Charge only while AC is not running

[Jeremiah Smith]

I recently moved to a new house and the electrical panel is on the opposite side of the house from the garage. It would be very difficult to run a new circuit to install my Openevse charger. However, the Air conditioning compressor is on the back wall of the garage, which is an ideal spot for the charger. It is a 50A circuit so it would be substantial enough to charge my Fiat 500e at 6.6kw but not while the AC is running. I could just make sure the AC is off while I'm charging the car but I would like to figure out a more sophisticated way to do it. I have thought of two possible ways to make this happen but I'm hoping that someone might have an even better suggestion. My first thought was to put a 24v relay in line on the pilot pin. When the air conditioning thermostat sends 24v to the compressor it would energize the relay and disconnect the pilot pin, dropping the pilot pin voltage to zero and stopping the charge. My other thought was to send Rapi commands to the charger via wifi. The advantage to this would be that  if there is enough overhead on the circuit I could issue a command to reduce the charging rate rather than stop it all together.  I haven't figured out how exactly to do this but I was thinking an electronic imp could do the job. 

[Adam Sharp]

Switching a supply between 2 different devices, even at low currents is very complex.  Your first issue is that you are dealing with inductive loads, which present inrush currents that have to be dealt with and predicted.  You must also keep the controls of your circuits powered up at all times to maintain their correct operation. This would require significant internal rewiring work, which i presume you are not qualified to action (?).

You will need to modify the equipment at hand to prevent both drawing inrush currents together.  You will also need to ensure that the switching process is break before make and these devices are very expensive and will need to be mechanical and have a fail safe condition of both off, should the control circuit go awry for any reason.  The 'switch' should probably occur at the zero crossing point and within 1 mains cycle, difficult for mechanical switching.

It is probably far, far easier to get a qualified electrician to run an SWA cable around the outside of your building from a new circuit, after performing diversity calculations on the total supply.  All complex issues go away and everything is safe.  Nothing is modified and invalidating any warranties or maintenance contracts etc.  Why complicate things? I can see no aircon, an uncharged car and the good probability of an electrical fire here with these sorts of energies being mishandled.  There are so many fail safes and regulations that you will have not thought of.  And that you are just asking for failure and why would an insurance company want to pay up for these unapproved modifications? - they just love any excuse not to pay out.

Sorry, but you have purchased an additional high current electrical device that you have not completed any of the ground work for and you are trying to 'bodge' a solution.  Buying an electric car requires the cost of a suitable charge point at your property.  You will need to pay for the correct solution. End of.

[Ben Franske]

I don't think it's quite as simple as the original poster thought but it's also not as complex as Adam has written. I do agree that in most circumstances it's probably easier to run a new circuit to the charging location than to mess around with this but it would be a bit easier than Adam is indicating. All of the switching at zero crossing within one mains cycle and worrying about the inrush load business might be true if you were trying to switch instantaneously between them, but that's not really a requirement for your situation. I'm assuming you are talking about a North American style whole-house air conditioner here as that seems most likely.

Keep in mind:

• There are already low-voltage control signals for both the air conditioner and the EVSE
  
• You can check to see if there is a call for AC by monitoring the 24V control line
• You can allow the 24V through or block it to the AC using a relay (to allow a time delay for the car to stop charging)
• You can turn the EVSE on and off by sending serial RAPI commands
• You have circuit breakers exactly for preventing overcurrent conditions
  

This is all you need. Should be fairly straightforward to do using an Arduino or similar, a relay, and a few misc parts.

You would want to actually interrupt the 24v signal to the AC while the car is charging and then monitor that line to determine if a call for air is initiated from the thermostat. This allows you to disable the EVSE, wait a second, and then start the AC so there is no chance of both running at the same time and is a way to avoiding all the inrush current issues and fast switching Adam mentioned. The simple relay method you suggested may not drop the EVSE load fast enough before the AC starts, that all depends on your particular compressor and EV, etc. Anyway, if you so determined a few seconds after the AC starts you could then restart the car charging at a lower current, etc. There should be no internal rewiring of the EVSE or the AC required.

You would definitely need some electronics experience and design skills to do this but it's also very possible. If I was stuck and couldn't get a line from the panel to a charging location I might look at doing something like this but I also doubt a little bit that it would be as hard to run a circuit as you think. I replaced my main electrical panel and needed to run a new grounding/bonding wire clear from one side of the house to another, including through some finished spaces, and was able to do it. So in the end I do agree with Adam's conclusion that it's probably faster and easier to run a new circuit. I just wanted to point out your solution is a bit more feasible than he had indicated.

-Ben

[Jeremiah Smith]

Wow. Take it easy...no need to be so aggressive. I had an a proper circuit at my old place and it's just not possible to install at the new place. I would have to run under a concrete driveway.  It not going to happen. I have a pretty good understanding of electricity and you are blowing these risks way out or proportion. There is a circuit breaker on the line that will pop if the loads get to high. The Open EVSE charger has a GFCI interrupt built into it. I don't think there is an electrical fire issue or any real safety risk if this is done properly.   Have a look at this blog post...this is essentially the same thing as I am trying to do...but just sharing the power with an air conditioner rather than other electric car chargers. 

https://store.openevse.com/blogs/news/112749572-charging-done-right-54-80a-adaptive-charging-stations-at-caltech

[Jeremiah Smith]

Ben thanks for the insight. Using an Arduino to add time delay to the switching between devices is a good idea. I also like not having to run a wire into the evse to interrupt the pilot. I could just put the Adruino right next to the thermostat. My hesitation with using the RAPI command method is that  if I have a WIFI network failure it wouldn't shut the charging down. However, maybe after sending the command via RAPI to shut down charging I could send another command to check the charge status...if no response is received...IE network failure...then no AC. 

I think for now I will program the Car to charge between 4-8 AM and program the thermostat to never run during that time period
. I think this will be fine while I implement the more high tech active solution. 

[Adam Sharp]

Sorry, not intending to be aggressive, but it did come out that way.

Multiplexing charging stations is not quite the same as sharing a supply, good short article though.  Multiplexing is a very common method of maximising diversity to control costs all round.  Working smarter is often a very good solution.  However, working simpler also has a huge amount going for it.

The concept of sharing the juice between the two is possible, but you are neither working smarter or nor simpler.  My statement of electrical fires is valid and the circuit protection does not always work and often behaves in unexpected ways.  Some points to address are:
- Is the wiring good (originally and still)?
- What would happen if the load exceeds the cable, but not enough to trip the protection?
- Is there an intelligent soft start in the AC?  If it does not spot the compressor motor starting within a few cycles, it will error.  And some hate 'ringing' on the supply after switching the supply.  Were I am, the switching of the grid to adjust voltages here, trip soft starters in heat pumps all the time.
- What fail safes are there you must over ride and implement to account for all failures.  The circuit protection device is really only a last resort and in UK regulations is only allowed to protect the supply cable and not the current using device.  If the device needs such protection, it has to provide it itself, or specify additional protection during installation by the installer.
- Do you understand what happens in cables, joints and protection devices when the current and voltage are out of phase with each other.  And what happens if a system injects DC into this feed or back EMF?
- If you have a fire and the insurance company finds modified equipment is the cause, are you still covered? Or if you move house, you are probably still liable for what you have left behind.
- What is the worst case scenario? Both on, the protection fails, your intelligence has crashed, communications have failed and a high impedance on the earth loop makes everything metallic live to the touch, AC supply gets into the LV side, etc.  Have you dealt with each potential problem, adhered to all the regulations and minimised the risks enough to satisfy yourself that you could stand up in a court and argue your 'modifications' were safe.  Really could you?  The worst case may probably never happen.  But if that was the case, then no one would ever be hit by lightning.

And so on.

Putting in a dedicated supply just seems so much easier and minimises the risk away from you to the insurance of the professional installer.  And it will work.  What I'm trying to say is that you are dealing with high grade energies and semi complex systems.  Once you start, can you back out comfortably?  What if you move away, or can no longer maintain your solution, could it be picked up by the average electrician or HVAC engineer?  Everything breaks and everything changes.

[Jean-Michel Olivier]

I think the safest way to get what you want is the second way you imagine.
Based on the OpenEnergyMonitor project, you could measure the total ampacity of the circuit and send RAPI commands, through a serial connexion, to the EVSE board to increase/decrease the ampacity allowed to the car in order to keep the total ampacity below 50A.
As OpenEVSE, OpenEnergyMonitor project also uses Arduino resources.
All you need is Emonlib library, and a very simple board with a coil to measure the current... and time to spend ;-)

[gnuarm.del...@gmail.com]

On Monday, February 26, 2018 at 7:09:53 PM UTC-5, Jeremiah Smith wrote:

I recently moved to a new house and the electrical panel is on the opposite side of the house from the garage. It would be very difficult to run a new circuit to install my Openevse charger. However, the Air conditioning compressor is on the back wall of the garage, which is an ideal spot for the charger. It is a 50A circuit so it would be substantial enough to charge my Fiat 500e at 6.6kw but not while the AC is running. I could just make sure the AC is off while I'm charging the car but I would like to figure out a more sophisticated way to do it. I have thought of two possible ways to make this happen but I'm hoping that someone might have an even better suggestion. My first thought was to put a 24v relay in line on the pilot pin. When the air conditioning thermostat sends 24v to the compressor it would energize the relay and disconnect the pilot pin, dropping the pilot pin voltage to zero and stopping the charge. My other thought was to send Rapi commands to the charger via wifi. The advantage to this would be that  if there is enough overhead on the circuit I could issue a command to reduce the charging rate rather than stop it all together.  I haven't figured out how exactly to do this but I was thinking an electronic imp could do the job. 

Hi, I guess you seem pretty adamant that you can't run any additional power.  I was going to suggest that you might be able to tap into another 120 volt line that is on the other phase from your garage (which I assume has a 120 volt connection).  But when you mentioned routing cable under a driveway, I'm figuring you have a detached garage.  Just a thought.  This would at least get you 3.6 kW without too much hassle if it is possible at all.  

  Rick C.

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