Local EV Charging Network

How we can get lots more charging stations – power sharing

Some electric car charging station installations require a power management system to ensure power consumption is constrained to the service panel capacity.  The “service panel” being the grey box containing circuit breakers for each circuit, and the master circuit breaker for the panel.  Service panel capacity is what constrains the total number of charging stations at a given facility, and when charging station requirements grow larger than service panel capacity the host site faces the expensive prospect of upgrading their service panel.

That is – Typical EVSE’s (charging station) today are designed to connect with a 40 amp circuit, providing 32 amps to the car.  At 208 volts (typical for commercial power) that is slightly over 6 kilowatts, and at 240 volts that’s 6.6 kilowatts.  In any case, a bank of 10 EVSE’s then requires 400 amps electrical service capacity.  (10x 40 amps = 400 amps)

But we’re facing a future of ever more electric cars being sold, and a constantly growing need for electric car charging stations.  That means host sites will like charging station products with which they can increase the number of charging stations, without increasing service panel capacity.

What if there were a way to have 20 or 30 EVSE’s on 400 amps of service panel capacity?

A few companies are working on systems to do that.  What I’m about to describe is my thoughts on a simple way to implement such a system.

Local EV Charging Network

Let’s start with this image.  You’ll see it has two connections to the electricity grid, one for the building power and the other for the local charging station network.  As I said in the immediately previous post – it’s useful to separate these for accounting purposes, such as achieving Net Zero Energy goals.  Let’s not get too hung up on that, and look at some other aspects of this system idea.

Instead of giving each charging station a simple connection to the service panel, with no coordination between the stations, I’ve drawn a box labeled “Local Charging Network Controller”.    Say what?

What we’re designing is a “charging station network” for a local site.  Let’s see what we can do without having to involve another company to remotely manage the charging facilities.

To that end, let’s design a small scale computer that would be tightly coupled to the service panel.  This computer needs connection to and control over every charging station.  It doesn’t have to be a big expensive computer server — today’s computer technology has managed to produce miniscule computers with the computing/transaction power of earlier behemoths.

Local Charging Controller

Here’s details on what’s going on between the car and the Local Charging Network Controller.  (Apologies for using a gas pump icon to represent the charging station)

Let’s take the top two points together

  • Watches the power consumption of all charging stations in the local network
  • Adjusts power consumption of each station up/down to keep the system within power constraints

The only way to share a 400 amp circuit with 30 charging stations is by adjusting the power consumption at each station to keep the whole system within 400 amps.  (The numbers have to be adjusted depending on local conditions)  Otherwise the host site can only support 10 charging stations off the 400 amp capacity, and the expansion to 30 stations requires 1200 amps of capacity.

Those of you complaining about the lack of charging stations at each site – this is the cause.  Service panel capacity constraints limit the number of charging stations at a site.

This design is meant to allow increasing the number of charging stations, without triggering the need for more service panel capacity.

The strategy is adjusting charging station power consumption on each circuit.  Staying with our 400 amp service panel, it can support 10 charging stations running at a full 32 amp rate.  As soon as 11 cars or more are charging, the network control computer has to start managing power load.  At 11 charging stations the safe rate per station is 29 amps, at 12 stations the safe rate is 26 amps per station, at 23 stations the safe rate per station drops to 14 amps, and so on.

The power rate is adjusted between the car and the charging station using the J1772 pilot signal.  Electric cars are supposed to respond to this signal and adjust the charging rate to match.

The normal purpose of the pilot signal is so a car with a powerful on-board charger can use a lower capacity charging station without causing a fire.  Otherwise the cars with an on-board 10 kiloWatt charger would overdrive a 3.3 kiloWatt charging station, heating up the wires, and if the circuit breaker doesn’t trip causing a fire.


The local charging network controller has to send a command signal to the charging station so it can in turn adjust the pilot signal so the car ramps down its charging rate.  The controller must do this based on the number of connected cars, sharing out power to keep it within the service panel capacity.

The mechanism for these command signals would best be powerline ethernet, unless there’s a smart grid control protocol which can be used.  The point is to implement data communication over the physical wires connecting the charging station with the service panel.  It wouldn’t be safe to use WiFi or other wireless protocol because the charging stations might be behind multiple concrete walls (signal propagation) or attackers could crack the system over WiFi.  Communications coming over that physical connection are more secure.

Lastly is the question of user authentication.  So far everything we’ve discussed can be handled within the confines of the local charging network controller.  It can be configured with service panel capacity and even the topology if there are multiple service panels.  But what about authenticating charging station users?

With the incumbent charging networks (Blink, ChargePoint, GreenLots, etc) there is a centralized authentication system reached over the Internet.  But we’re trying to avoid using such a centralized system, and keep control at the local level.

Instead of an RFID card with a card reader that might break down – what about a PIN code entered over a Bluetooth connection?

The local charging network controller could have an iPad app that a network administrator uses to configure the system.  One task would be assigning PIN codes, and ensuring employees knew how to authenticate using the PIN code.  Another would be designing the overall access policy – such as turning off access control in the evening or weekends to allow the public to use the charging stations.

Having the charging rate fall so low (10 amps?) is suboptimal, but it’s better than a 0 amp charging rate.  Currently I hear some workplaces see real competition erupting between coworkers over access to a limited number of charging stations.  Easing charging station access is therefore crucial.

The system as described might not qualify under current electrical code.  I’ve been told that each charging station must be on a dedicated circuit, and the required service panel capacity is a simple calculation – the number of circuits multiplied by the rating on each circuit.  (10 circuits, 40 amps each, 400 amps total)

For the system described here to work, there must be multiple levels of protection.  For example the charging network controller must monitor lines, and keep the system within parameters even if one or more charging stations starts misbehaving.  The measure of last resort is to completely shut off power to charging stations that don’t respond to control commands, and to completely shut down the system in extreme cases.

We going to have lots of electric cars on the road soon enough.  That’s exciting, but how will there be enough charging stations?

About David Herron

David Herron is a writer and software engineer living in Silicon Valley. He primarily writes about electric vehicles, clean energy systems, climate change, peak oil and related issues. When not writing he indulges in software projects and is sometimes employed as a software engineer. David has written for sites like PlugInCars and TorqueNews, and worked for companies like Sun Microsystems and Yahoo.
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About David Herron

David Herron is a writer and software engineer living in Silicon Valley. He primarily writes about electric vehicles, clean energy systems, climate change, peak oil and related issues. When not writing he indulges in software projects and is sometimes employed as a software engineer. David has written for sites like PlugInCars and TorqueNews, and worked for companies like Sun Microsystems and Yahoo.


  1. Hi David, You might be interested in this…
    I’m part of a group of EV owners in the UK looking to start our own network and have been researching the market for exactly what you describe. This one is based on AC, public charging facility. Sold in the UK under the APT brand.
    The best solution of course is a multi head DC system that will allow up to 50kW per car – obviously with the caveat if there are 50 cars parked you get 1kW each! It’s the logical approach to me… not everyone will arrive at the same time and the AC>DC kit being left out of the car and part of the stationary charger is surely the way to go in the [long term] future to reduce cost.

  2. Arnold Offner @Phoenix Contact in USA

    Hi David,
    At Phoenix Contact we have a the solution close to what you describe. Our EV Charge Controller (www.phoenixcontact.com/catalog/2902802) is able to networked with a Modbus master controller (we make those too), and can be configured to the maximum current available, and if needed vary the EV Charging current from a SAE required minimum of 6A to the maximum capability of the EV connector (most SAE J1772 are rated at 32A) or the maximum required by the car. The Modbus Master controller communicates locally with all the EV Charge cords, and can vary the output power based the signal from each connected car. We just deployed such a solution for employee workplace charging and can share more of the concept with you and your readers if you wish.
    Our design is modeled on a 25kW (240 Vac @ 100A) and 4 EV charging cords that can deliver the combined current output. Since not every EV our employees have 32A charging capability, we do not yet have to reduce output power. The Chevy Volt and Ford Fusion Energi draw 13A, and the e-Golf 32A.
    Of course, as a previous comment mentions not every arrives or departs at the same, and all have differing charging levels based on their commute to work. Also priority can be given to folks who will not be parked all at the office. The power sharing idea is also possible when combined with solar and battery storage, and the combined power requirements of the home or other installation.
    It s a wonderful topic, since the modularity we offer to the EV Charging community requires a system integrator or electrician to wire up and install the appropriate components, and software skills to make it all work.

  3. Thank you David for this excellent article.

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