Like an increasing number of people, we have one home EV charger but have two EVs.
My 2015 Nissan Leaf has a range of about 60 miles, which is perfect for daily school runs, and trips to the office. However, this means that most nights I need to put it on charge.
My Wife’s EV on the other hand is a more modern Nissan Leaf with a range of about 200 miles, which is more than enough for 99% of all day trips out, requiring no charging from public charging networks.
While it has a range of over 200 miles, it does need to be charged about once or twice a week depending on how much it has been used, and that means that my poor old leaf with its type 1 cable needs to be disconnected from the EVSE and replaced with her type 2 cable.
Can you join two EV charging cables together?
While we could get a second EVSE fitted to the house, to allow the charging of two EVs at home concurrently, there is no reason why you can’t have two EV chargers on one domestic circuit (in the UK). Our incoming fuse fuse from the distribution network is 100 A which means that in theory, the maximum current we should pull from the grid is 60 A at any one time.
A single 7 kW (about 36 A maximum available current at 230 V AC 50 Hz) charger paired with a 32 A cable is the most it is sensible to attach to a residential supply, to avoid getting too close to the 60 A maximum draw while also powering things in the home. Our PodPoint solo 3 is fused at 40 A in the meter cupboard.
While it may be tempting to consider cutting the two cables and putting a junction box in the middle, like a weird EV extension lead, the CP and PP signals in the cables would no longer work correctly and one car may charge if the other end was not plugged in, chances are that with both cars plugged in at the same time, the EVSE would enter a fault state and refuse to supply current to either.
How to split an EVSE between two EVs?
I previously worked for a market-leading EVSE manufacturer in R&D and Test and Development… so I know my way around IEC 61851 and SAE J1772, building on the theories covered in Minimalist EVSE Charger Circuits and DIY Manual EV Emulator we can design a box that plugs into our EVSE and then allows us to plug both vehicles in at the same time.
The box will split the supply current between the two vehicles while they both demand current until one of the vehicles is full, then redirect most of the available current to the one that is still demanding current.
b. EVSE splitter
c. Cable
d. Vehicle connector
e. Vehicle coupler
f. Vehicle inlet
g. Charging station
h. EV socket-outlet
i. EV Plug
The OBC (On Board Charger) is responsible for converting the AC from the EVSE to the DC from the batteries, and responsible for only drawing the maximum current advertised on the CP line.
The upstream current would come into the box along with the Control and Proximity pilot signals from the EVSE and the cable, which an onboard microcontroller would use to determine the maximum available current to supply to the two downstream connections.
By reading the current carrying capacity of the cables plugged into the two outlets, and using two sets of CTs (Current Transformers) the two downstream Control Pilot signals can be adjusted in real-time to make sure that the total advertised current between the two ports never exceeded the available from the upstream supply.
When a vehicle battery SOC (State of Charge) is nearing 100 % the OBC reduces the amount of current being pulled. Since we have a CT on each port, we can monitor the real-time current draw, and then reduce the advertised capacity on that port to just above the demanded current we can adjust the current availability on the other port.
Safety
If we are using BS EN IEC 61851 Part 1 Case B we will know if a cable is plugged into each downstream socket on the box by reading back the current carrying capacity of the cable via Proximity Pilot, if we are using Case C, then we will need to hard code the maximum carrying capacity of each cable.
The GFIC and RDC safety requirements will be met by the upstream EVSE, so we don’t need to re-implement them in the splitter box. At least for version one.
Octopus and Photovoltaics
Our electricity supplier is a company called Octopus Energy, Octopus has tariffs that vary the cost of electricity depending on how much demand there is on the grid, and the availability of power. It would be nice to be able to limit the charging speed when the cost is high and charge at a higher rate when the power is cheaper. This variable charging speed would also be beneficial when we get Photovoltaic cells so that we can use the self-generated power to charge the cars for free.
Conclusion to Charging two EVs from one EVSE
I will write a post soon with more details, block diagrams as well as representative circuits shortly.
An important question is what should I call it? I wanted to go with either Hydra or Madusa however both names were vetoed by my wife… so I need a name for this project.
26 thoughts on “Charging Two EVs on One EVSE”
I think Hydra is the only viable name for this. I’ll fight your wife if necessary.
Went with Hydra in the end, at least at the moment – https://philipmcgaw.com/ev-connectors-for-bevs-phevs-and-evses/
With regard to microcontrollers, if it needs WiFi then I’d use an ESP32, but ESP32 isn’t particularly well suited to PWM capture so I’d be tempted to go the dual controller route, putting the important logic in a STM32, and adding the ESP32 for optional network connectivity.
Thank you for that, I was working on the assumption that most microcontrollers could manage a 1 kHz square wave and accurately measure the mark space ratio of the CP signal.
Obviously, I was going to use a function generator and oscilloscope to validate my code and hardware before letting it loose on either an EVSE or a Vehicle.
Ultimately, this is less a “splitter” and instead, two fully functional EVSEs with load sharing capability, powered from another EVSE 👍
I think it’s a super useful idea (though I only own one EV) 🙂
It relies on upstream protection rather than including it.
Also can be used with a granny charger
I get that this is a fun project to you but honestly, if you want to be practical, selling your single port charger and buying a dual port charger is the better option.
If you want the challenge, then go for it.
“I want to build a small box that plugs into our installed EVSE and then allows us to plug both vehicles in at the same time, it will split the supply current between the two vehicles while they both demand current and then put the full available current on the one that is not full.”
You don’t need a single digital thing. The more discharged battery will pull more of the current anyway because the voltage differential is greater.
Now, if your charger can’t handle the full current draw of both vehicles, all you need is a simple ANALOG current regulator. It will drop the output voltage (and thus the current draw) to keep the current draw within tolerance. Again, the more discharged battery WILL pull more current naturally.
IF you absolutely, positively want only the more discharged battery to pull the FULL current a simple op-amp (LM741) wired as a comparator and a relay can do that. But, it will be tricky since I guarantee that each car battery is not the exact same voltage when fully charged. But that is even manageable with a some voltage dividers.
and you have just told us all that you don’t know how SAE J1772 works.
I don’t see anywhere in your ‘specs’ where your box has to be compatible with SAE J1772.
You provided nebulous specs, I provided a nebulous answer.
Arvon, everything about this answer is wrong. AC chargers just don’t work the way you’re suggesting. The post clearly states what AC supply equipment it needs to work with. The current splitting needs to be done digitally.
So DC batteries now take AC?
…and even still, I know how to determine whether something is a supply or a load of AC current so an analog function will still work for AC supplies.
The OBC is in charge of managing the current to the vehicle batteries: https://philipmcgaw.com/charging-an-ev/
The CP signal needs to be read to confirm the maximum current available from the upstream EVSE, and the cable needs to be read to confirm the maximum available current carrying capacity of the upstream cable.
Then based on the demand by each of the EVs plugged into the unit to be designed, and the current carrying capacity of the cables between this unit and the EVs, the downstream CP needs to be generated to advertise the available current to each vehicle.
Also, the unit will need to confirm that a vehicle is attached, and ready to receive charge before closing contactors to the EV outlets. so we will need to read back vehicle status, and the maximum current carrying capacity of downstream cables.
So after going through all of these specs, I see no explanation of how the charger knows the charge level of the car.
How do you plan on detecting that to determine which is discharged the most?
the amount of current the OBC will change depending on the SOC of the battery in the vehicle.
By monitoring the current each vehicle is pulling (against the amount available) the CP signal can be split to allow each vehicle to pull a different proportion of the available upstream current.
The CP signal tells the vehicle what the maximum available current is from the charger. The only vehicle CP feedback I see are the 5 base status resistances.
I don’t see any spec that allows the vehicle to tell the charger ‘how much to send’.
When a vehicle battery SOC (State of Charge) is nearing 100 % the OBC reduces the amount of current being pulled, Reading back the drawn current using a Current Transformer, the box can reduce the advertised capacity on that port to just above the demanded current. Thus increase the advertised current available on the other port.
But you only have one port so the total output will be reduced.
It will have two output ports…?
Your supplying charger only has one output.
How are you going to prevent a hot-plug situation?
How are you going to prevent an over current situation?
It works as follows:
1) During a hot-plug-in, the adapter will reduce the other car’s allowed current to 50%, then allow the newly plugged car 50%. If one or the other draws less than its 50% entitlement, it will decrease its entitlement and give it to the other vehicle.
2) If an overcurrent is detected (ie one vehicle draws more than its allowance), you just turn off that vehicle’s supply.
With regard to “you’ll be plugging in a hot connector on the second”, it would probably be bad practice for this device not to have its own relay per outlet, but it would absolutely work. In that case you’d be putting more trust in the vehicles not to misbehave, but ultimately, the upstream EVSE would shut down if the overall limit was exceeded.
With regard to “How are you going to ‘split the CP signal’?” – that’s literally the point of this device, and that’s why electronics are required. You detect the allowed current from the upstream EVSE, and generate 2 new CP signals downstream.
Each output will have its own contractor.
So, now that I know more of your parameters, I know exactly how to do this and it’s complex.
You DO NOT want to hot plug, that’s how you burn-up contacts. You do NOT want to exceed the charger’s power limits and rely on the charger’s safety circuits.
You’re going to have to emulate a car (to the charger) and emulate two chargers to the car.
You’re going to have to program it to detect car connections/disconnections.
When the first connection is made, the circuit will emulate the car and the charger and everything works normally. AND it needs to read the current pull.
When the second connection is made, it needs to:
1) Drop the 1st connection.
2) Establish the 2nd connection and read the current.
3) The decide:
a) Which connection has the higher current draw.
b) Does the total of both connections exceed the limits of the charger.
IF both connections exceed the limit of the charger, it needs to just make a single connection back to the higher draw car to get it charged up. AND it needs to drop this connection and ‘renegotiate’ after a certain interval (to see if both cars could be brought ‘online’).
IF both connections do not exceed the current draw, it needs to make both connections (emulating the charger’s control signals to each car).
As I said in the original post, there is no need to adjust current from one to another because each car will pull the amount that it needs and as long as that amount is less than the limits of the charger, you don’t care.