The J1772 Hydra - charge two EVs from one EVSE or circuit

The J1772 Hydra is a sort of outgrowth project of OpenEVSE. There are two variants of the Hydra. The original "splitter" variant has a J1772 inlet and two plugs. It's intended to be used on a "host" EVSE and allows connecting two cars at once. As long as both cars are charging, each is allotted half of the available current. When one car finishes, the other is brought up to 100% power (there are also per-car limits in the code, so that the design maximum of each car's high current path cannot be exceeded accidentally).

The second variant is a straight-up two head EVSE. It connects to either 120v (but why would anyone bother?) or 240 VAC power and has a UI to set the available current (much like OpenEVSE). Like the splitter Hydra, when both cars are charging, each is granted 50% power, but when one stops, the other gets 100%.

Both Hydras also have a sequential mode that can be used if desired. In sequential mode, only one car is given power at a time. When the first car finishes, the other car is given a chance.

The EVSE variant has a real-time clock built in and you can set up up to 4 time events with day-of-week flagging. The events can "pause" or "unpause" the Hydra, for those who want to connect two cars up in the early evening and have them start charging when the rates are low and have both of them finished in the morning (without having to move the plug at 2 AM).

The reference design uses 30A 12VDC coil relays, but I also have "contactor adapter" boards that use an opto-isolated triac to drive a line powered contactor. Using those, I built what I call "mega-Hydra," which is rated to charge a single car at 50 amps or two cars simultaneously at 25A.



Anyone who is moderately handy with tools and basic wiring can build a Hydra. If you buy the logic board fully assembled, then no soldering is required at all.

More info is at the OpenEVSE wiki: https://code.google.com/p/open-evse/wiki/Hydra

Awesome! I read about this project before on the openEVSE page but haven't seen many pics of actual builds. That looks like great work. This is the sort of thing - perhaps scaled up a bit - that I think every office building should have for its employees. Of course as usual the community is way ahead of the big industry players here.

Naturally, I want a bigger one... 80 amps total, 40 amp split.

Wouldn't I just need two 80 amp relays?

TonyWilliams said:

Naturally, I want a bigger one... 80 amps total, 40 amp split.

Wouldn't I just need two 80 amp relays?

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You would need 80A contactors, 80A J1772 cable and plug assemblies and 4 gauge wire, which is a problem because 6 gauge is the biggest you can squish through the GFI coil.

But 6 gauge would be good for 75A, which is pretty close.

Unfortunately, the biggest cables I can find are 60A rated ones from eMotorWerks. But that, 60A contactors, and a 90A circuit breaker would yield the ability to charge 2 cars at 36A or one at 60A. That, I believe, is the highest you can presently go without getting and building custom components.

100 amp breaker would let you charge 2 cars at 40 or one at 75 amp, Tony has some on the 75 amp cables from when they were available.

nsayer said:

TonyWilliams said:

Naturally, I want a bigger one... 80 amps total, 40 amp split.

Wouldn't I just need two 80 amp relays?

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You would need 80A contactors, 80A J1772 cable and plug assemblies and 4 gauge wire, which is a problem because 6 gauge is the biggest you can squish through the GFI coil.

But 6 gauge would be good for 75A, which is pretty close.

Unfortunately, the biggest cables I can find are 60A rated ones from eMotorWerks. But that, 60A contactors, and a 90A circuit breaker would yield the ability to charge 2 cars at 36A or one at 60A. That, I believe, is the highest you can presently go without getting and building custom components.

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We build J1772 cable assemblies (and have sold ours to eMW), so that part is a "go". We won't use 6 or 4 gauge, but instead parallel 10 or 8 gauge wires for each power pin.

I have two 75 amp SquareD relays (240 volt powered) sitting on my shelf in the garage. I also have several of the previously available 75 Leviton cables.

So, what if I'm pumping 75 amps into my twin charger Rav4 EV and then somebody plugs in an 12 amp Prius. Will I get 75-12 amp into the Rav4 EV? When the Rav4 EV starts ramping down to 20 amps, will another car automatically get an ever increasing pilot signal?

TonyWilliams said:

So, what if I'm pumping 75 amps into my twin charger Rav4 EV and then somebody plugs in an 12 amp Prius. Will I get 75-12 amp into the Rav4 EV? When the Rav4 EV starts ramping down to 20 amps, will another car automatically get an ever increasing pilot signal?

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I believe the Hydra project as-is takes the simple approach of either producing a 100% (1 car) or 50% (2 car) pilot signal, which unfortunately means the Rav would get bumped down to 40a when the PiP plugs in. But what you describe is certainly possible with CTs to determine each car's draw and slightly more sophisticated software to continuously monitor actual current draw and adjust pilot signals accordingly.

fooljoe said:

TonyWilliams said:

So, what if I'm pumping 75 amps into my twin charger Rav4 EV and then somebody plugs in an 12 amp Prius. Will I get 75-12 amp into the Rav4 EV? When the Rav4 EV starts ramping down to 20 amps, will another car automatically get an ever increasing pilot signal?

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I believe the Hydra project as-is takes the simple approach of either producing a 100% (1 car) or 50% (2 car) pilot signal, which unfortunately means the Rav would get bumped down to 40a when the PiP plugs in. But what you describe is certainly possible with CTs to determine each car's draw and slightly more sophisticated software to continuously monitor actual current draw and adjust pilot signals accordingly.

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That's the beauty of OpenEVSE and Hydra - if you have the programming skills you can make it do whatever you want. If it has measurement capability, I would think it would be easy to program it to give the first car whatever it can draw, measure it, then give the leftover capacity to the second car. Then, maybe measure the draw every minute or 5 minutes and re-adjust the pilot signals.

Has anyone ever tested what the RAV does when it encounters a changing pilot signal? The Model S used to go down only and never go back up automatically. That was changed in a recent firmware release so that the charge current automatically goes up and down.

miimura said:

Has anyone ever tested what the RAV does when it encounters a changing pilot signal? The Model S used to go down only and never go back up automatically. That was changed in a recent firmware release so that the charge current automatically goes up and down.

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Yes I've seen it. My work recently installed charepoint EVSEs that have a 6.6kW max output but two j-plugs that share the max output. Not realizing this I plugged in next to a model s and wondered why I was charging at 3.3 kW. Since they charge by the hour it actually makes quite a bit of difference in cost. I guess I might have pissed off the model s owner because 15 minutes after I plugged in he unplugged and the charging rate went to 6 kW. Still not quite sure why I couldn't get the full 6.6 kW.

yblaser said:

miimura said:

Has anyone ever tested what the RAV does when it encounters a changing pilot signal? The Model S used to go down only and never go back up automatically. That was changed in a recent firmware release so that the charge current automatically goes up and down.

Click to expand...

Yes I've seen it. My work recently installed charepoint EVSEs that have a 6.6kW max output but two j-plugs that share the max output. Not realizing this I plugged in next to a model s and wondered why I was charging at 3.3 kW. Since they charge by the hour it actually makes quite a bit of difference in cost. I guess I might have pissed off the model s owner because 15 minutes after I plugged in he unplugged and the charging rate went to 6 kW. Still not quite sure why I couldn't get the full 6.6 kW.

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This is one very good reason why charging by the hour is not good. Even with a dedicated charging station, if your car can only take 3.3kW, why should you pay the same as someone who can suck down twice as much energy in the same amount of time?

TonyWilliams said:

So, what if I'm pumping 75 amps into my twin charger Rav4 EV and then somebody plugs in an 12 amp Prius. Will I get 75-12 amp into the Rav4 EV? When the Rav4 EV starts ramping down to 20 amps, will another car automatically get an ever increasing pilot signal?

Click to expand...

The current firmware doesn't attempt to "balance" the pilot to the car's demand - giving unused capacity back to the other vehicle.

I've considered such a feature in the past, but in the "worse is better" software development paradigm, that's a feature that simply didn't make the cut.

I may still add it. I would probably call it "cinch" mode, and the concept would be that if one car was drawing substantially less power (say, a more than 5 amp gap) than it was being offered for more than 5 minutes, then the Hydra would "lower the ceiling" for that car and give that power to the other car. This would also allow the Hydra to detect cars that were balancing and give all but 6 amps (6 amps is the minimum the specification allows) to the other car.

I would still leave the current "shared" mode as an option, because I can just envision a car that consistently drew 6 amps less than the pilot, and as the ceiling was lowered it would draw less and less. Shared mode is more or less guaranteed to be compatible. The problem with J1772 is that there is no "request" channel for the car to send information about what it wants.

I've not added this feature until now because the bulk of the Hydras that exist in the universe today are the 16/30 reference design, and there's not a whole lot of benefit to be had (because not many cars draw substantially less than 16 amps except during balancing, and that's a limited amount of time). But if people are going to start building Mega-Hydras, then it becomes a much more interesting idea.

miimura said:

If it has measurement capability...

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It does.

Has anyone ever tested what the RAV does when it encounters a changing pilot signal? The Model S used to go down only and never go back up automatically. That was changed in a recent firmware release so that the charge current automatically goes up and down.

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We've got a couple of Ravs that use our Hydras at work. They do the right thing.

yblaser said:

... charging rate went to 6 kW. Still not quite sure why I couldn't get the full 6.6 kW.

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208 volts multiplied by 30 amps.

nsayer said:

But if people are going to start building Mega-Hydras, then it becomes a much more interesting idea.

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Big loads are where the biggest likelihood for cost effective gains.

I would put Hydras in my garage, probably two, each on 100 amp circuit. One more feature I'd want is manual override, where I can limit both cars to XX max amps, or limit either car to XX amps.

So, I pull in and plug in Model S, but I need all the power I can for my next trip. Driver B pulls in with a Model S, but is there all night, and only needs 16-20 amps max.

Otherwise, both cars would get 40.

Then, I would make the manual mode automatically reset to auto when both cars are disconnected.

TonyWilliams said:

So, I pull in and plug in Model S, but I need all the power I can for my next trip. Driver B pulls in with a Model S, but is there all night, and only needs 16-20 amps max

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Sounds like a great use case for "sequential mode", which was mentioned as an existing feature.

fooljoe said:

Sounds like a great use case for "sequential mode", which was mentioned as an existing feature.

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Model S is a special case. Even if the car that would be there all night got there first, he could manually set his current low and the second car could still charge fast as long as the firmware would give all available current to the second car. Manual mode is needed for cars like the RAV that do not have user adjustable charge current.

Well, even with Sequential Mode, you could just unplug the low priority car that was there first, plug in the high priority car, then re-plug the low priority car.

Also, what happens when you plug in a car that has the vehicle charge timer enabled and then plug one in that is set to charge immediately? Naturally, you would want all but the minimum pilot to go to the car that wants to charge now.

miimura said:

Well, even with Sequential Mode, you could just unplug the low priority car that was there first, plug in the high priority car, then re-plug the low priority car.

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When we start talking about "priority" one could envision that translating into cost. For example a Hydra at an airport parking lot might have tiered pricing - if you're going to be there for a long trip you pay only 5c/kwh or charge for free but get the slowest possible charge if anyone else is charging; but if you're just there to pick up / drop off and need a quick boost to get home you could pay 25c/kwh or some fixed fee and get full 80a charging in your dual charger Model S.

It's pretty much all just software, and there's plenty of room for innovation in this space, but it'll probably be quite a while before there's enough of a market (and enough consumers/businesses that know enough to care about such a thing) to get things going.

fooljoe said:

miimura said:

Well, even with Sequential Mode, you could just unplug the low priority car that was there first, plug in the high priority car, then re-plug the low priority car.

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When we start talking about "priority" one could envision that translating into cost. For example a Hydra at an airport parking lot might have tiered pricing - if you're going to be there for a long trip you pay only 5c/kwh or charge for free but get the slowest possible charge if anyone else is charging; but if you're just there to pick up / drop off and need a quick boost to get home you could pay 25c/kwh or some fixed fee and get full 80a charging in your dual charger Model S.

It's pretty much all just software, and there's plenty of room for innovation in this space, but it'll probably be quite a while before there's enough of a market (and enough consumers/businesses that know enough to care about such a thing) to get things going.

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I was assuming a home scenario where you have full control over the cars that are charging. Public use is a whole different animal and it gets way more complicated when people are paying. I understand Nick is using this at his workplace, probably with free charging, so the only fair thing to do there is to split the capacity equally when two cars are charging and when one is done, allow the car that's still charging to get all the current. When a different car plugs in, it goes back to 50/50.

TonyWilliams said:

yblaser said:

... charging rate went to 6 kW. Still not quite sure why I couldn't get the full 6.6 kW.

Click to expand...

208 volts multiplied by 30 amps.

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I realized it was probably at 208V, but was confused why it would pull 32A (2x16A) when two cars are plugged in and only 30A when one car is plugged in. I guess the particular parts in each half of the EVSE are only rated at 30A while the supply circuit can go to 32A.

yblaser said:

TonyWilliams said:

yblaser said:

... charging rate went to 6 kW. Still not quite sure why I couldn't get the full 6.6 kW.

Click to expand...

208 volts multiplied by 30 amps.

Click to expand...

I realized it was probably at 208V, but was confused why it would pull 32A (2x16A) when two cars are plugged in and only 30A when one car is plugged in. I guess the particular parts in each half of the EVSE are only rated at 30A while the supply circuit can go to 32A.

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The Hydra reference design is like that. I use 30A rated relays and J1772 plugs and #8 SOOW input cable. The result is a 20/30 (20A each car shared, 30A for a single car) capable unit. Done as a "splitter," the reference design has a 50A rated inlet, but the typical result is 15/30, because most public L2 chargers will only do 30A. Except for Blinks, which are still under a universal de-rate to 24A.