OCPP or Modbus?

I still get excited about birthdays.  Mine was the other day.  I like the silly bits the most – funny hats and off-key singing.   So last week when a friend asked, “What’s your favorite dessert?”  I knew that she wanted a single answer so that she could make a sweet treat for my big day, but the problem is that my favorite varies by season – peach ice cream in summer and hot apple pie in winter – so the complete answer would be, “It depends.”  Instead, since it's June I replied, “Peach Ice Cream,” (and it was excellent).

A client asked me nearly the same question, but not about desserts.  Instead he asked, “What’s the better communication protocol for EV chargers – OCPP or Modbus.”   He wanted a single recommendation so that his organization could select a standard for all their chargers.   I hate disappointing clients, but just like with desserts, my answer was, “It depends.”  There are similarities between the two standards that lead people to believe that they’re interchangeable, but I believe that there are advantages and disadvantages to each.

Both standards enable customers to control their EV charging to meet their business goals.  Both enable communication between an EV charger and a control system.  Control inputs and meter data can be transmitted by both.   Global manufacturers support both.  Both can authenticate charging via an RFID card.  Both allow customers to charge at a reduced power level to lower energy costs.  At a high level, they sound the same, but are they?

Pluses for both

OCPP was developed specifically for communicating with EV chargers.  Its design simplifies setting up and operating public, networked chargers in a couple of ways.  First, the chargers can be factory configured to connect to a central controller via an on-board cellular modem thus keeping commissioning costs low.  Second, its messaging format focuses on the important steps of public, fee-based charging: authorizing charging and reporting on the energy delivered during that charge.  To reduce unneeded data traffic, the charger limits communication with the server; mainly exchanging data at the beginning and end of a charge cycle.

Modbus was developed for communicating process control data in an industrial environment.  It was designed as a simple yet effective way to connect various devices to a central controller, and that simplicity has allowed Modbus to remain a standard for control systems for over 40 years.  Modbus’ design allows the server to poll for data or issue commands to chargers as needed which improves its energy management capabilities.  It can communicate over low-cost, serial networks and also over  faster communication protocols such as TCP/IP which has enabled Modbus to control complex devices over the internet.   Modbus is very flexible.  It enables device manufacturers to easily add custom features without requiring control providers to customize their software.

And some minuses

The intention behind OCPP’s design was to standardize communication so that a single protocol could work with any charger.  The OCPP standard has undergone several revisions since it was first released, but almost no manufacturer has adopted the latest release. Instead most support OCPP version 1.6 which was released back in 2015.  The 2015 version was OK, but it lacked specifics in certain key areas which has forced EVSE manufacturers to make assumptions in their OCPP deployments.  These assumptions vary by manufacturer which means the goal of a universal protocol has not yet been achieved, and control systems must support custom OCPP variations for each manufacturer’s devices.  Finally, by limiting the ability to change the address of the supervisory server, some OCPP chargers are “locked” to a single software system which could force customers to operate multiple control systems – a different system for each manufacturer's chargers.  Coordinating control signals between such systems to achieve energy savings would be extremely difficult.

Modbus’ simplicity avoids those issues, but it too has negatives.  Every Modbus device must have a unique address which adds a need for skilled labor to commission each charger. Also, while the newest version of the Modbus protocol allows Modbus commands to be issued across the internet,  Modbus was not designed with security in mind.  Therefore, special steps or additional hardware would be necessary for a Modbus charger to securely transmit bank card data for public charging stations.

So, what’s better OCPP or Modbus?

Like my favorite dessert, “It depends.”

For deployments that primarily focus on  public charging, OCPP’s focus on secure authentication and reporting total energy delivered per charging event works best.  Chargers can be shipped pre-configured so all an electrician needs to do is provide power.  The data flow mirrors the gas station experience (authenticate, fuel, charge based on usage) making it best for public (credit card) or charge network (membership) charging.

Modbus offers flexibility that better supports more accurate control and interoperability.  It can operate over low-cost serial networks.  For sites that wish to minimize energy costs, Modbus’ client / server design allows for more accurate energy management making it best for fleet charging depots.

The client who called me is deploying a pilot fleet of Electric School Buses in a high demand cost state.  Reliability and energy cost management are critical to this pilot projects' success.  They may mix EVSEs from multiple manufacturers.  Therefore, in this case my recommendation was easy – Modbus.  Then he mentioned adding a couple of public chargers at the court house, and for those I suggested OCPP.

Like I said, "It depends."

I suggest that communication protocols be matched to the vehicles and how they will be used.  For public / fee chargers where the primary control function is authentication, OCPP will meet the needs, and should make deployments easier.  For sites where there's a single payer for electricity, then energy savings will become the primary goal, Modbus will best support control operations.  Luckily, several manufacturers have announced support for support both standards.

Anything else?

Recently I received information about a charger that has an option for using another proven control system protocol – OPC-UA. More to follow.

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