Building a "Juicebox" 240v level 2 charger


The more I researched how electric vehicles are set up, the more PO'd I got over the high costs of the "charger".

As I mentioned before, the "charger" is really nothing than a relay that connects 110v or 240v to the car. That's really it. There is a line that apparently can send data, and you can use it to turn the "charger" on and off, signal end of charge, etc.

So, after researching a bit, I found a couple of companies that make a "kit" for a level 2 charger. My Fiat can charge to a maximimof 6.6 kilowatts, so that's about 30 amps at a nominal 220v.  This also makes sense that the level 2 chargers often have 30 amps indicated, and the charging cables are rated at 30 amps.

I purchased the basic kit, "Basic Juicebox" from Electric Motor Werks and anxiously awaited it's arrival. I was already on the new time of day rates from San Diego Gas & Electric, and the optimum rate is in the window from midnight to 5 amp, not long enough to charge completely when I have put a few miles on the car.

Another alternative for a level II charger

Note: The kit is no longer available, only built up, and apparently the company has changed hands. After about 6 years, my charger has quit, so I'm using components from which is back in the "spirit" of DIY and low cost. The main module has shrunk and power supply on board. See this page.

Preparing to build the charger

Opening the box, no manual or disc, so the papers enclosed gave me a link to the manual in a Google document.

Right off it was a pain to get the Google document, I don't use Google docs much, but on an older laptop the damn thing kept insisting I reload the document... I finally got smart and downloaded it and then viewed it in Word. Not so fast! I had an older version of Word and had to load a converter. Suggestion: put the document on a web site, to be downloaded and put it in an old form of Word, or better yet make it a pdf.

The "manual" is here:

Suggestion: be sure to watch the videos, trust me.

Build experience

Well, after that mess, I started to assemble the unit, and I had watched a video on building the basic unit before on Youtube.

First off, the manual was not up to date, some small changes, easy for me, I'm an engineer, but sure to cause issues.

Power supply differences

There is a small 100-240v switching supply that provides the power for the Arduino processor. Apparently in the first versions, it had screw terminals, but now it has 2 connectors, and the wires are already in the connectors... great, made it much simpler to wire the power to the circuit board, and the 2 wires the power the power supply were already in the other connector and routed around the relay.

The power supply was also already mounted to the case, nice.

Connecting to the relay

First, getting the cover off the relay was a pain, that is until you realize it does not pop off, but there is a screw in the top center of the clear cover, hah! Should be in the manual.

The supplied washers are too big to fit in the recesses near the screws, especially the "lowest" terminals, and if you don't pay attention you could break the plastic or not screw things down tight. Be sure to find some smaller washers that will clear into the recess.

In the videos, the small gauge wires that go to some of the relay terminals are made into a loop and soldered. I don't think this is a good idea, tightening them under a screw head will just split the loop apart, solder is NOT a mechanical connection. I took small crimp on ring terminals (color red) and took off the insulating plastic and crimped and soldered the wires. Then I put the screw through this ring terminal, then a washer and then into the #6 crimp lug, very solid and very good contact.

Adding the optional current sensor

I purchased the optional current sensor, since I expect to upgrade the kit later. Here's where I got into trouble. There is a single current sensor on the circuit board, and you can add an optional one. The "stock" sensor is used for GFI sensing, and the "optional" one allows charge current sensing. The idea was to purchase and install anything now that might be a pain later. For $15 I decided to get this now, so I would not have to remove the circuit board and add it and re-route wires later.

This is where the documentation/manual is terrible. The main build instructions says "put the wires here", unless you have the optional sensor... and then at the end of the main manual, there are sections that describe options, and there is one for the sensor. There is text that refers to a picture embedded which does NOT show the wiring, or the extra sensor.

Bottom line, the manual is a mess. I re-read the 2 sections several times, and the unit still did not work properly. The video on youtube does not show the wiring of this optional sensor.

When I turned the unit on, it cycled the relay once. Connecting it to the car made the relay click once and then release (charging current is only applied when the relay is energized). The Fiat responded with the dreaded "goal post" display on the dash (outside 2 charging leds lit)... the instruments indicated charging system problem.

Then I figured, maybe the youtube "manual" for the premium unit would have this information in it, since it must already include the optional current sensor. BINGO! By looking at the video, you learn that the "original" sensor is the GFI sensor, and both of the charging cable wires go inside, since it compares BOTH wires for an imbalance in power.. OK, so both power wires goe through it. Got it.

Next the optional sensor, CS1, which measures current, must only have one wire, so the other "hot lead" does NOT go through it.

So the #1 hot lead, usually red, goes through BOTH sensors. The #2 hot lead, usually black, SKIPS the nearest sensor (CS1), and goes through the further, original sensor, CS2.

Tuning the charger

By getting the sensors "wired" properly, the unit no longer shut itself off and there was a charging indication on the Fiat, and it did also indicate a level 2 charger, hurrah!.

But, first inspections indicated not much current flowing. Now what the manual says comes into play, you need to move the wires in the GFI sensor and minimize the voltage on one of the Arduino pins, A1.

This took a bit of tweaking, but I got mine down to 0.009 volts. Then you carefully squirt RTV silicone rubber into the sensor and let it set. I had to tweak it a few times after adding the RTV, then waited a bit to let it set up some and tweaked it again.

Now it works fine.

I tested it on both 110v and 240v and adjusted the trim pots down a tiny bit (go slowly!) and then up until they did not lower the current delivered, and then turned each up a little bit.

I used a clamp-on AC ammeter, and was getting about 12 amps on 110 and 25 amps on 240, but the battery was not down a lot.

My modifications/tweaks

I'll list up what I did differently from the instructions.

  • crimp and solder ring terminal to the small gauge wires that attach to the relay
  • trim the relay cover bottom so it will fit on the relay with the lid closed. Trim the bottom edges about 3/32". Put the cover on the relay, you might have to re-align some of the terminals to make it fit, and I had to deepen 2 of the "slots" to clear wires. Sight across the box and trim the relay cover the amount it sticks up over the edge of the box. You will also need to take a little bit off the center post in the cover. Once you have the top of the cover flush with the edges of the box, you are good.
  • I used a 1" length of the cover stripped from the J1772 cable to center the wire in the current sensing coil. Take the insulation, cut it lengthwise, and wrap it around the wire inside the sensor. Easy way to center the wire, and it helps stabilize that wire as it enters the critical GFI sensor.
  • Be sure to really tighten the gland nuts in the case first, because you will have to crank down the compression nuts a LOT to hold the cables, and DO crank them down.

  • I already had a 240 volt dryer type outlet in my garage, so I added a nice long cable of 3 conductor, 10 gauge rubber jacketed wire.
  • I made an adapter that my 240 volt plug plugs into to allow me to connect the same charger to 110v.

Misc Info

Nice tips on keeping someone from stealing your charger if you leave it outside the car:


J1772 interface explanation:



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