Sunday, May 4, 2014

Hydra II EVSE variant now available

If you've ever wanted to use a Hydra at home, but wanted to wire it directly to a dedicated circuit or plug it into a dedicated 240 volt outlet instead of connecting it to a host EVSE, then your day has come!

The Hydra II EVSE variant boards have now arrived in the store! This variant trades the inlet handling and outlet proximity signal for a GFI and a real-time clock chip. With it, you can build your own double-head EVSE capable of connecting to two cars simultaneously and either charging both with half the current allowance of your circuit or sequencing the two vehicles automatically.

Not only that, but the real-time clock chip allows you to set up to 4 events, each with a time-of-day and a day-of-week mask - enough to set up two independent time-of-use charge schedules - perfect for weekday and weekend scheduling (if you want more, you can simply change a constant in the firmware and re-upload). There's even timezone support so that the clock will automatically correct for daylight savings time.

As with the original Hydra II, if you order the assembled and tested board, then you can complete your Hydra without doing any soldering at all (you can buy a pre-wired button from the OpenEVSE store - just tell Chris you need long wires), and it takes about a half a day of basic mechanical assembly.


  1. Hi Nick,

    I was wondering if it would be possible to program the Hydra to detect when one of the 2 vehicles doesn't require the full amount of power that it is alloted and then reduce the power accordingly, while increasing the power offered to the 2nd vehicle. I believe you have amp meters on each outlet, so you might be able to do it.

    For example, let's say that there are 2 cars and a 40 amp Hydra II. Car A requires 32 amps, so when plugged in alone, it gets the full allotment. However, when a second car is plugged in, the 40 amps are shared equally: 20 amps for car A and 20 amps for car B. However, car B is a PHEV and can only charge at 16 amps. Instead of "wasting" 4 amps, the Hydra could sense that car B never pulls more than 16 amps and then adjust the pilot signals so that it offers 24 amps to car A and 16 amps to car B, therefore allowing car A to charge slightly faster and maximizing the use of available power from the Hydra.

    I don't know if it's doable, but it would certainly be a great feature that would take advantage of your design and be more advanced than what the current generation of EVSE offer.

  2. I have thought of arrangements like that, but not implemented them only because at least from my perspective the cost:benefit ratio doesn't work out.

    The reference design calls for a 40A circuit, so can deliver up to 32A, shared between two vehicles at a maximum of 30A for any one. Most cars that draw less than 30 will draw close enough to 16A that trying to optimize isn't going to be of much benefit in terms of charge time - a few minutes out of 3-4 hours.

    There's also balancing time. Most cars will draw 100% of their offered power until the battery gets close to 100% and then drop down to 6A or less (6A is the lowest offer allowed by the spec). It might be possible to detect, say, 5 minutes of sub-6A draw and then just drop them to a 6A pilot and give the rest to the other car, but it adds complication to the state machine for a potential benefit of giving around 10A to the other car for a half an hour perhaps.

    In the "worse is better" school, tremendous value is placed on simplification of the design at the potential expense of extra features. That's one that just didn't make the cut so far.

    That said, the firmware is on GitHub, and I certainly am not adverse to pull requests.

  3. Oh... One more thing. I read your comment again and you spoke of 20A when sharing 40A. Don't forget that you must derate a 40A circuit breaker 20% because EVSEs are continuous duty. You're only going to be able to charge at a total of 32A, not 40A. This goes on up too. A 50A breaker affords 40A for charging. To charge at the maximum rate the spec allows - 80A - you must use a 100A breaker.