to or from pause mode at the configured time. In this way, you can configure the Hydra to enable charging when electricity ToU rates go down.

If there is an error, the display will turn red and disable charging for one or both vehicles. To clear an error, disconnect and reconnect the vehicle. When all errors are cleared, the display will stop showing red.

Possible errors are:

• G - Ground fault detected
• F - Ground impedance test failure
• R - Stuck relay detected
• E - Pilot state error - invalid vehicle state detected
• O - Overcurrent error - vehicle drawing more current than allowed
• T - Illegal transition - vehicle requested charging when not enabled (Sequential mode)

The Hydra will perform a GFI self-test any time there is a transition from no vehicles to one vehicle charging. If this test fails, the Hydra will lock up with an explanatory message on the display. If a stuck relay is detected at power-up, then that will similarly lock up the Hydra and display a message.