Is it safe to switch off charge current suddenly?

[donald]

I have found a timer with a suitable nominal rated current that I want to rig up to my charge station, so that I can take control of when it is powered up.

I'm not sure if it's relay is a SSR or a mechanical type. Let's assume it is mechanical, and so it can switch off at a cycle peak, for the sake of discussion.

One thought I have had is whether it is safe to disconnect the supply current to the charge station suddenly, whilst the charger is in operation. The opinion expressed before (when I asked about power factor) is that an EV on board charger is not reactive, and I presume the charge station isn't and simply acts as a glorified relay itself. So, I suppose it would be ok, then?

This is a different scenario to pulling a plug out, which may have features that will, I presume, disconnect the power before the powered terminals disconnect and arc.

I can only presume it is fine, because when I turn the key on the charge station there is a mechanical clonk from what I presume is a relay doing exactly what I would suppose a timer relay would do. Or does it send a control signal to the charger to stop its charging first, before disconnecting the supply current?

Am I mistaken? Is there no issue to cutting the charge current suddenly, or are there problems I have not yet begun to appreciate?

 

[TomT]

It may - repeat MAY - cause a charging fault on the vehicle... If it were to happen though, it is not permanent and will clear the next time the vehicle is power cycled... More problematic is that most such timers are not really designed to handle that level of power and I suspect that it might quickly destroy itself...

 

[donald]

That's why it doesn't seem trivial to find one. I have found one rated for 30A, but that'd be for a resistive load so I'm just wondering if it will be OK to switch off that much current to an on-board charger. I presume there will be no problem for the timer to switch the current off, as [I presume] any reactive response in the circuit will be on the charge station/on-board charger side of the open circuit.

 

[davewill]

Personally, I wouldn't do it. The EVSEs that have timers handle it by shutting down the pilot signal, which causes the on-board charger (OBC) to stop drawing current, THEN the contactors in the car and EVSE are opened. When the LEAF's timer shuts down, the OBC stops drawing current, and THEN the contactors are opened. Suddenly interrupting the current is a fault.

We've had enough OBCs die for dumb reasons that I wouldn't take the chance.

What is it you're trying to accomplish? I bet we can come up with a way to do it using just the LEAF's timer.

 

[donald]

I have a Renault Fluence that doesn't have a charge timer.

.... that's real dumb, eh!?

So I'm looking at whether I might want to try a Leaf next, after the lease expires. But in the meantime still looking for a way to control timings.

 

[smkettner]

Let's get the whole story in case there is another solution.

How long are you charging for? Why end the charge early?

Otherwise I don't think it is a good idea on a regular basis. Does the owner manual have anything to say?

 

[planet4ever]

I have a Renault Fluence that doesn't have a charge timer.

In that case your best bet is to get an EVSE with a built-in timer. That will shut the charging down gracefully, as davewill said.

Ray

 

[davewill]

I have a Renault Fluence that doesn't have a charge timer.

In that case your best bet is to get an EVSE with a built-in timer. That will shut the charging down gracefully, ...

If you are really handy with electronics, you could hack a timer inside your existing EVSE that would properly stop the pilot signal. It's not for the faint of heart. I'm confident it could work, but I'm not qualified to advise you on the details (software is my specialty not hardware). Best to buy an EVSE that already has this feature as Ray suggests.

 

[johnrhansen]

I don't see why not as long as the contactors you use to cut the current are rated for it. If you use a double pole contactor, it will be easier on the contacts because you have the breaking volltage split between the two of them. I't's not exactly a resistive load, so I think it would be prudent to use a 50 amp contactor. As for it causing a fault in the charger, I'd say it had better not. Power failures happen all the time, and I'd sure hope they's designed around it. I have a AV charging station and a Bosch power Max, and both of them mentioned power failures in the manual if I remember right.

But instead of going to all the trouble of building a timer, I think all I'm going to do is set the charge timer to turn off 90 minutes AFTER I plan to leave for work in the morning. Set the car to charge to 100 percent, and with it ending 90 minutes early, It should have about 85 - 90 percent by the time I leave. I'll just manually stop the charge and pull the cord.

 

[RegGuheert]

I asked a similar question a while back. The bottom line is that it is not recommended. Please have a look at the thread for details.

 

[johnrhansen]

Interesting. I still wonder though why a motor contactor that is designed to brake highly inductive loads like a motor won't safely work breaking current from what I see basically as an oversized switching power supply. How big of an inductor is it inside the car charger? It can't be bigger than one in a small sized electrical motor. This inductance is what generates those high voltage spikes. If you are worried about voltage spikes you can always stick a low pass filter or a surge protector in between the contactor and the evse. Besides, the current drops to zero 120 times a second anyway. The spikes can't be that long in duration. It would be nice if some expert weighs in on this. I'd like to know. It just seems easier switching the power source than it would be to tear the evse cord apart to access that button circuitry.

The way I read it, you were looking to build some kind of automatic timer that breaks the current without you there? If not and you just want to manually stop the charge, I think the best way is to push the button down and take the plug out of the car first.

 

[donald]

Let's get the whole story in case there is another solution.

How long are you charging for? Why end the charge early?

reasons for wanting to control timing include;
-limiting the charge
-stopping the vehicle/charger from drawing power after the battery has reached full charge
-stopping the charger station so it doesn't waste standby power,
-controlling the charging such that in cold weather it finishes its charging (or gets to the desired level) immediately before the vehicle is required, thereby warming the battery pack up (which warms the cabin up).

My usual MO is to plug in after arriving home, and manually unplug after a few hours before getting ready for bed. This is neither ideal for what I really want to do, nor is it very good to avoid overloading the network at peak times. (It's also not ideal for CO2 emissions either, because night time juice is usually lower CO2/kWh.)

 

[donald]

Besides, the current drops to zero 120 times a second anyway.

That's why I included in the question a discussion on whether SSRs would be OK, even if mechanical relays aren't. SSR's should switch off at zero-turning. But SSRs are lossy, particularly at high currents, as they are back-to-back triacs (as far as I am aware) and so there is a little bit of the voltage cycle that they block.

 

[RegGuheert]

That's why I included in the question a discussion on whether SSRs would be OK, even if mechanical relays aren't. SSR's should switch off at zero-turning.

Good point!

 

[TimLee]

The button most definitely does drop the power to zero. There is no indication that this is happening, no noise, no lights, but if you monitor the current (have someone watch your electric meter) you will see that it instantly does drop the power to zero. Only upon removal of the handle does it change any status indicated on the car/EVSE.

Thanks for telling me that! I will certainly test this out further!

Well, just as you said, the charging current drops to zero when the EVSE handle button is pressed. But it really fooled me! There is ABSOLUTELY no clue from the LEAF or the EVSE that any change has occurred!

This is good to know. Frankly, I had always wondered if the charging current was interrupted and therefore I was concerned that arcing was happening between our $1000 EVSE and our $35,000 EV. That incorrect belief is one reason I thought throwing the breaker would be O.K. In fact, I thought it might be better because I would rather replace the breaker than the EVSE or the car. Thanks again for setting me straight on this!

So pushing the button on the EVSE handle stops current flow without interrupting the timer or changing any of the indications on the EVSE that charging is on going.
If you release the button, does current flow resume?

 

[davewill]

So pushing the button on the EVSE handle stops current flow without interrupting the timer or changing any of the indications on the EVSE that charging is on going.
If you release the button, does current flow resume?

Yes. It takes several seconds, but it does start back up, all without any outward sign.

 

[smkettner]

Let's get the whole story in case there is another solution.

How long are you charging for? Why end the charge early?

reasons for wanting to control timing include;
-limiting the charge
-stopping the vehicle/charger from drawing power after the battery has reached full charge
-stopping the charger station so it doesn't waste standby power,
-controlling the charging such that in cold weather it finishes its charging (or gets to the desired level) immediately before the vehicle is required, thereby warming the battery pack up (which warms the cabin up).

My usual MO is to plug in after arriving home, and manually unplug after a few hours before getting ready for bed. This is neither ideal for what I really want to do, nor is it very good to avoid overloading the network at peak times. (It's also not ideal for CO2 emissions either, because night time juice is usually lower CO2/kWh.)

Limiting charge might be accomplished with a start timer.
Cutting power after reaching full charge is probably not a big deal.
Standby should be minimal but if current is low it should not be an issue to cut off.
Ending at full charge just before departure would be done more with a start timer so no worries there either.

Sounds more like you will have a timer to delay the start time vs cutting power during the bulk charging period. No worries. With a little practice you should get pretty good with setting the start time.

 

[donald]

I agree that, in general, 95% of the time, my objectives can be achieved by timing the start, rather than the stop, and therefore the circumstances of shutting down the charging are infrequent. However, it may still happen and in that case I was trying to figure if a mechanical relay (0 effective voltage drop, but risking arc-cutoff or inductive surges) could be used.

My wall charge station consumes 16W on standby. That's ~600 EV miles wasted, annually, just on keeping the thing warm!!!

 

[RegGuheert]

My wall charge station consumes 16W on standby. That's ~600 EV miles wasted, annually, just on keeping the thing warm!!!

That's one reason I went with EVSEupgrade for our EVSE. It uses almost no power on standby.

 

[johnrhansen]

could you use both a mechanical relay and a triac in parallel to have the best of both? Make the triac first, then make the mechanical then break the mechanical then break the triac. This way the mechanical relay could be much smaller and you wont have to worry about losses or voltage spikes.