Stop and start granny charging via power lead

[Coulomb]

When the Shelly relay switches on the granny charger doesn't respond for 6 seconds, it then draws 7.3 watts for exactly 8 seconds and then draws the full 2.16 kW.
So I don't think the relay contacts are getting hammered.

That's at switch on. There is no problem there. The problem us when you want to stop the charge and it's drawing some 10A. If you happen to time it such that the contacts are opening right around a zero crossing (50 Hz AC crosses zero 100 times per second), then that's not too bad. But you could equally be opening the circuit when the current is near 14A. The peak of a 10A sine wave is ±(sqrt 2)*10 ≈ 14.14A.

If you always charge to 100% so that the car turns off the charge electronically, then there is no problem, provided that you allow enough time to ensure that the battery does get to 100%.

There will always be cases where the current gets interrupted, for example a blackout, the breaker trips, and so on. The on-board charger can handle a certain number of these, I don't have a feel for how many. It might be a hundred average events, it might be ten thousand. But if you sre regularly interrupting a charge on a timer, then you are reducing the life of the on-board charger. It will cost at least £1000, possibly double that or more, to have it replaced. Maybe you can get that done under warranty, maybe not. I think it's crazy to risk this, when there are usually alternatives.

If you are handy with power electronics, it might be a moderately inexpensive repair (the movistor costs only a few pounds, but when it becomes ineffective, something far more expensive dies). Few people do this sort of component level repair any more. And the information needed to aid such a repair, like a schematic diagram, is often hard to obtain.

It's a worry that so few people are aware of this problem. There are probably thousands of timers on EVSEs right now, and because the problem doesn't show up immediately, it's natural to assume that all is fine.

 

[Coulomb]

do you know if these are the only possible fixed currents, or is it completely analogue depending on mark-space ratio?

It's two straight lines. Above about 48A (I forget the transition point) there is s steeper slope, so that Americans can get to about 19kW with a single phase.

There are EVSEs that allow 1A steps, so there are certainly more steps than in that table. I believe that the steps are essentially smooth, so I'm pretty sure that you could set 8.25A, say. One car might draw 8.4A, another 8.1A at that setting.

Precision isn't guaranteed, and the system was designed for protecting wires, which are affected by a host if other factors, and hopefully have decent safety margins.

 

[Coulomb]

However I think I may be able to solve this problem.
The Home Assistant system enables me to open and close the car doors.
When the car is charging opening the doors stops the charge - I can see that the current going into the granny charger goes down to 1.5 watts for 40 seconds and then returns to the full 2kW (I think this is the car re-locking itself).
So if I open the doors, and then make the Shelly relay turn off power to the granny charger then hopefully all should be well.

Yes, that's a reasonable solution. Great that home assistant has this capability.

 

[Coulomb]

I thought you had to lock the car to lock the charger in place? Then once you’ve started charging you can unlock the car again and wait inside if you want.

My understanding is that locking the doors with the fob or door buttons always locks the charge cable if a plug is detected. Unlocking the doors always pauses the charge and unlocks the cable, but only for some 10-30 seconds. Then the cable automatically re-locks with a ziiiip sound, and charging resumes thereafter. During that pause, the screens typically don't show the charge as stopped, so people often assume that it continued. But if you check the battery current and/or the wall current, you'll see the pause. This seems to be MG simplifying things for the user, or simply poor software; personally I'd prefer to know what's actually happening.

There are conditions when the charge cable will lock without the key fob or door buttons being used, so if you use the right charging sequence, it won't be necessary to lock the car as a separate step.

 

[Everest]

@Coulomb - thanks that's all a lot clearer now. I might as well just get an OpenEVSE when I get the car then, even if it is temporarily connected to a 13A or 16A circuit.

 

[Jto]

Will the MG4 automatically start charging when power is applied via a 'granny' lead or does something have to be set within the car to initiate charging?

If the latter, is it possible to stop and start charging simply by switching the power to the granny lead off and back on, without having to do anything in the car?

Reason for the question is, I am thinking when I get the car and before I install an EVSE, can I just control charging by automatically switching the power socket? I will want to do that to limit power drawn by the car from my inverters depending on other house loads.

I use a BG smart water proof double scocket for a number of years which has an app as it will connet to your WiFi. If you wish to have it come on and off you can program the scocket to do this as many times as you wish. Now I have the ZS I just have this on all the time and set the car to come on and off for charging.

 

[Moonraker244]

Will the MG4 automatically start charging when power is applied via a 'granny' lead or does something have to be set within the car to initiate charging?

If the latter, is it possible to stop and start charging simply by switching the power to the granny lead off and back on, without having to do anything in the car?

Reason for the question is, I am thinking when I get the car and before I install an EVSE, can I just control charging by automatically switching the power socket? I will want to do that to limit power drawn by the car from my inverters depending on other house loads.

I plug in a granny charger and set the battery to charge to 80% on the car, as well as setting the time from 00.15 to 7.15am - no problem.

 

[gwatuk]

... I think I may be able to solve this problem.
The Home Assistant system enables me to open and close the car doors.
When the car is charging opening the doors stops the charge - I can see that the current going into the granny charger goes down to 1.5 watts for 40 seconds and then returns to the full 2kW (I think this is the car re-locking itself).
So if I open the doors, and then make the Shelly relay turn off power to the granny charger then hopefully all should be well.
(Need to do some more research.)

Yes, that's a reasonable solution. Great that home assistant has this capability.

For the benefit of anyone else finding this...
I've just discovered that Home Assistant is able to send a command to the car to start or stop charging. (And quite a few other commands.)
It's extremely simple once you know what you're doing

 

[campbell taylor]

So I am still waiting for my CCU to be changed, it is booked in at the dealers on 27th February. So still having to use Fast charging, can’t use charging at home.

Two questions.
Remember I am no kind of technician and did not do any science at school so have no education of electricity. The battery will only take DC charging so the CCU converts AC - when I am charging at home - into DC, is that right?

So secondly I have been reading about switiching the charger at home on and off. I haven’t been using the app, my car will not accept my instructions from the app. So I have just been gaily plugging the cable in the wall, then plugging the car in to charge, when nearly full, just switching the plug at the wall off and then unplugging the car. Have I been messing the contactors inside and will that have damaged the CCU? When my car is fixed, should I not be interrupting the charger?

Many thanks for all your help.

 

[Everest]

The battery will only take DC charging so the CCU converts AC - when I am charging at home - into DC, is that right?

yep.

So I am still waiting for my CCU to be changed,

Out of interest, how long has it taken since the problem occurred to get fixed?

Have I been messing the contactors inside and will that have damaged the CCU?

I doubt it, but...

When my car is fixed, should I not be interrupting the charger?

... from what @Coulomb has said (further up this thread), it's best to stop charging via the car, internally I guess, if you don't use the app.

 

[campbell taylor]

yep.


Out of interest, how long has it taken since the problem occurred to get fixed?


I doubt it, but...


... from what @Coulomb has said (further up this thread), it's best to stop charging via the car, internally I guess, if you don't use the app.

I first had the problem in October. It took 3 visits to the garage to get them to agree that it was the CCU, by that time Christmas was upon us, so I was told the new part would be in, in January and that they would let me know. I had to ring and it is booked in, on February 27th. Mainly because I need a courtesy car as mine will be in all day, and those cars just don’t seem to be a available all the time.

Thanks for your comments.

 

[Everest]

OMG - so did you have to fast charge away from home for all that time That's a pretty slow response for something that is a 'known' problem.

 

[Martinonline]

@campbell taylor, you should use the battery level settings in the car if you can't use the app to end the charge. By terminating the charging early the battery does not balance/equalise and you are not making optimum use of the charging/battery.

 

[Coulomb]

@campbell taylor, you should use the battery level settings in the car if you can't use the app to end the charge.

Or you can just use the key fob to unlock the car. This will electronically and safely pause the charge, and unlock the charge plug. You probably want to unplug at that point anyway, so just get into the habit of unlocking before you switch off the "charger" at the wall.

As per the post above, allow the charge to finish and leave it plugged in once every month or two.