Multi car EV chargers

Electric Vehicles are moving up, at least here in California, and it's gotten to the point that EV drivers are finding all the charging stations where they want to go already in use, forcing them to travel well outside their way, or to panic. Sometimes not charging is not an option. Sometimes the car taking the spot is already mostly charged or doesn't need the charge much, but the owner has not come back.

Here in Silicon Valley, there is a problem that the bulk of the EVs have 60 to 80 miles of range -- OK for wandering around the valley, but not quite enough for a trip to San Francisco and back, at least not a comfortable one. And we do like to go to San Francisco. The natives up there don't really need the chargers in a typical day, but the visitors do. In general, unless you are certain you are going to get a charger, you won't want to go in a typical EV. Sure, a Tesla has no problem, but a Tesla has a ridiculous amount of battery in it. You spend $40,000 on the battery pack in the Tesla, but use the second half of its capacity extremely rarely -- it's not cost effective outside the luxury market, at least at today's prices (and also because of the weight.)

Charging stations are somewhat expensive. Even home stations cost from $400 to $800 because they must now include EVSE protocol equipment. This does a digital negotiation between the car and the plug on how much power is available and when to send it. The car must not draw more current than the circuit can handle, and you want the lines to not be live until the connection is solid. For now that's expensive (presumably because of the high current switching gear.) Public charging stations also need a way to doing billing and access control.

Another limit on public charging stations, however, is the size of the electrical service. A typical car wants 30 amps, or up to 50 if you can get it. Put in more than a few of those and you're talking an upgrade to the building's electrical service in many cases.

I propose a public charging pole which has 4 or even 8 cords on it. This pole would be placed at the intersection of 4 parking spots in a parking lot. (That's not very usual, more often they end up placed against a wall, with only 2 parking spots in range, because that's where the power is.) The station, however, may not have enough power to charge all the cables at once. Instead, the charging station would feed out the power to cars in turn. We'll examine the simple case of 4 cords but only 10kw of power, which gives the ability to charge only one or two of the cars. We'll consider this as well in a business or airport parking lot where cars are often present all day or longer. (In the airport case, the solution is usually valet parking.)

Without anything fancy, such a station could just charge cars in the order they arrived. In particular, once a car gets closer to full, it can't actually take a full current charge any more, and so the station could offer some juice to the next car in line. When you arrived, the station could display when you would get power and how much, based on who is ahead of you and how much power they need. (If they know how long the car will be there, this could be added.)

A marketplace for the power

It gets more interesting if you create a marketplace for the power. Of course, the first thing many would like would be a way to reserve a charging slot in advance. That doesn't happen much today, but if it did, you could take off on a trip knowing (almost) for sure that you're going to get your charge. (Almost because people could still physically occupy the spaces even though the machine won't charge them.)

The marketplace could be a lot cleverer the more you are willing to tell it. Bidders could request a slot and a current curve. If there is a spot that is not contested -- they are the first -- they would get it for the base price. You would pay even if you don't use the slot. If your slot is not available, you would have to buy a slot from somebody else who already got one. The previous buyer, the owner of the charging station and the marketplace operator would all take a cut of the profit on the resale.

For example, if you know you're staying there all day, you can tell your phone that and so it might be happy to buy the charging slot from 2pm to 4pm if the earlier slots are taken. The guy who arrived with you at 9am but is leaving at noon wants the early slot more, and will pay more, and will buy it from you at a small premium if you have it, but only those who know they are there for a while would buy the later slot.

You (your agent in your phone or car, really) could buy an early slot, but be happy to sell it off to somebody who arrives later and is willing to pay a premium, as long as you know you are sticking around. You could be personally pinged to confirm this if your schedule is in flux.

In addition, you could sell off some of your charge slot if you know you don't need it as much as others. For example, if you have a Tesla S with plenty of juice to get home, you might sell off your slot and take no charge at all because somebody more desperate is willing to pay. You should get some reward for your $40,000 battery! You might also sell off for a very high price if you know you can get a supercharger slot on the way home, though it costs you time.

Often if you buy a later slot, you may find the station willing to charge you earlier because nobody else is there. It may even reserve the right to insist on doing that, and you can't really complain. Owners of stations will probably not want people buying slots they know will be popular just to speculate on them, even though the station owner will get a cut. For example, when I get there at 9, I might deliberately grab 11am to 2pm, knowing that's a peak time for visitors.

Naturally, the goal would be to not personally involve you very much. You would provide basic inputs -- how long you expected to park, and how certain you were of that duration. Your car would know how much charge you need, you might need to add a bit more info about your other plans for the day, but otherwise it would presume your main goal is to get back home or follow your typical daily driving pattern from this location. Complex input would probably only come when you go to a place regularly, so the system can learn from your habits. Direct input would only be used in unusual situations -- such as a bid above a certain threshold to buy your slot, or a parking lot you need to park in that has no capacity for you.

With most level 2 charges taking about 4 hours (except the Chevy Spark EV, which is a great vehicle except for its low power charger) 8 cords on a pole is too much, but 4 might be about right since most cars at public lots don't need a complete charge, and they don't draw as much power when topping up. In an airport lot, 8 cords might be too few, and valets would do the rest. In airport lots, even 115v charging can be sufficient.

Whether any of this makes sense depends on the balance of cost between EVSE switching equipment, high current wiring, extra cords and plugs, and higher capacity service into a facility. Right now the EVSE switches are expensive enough that the sharing isn't that useful.

Without all the extra cords and switches, it would still be possible to implement a system like this "the hard way," which is to say that when it's time to switch from one car to another, one of the car owners or a parking lot attendant has to come down and physically switch who is plugged in to a single cord station. That may not be as much work as imagined. If the person getting the charge is leaving, they can be told where to put the plug next. Or once their car is full enough, they can be told that they will keep paying until they come and switch the plugs, and if they don't do it, perhaps the owner of the next car will pop down, or failing that an attendant or even a random taskrabbit type worker might do it for a few bucks. Right now many EV charging stations in busy lots limit the spot to 4 hours to make sure people come down and yield to other EVs; this would be better. Finally, you could also build a system which still offers 115v charging to cars who aren't plugged into level 2 when the capacity is available. 115v plugs don't talk EVSE, they are assumed to be always on and always 15amp, but they are much cheaper to provide, though they tend to provide under 5 miles of range per hour plugged in.

I should note that the EVSE protocol is a remarkably stupid protocol for something designed in modern times. It's darn close to analog, and about all the station can communicate to the car is how much current to draw. Officially it runs from 6 amps to 80 amps. A digital protocol would have been so much nicer, allowing the car to negotiate payment over the cable and to send information like this. Of course, that can be done over bluetooth or wifi, but you then have to kludge the authentication that you are talking to the same car over both channels.


How many miles per hour of charging would *wireless* charging accomplish?

I imagine it's so low that it would make sense only in an airport's long term parking.

Inductive charging systems are able to do full level 2, I don't know of anybody who has built an inductive fast charger. They lose about 10% of the energy.

Their main advantage is you just park, you don't have to hassle with cords, and that is attractive, but it comes with a cost. I don't know if anybody has made a protocol for payment at inductive chargers, or if there are any seriously deployed ones.

They have some attraction for robocars, which can't handle cords at present, though robotic arms to plug in cords are not science fiction (there are already systems in use which have robot gas pumps refill your gas tank.)

Wireless charging seems like it'd make sense for public parking areas where every spot has a charger. Right now only a few spots per large garage or lot offer charging, so anything that works is a feature. Eventually it'll be better to get rid of boxes and cables and have all that built into the ground where it's more difficult to wear out or be tampered with. No moving parts means much less maintenance.

The lost energy in that scenario will probably be offset by the lower maintenance costs. In the home garage, people may remain willing to plug a cable in order to save that cost.

To get rid of the sidewalk box, there would need to be some way to pay that gets initiated automatically from the car itself, or phone inside it. As smartphone adoption is moving much faster than EV adoption, presumably this will be a non-issue in a few more years.

It's good, but it's a lot more expensive to install. The proposal above is how to serve a lot of cars without a lot of money (and installing a crazy amount of amps of service.)

How about parking the car on a rotating plate or elevator so you don't have to have a person moving the cars? Or what about a switch that automatically feeds first one outlet and then another each in turn, so the cars dont have to physically move at all? Isn't there a better way to take turns than paying a valet?

A switch is exactly what I am talking about. The turntable would be hugely expensive, much more expensive than just building more electrical capacity, and it wouldn't solve anything because you still would need to move the plugs.

So... Not to spam or anything... I hope...

But I'd like to tactfully point out my little product. It's the J1772 Hydra, and it's a double-headed EVSE. It comes in two variants. One is just a straight EVSE with an AC power connection and two J1772 plugs/cables. The other is the "splitter" variant. It has a J1772 inlet and is intended to allow you to share an existing EVSE with two vehicles.

The design of the firmware and hardware take every safety requirement and precaution into account. The firmware can operate in two modes - shared mode and sequential mode. Sequential mode simply waits for one car to finish and offers power to the other. The benefit is that it can do the switching immediately without waiting for a human to come move the plug. Shared mode is more sophisticated. It allows a single car to have 100% of the available power or both cars to have 50% each. When one car finishes, the other gets 100%. When the second car requests power, the first car's power is reduced to 50% dynamically.

So far as I am aware, the Hydra is unique. The only other single-circuit double-headed EVSE available is one from ChargePoint that is intended only for commercial installations.

More information is available at the OpenEVSE wiki, and the Hydra board sets are for sale at my Tindie store at

Not spam at all, and in interesting product. Why 50-50? Can't you notice what the prime car is using, and offer the remaining amps to the 2nd car, so as the first car starts drawing less, you can switch the other car to more?

Much cheaper than I expected it to be, but of course that is just as a board.

In general, many charging stations have more capacity than many of the cars that come up to them, so it should always be possible to give a little charge to the 2nd car unless the first car has a high end charger that exceeds the overall capacity of the station.

Since I built a bunch of splitter Hydras for the office, I've had a chance to observe the behavior of a lot of different cars. Every single one draws as much power as you offer (up to a maximum) until the battery is full, and then drops down to usually 6A or less while the pack is balanced, usually for around a half hour.

Given that profile being the norm, there isn't a lot of benefit to other than a straight 50-50 split. I have considered an extension to the rules such that if a car draws under 6A for 5 minutes, it's pilot is dropped to 6A and the rest given to the other car (6A is the J1772 minimum current offering), but "Worse Is Better" prevents me from doing the work. ????

A fully built Hydra typically costs around $700 and an afternoon of labor. I would sell complete ones, but don't have the tens of thousands of dollars it would cost to seek UL approval, and lack the proper liability insurance as well. That said, I have every confidence that the design is safe.

Typical cars have a 6.6kw charger (30a), though the Tesla has a 10kw and a few have a 3.3kw like the Spark EV, Volt and some leafs. So surely they don't draw more than 30 amps if you offer 50? Also, I had not expected that when the packs were approaching full that they could take the full amount. But if you have a 30a port, I could see one car taking all of it for a while.

If you could really talk to the cars (over wifi or their phone app) it would be good if you knew how much they really need and how long they are going to be there, but your device is a good start. Of course, $700 is the cost of a typical charging station, but there is also the cost of the extra amps, which is what I was talking about.

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