Granny charger using a smart plug with usage monitor

[jayD]

Hi all,

Looking for some advice, I have a 23 plate MG ZS EV long range. We have two electric cars in our household and only one 7kW wall charger. The other car is a Citroen and doesn’t have a programmable charging time option.

As a result I often use the 3pin granny charger from the garage which is on its own ring. We live in a new build house meaning the wiring and circuit board is only 3 years old.

I want to be able to firstly calculate how much each charge is costing using the granny charger. I would also like to be able to programme it to come on and off to utilise our off peak tarriff.

Would a smart be suitable in these circumstances the Tapo plugs all say not suitable for EV charging but have the functionality I require.

Any advice/thoughts gratefully received,

Many thanks

Ps sorry if this has been posted before…

 

[Coulomb]

Would a smart be suitable in these circumstances the Tapo plugs all say not suitable for EV charging but have the functionality I require.

I assume you mean a smart plug, i.e. one that has a timer and/or can be switched on and off remotely.

I strongly don't recommend anything that interrupts an EV charge by switching off the AC power under load. There is a substantial inductor in the car's on-board charger, and interrupting current to a large inductor causes huge voltage spikes. These spikes are hard on the switch or relay contacts trying to interrupt the current flow; the resulting arc will pit the contacts, eventually leading to high resistance and a fire risk. Worse, the voltage spike causes problems in the very expensive on-board charger. Devices like Metal Oxide Varistors (MOVs) degrade slightly with each transient that they have to absorb, and eventually their protection wears off. Then the transients are free to affect more expensive components in the on-board charger, which will then fail, and AC charging will no longer work. Component level repair of on-board chargers, or any other vehicle electronic module, is almost non-existent these days.

Edit: I forgot to say that if the charge is terminated electronically, then there is no issue. It's the abrupt mechanical disconnection that is the problem. Electronic termination happens if the car terminates the charge (by schedule, app, front screen, door unlock etc), or if the control pilot signal is disconnected. The better EVSEs (AC "chargers") will do this.

 

[GaryMG4]

I assume you mean a smart plug, i.e. one that has a timer and/or can be switched on and off remotely.

I strongly don't recommend anything that interrupts an EV charge by switching off the AC power under load.

Isn't that what happens with a Zappi when using solar excess and the excess drops below 1.4kW, or perhaps it does a soft disconnect. Maybe the answer is to set the timer to power up the granny 5 minutes before the off peak starts and use the car/app schedule to start/stop the charge the charge and let the timer run on 5 minutes after the charge has stopped that way the peak current only passes when the contacts are closed.
The other thing I've noticed is that the car doesn't instantly take 2.2kW when the power is switched on it seems to ramp up over 10 or so seconds.

 

[Gadget Geek]

Isn't that what happens with a Zappi when using solar excess and the excess drops below 1.4kW, or perhaps it does a soft disconnect. Maybe the answer is to set the timer to power up the granny 5 minutes before the off peak starts and use the car/app schedule to start/stop the charge the charge and let the timer run on 5 minutes after the charge has stopped that way the peak current only passes when the contacts are closed.
The other thing I've noticed is that the car doesn't instantly take 2.2kW when the power is switched on it seems to ramp up over 10 or so seconds.

If you look at diagrams of charge plugs, one of them will be marked as "Pilot". The pilot signal is a square wave voltage which tells the car's control circuit that the plug is safely connected. Smart chargers and the car itself, use the pilot signal to ramp up and ramp down the onboard inverter safely.

 

[Coulomb]

Isn't that what happens with a Zappi when using solar excess and the excess drops below 1.4kW, or perhaps it does a soft disconnect?

I forgot to say that if the charge is terminated electronically, then there is no issue. It's the abrupt mechanical disconnection that is the problem. Electronic termination happens if the car terminates the charge (by schedule, app, front screen, door unlock etc), or if the control pilot signal is disconnected. The better EVSEs (AC "chargers") will do this.

I've edited my post above to include the above paragraph. I may point to it later; this issue seems to come up fairly regularly. For most loads, interrupting the power mechanically is fine, and it's convenient, so it's one of the first things people think of.

 

[Brian0]

Surely the "physical disconnect" is what happens if you switch off at the mains socket anyway, so why would a remote switch be any different?

 

[Gadget Geek]

Surely the "physical disconnect" is what happens if you switch off at the mains socket anyway, so why would a remote switch be any different?

Yes, so don't switch off at the mains! Use the app or do it in the car to stop the pilot signal and end the charge.

 

[Brian0]

Yes, so don't switch off at the mains! Use the app or do it in the car to stop the pilot signal and end the charge.

Thanks for that. I only used the granny charger for a couple of weeks until we had the Zappi installed, and even then just left it charging until the car shut it off.

 

[Everest]

I would also like to be able to programme it to come on and off to utilise our off peak tarriff.

Does the MG4 not allow time scheduled charging when charging via a granny lead

 

[GaryMG4]

Does the MG4 not allow time scheduled charging when charging via a granny lead

Yes it does via the app or from within the car.

 

[Brian0]

To get back to the original question(s)...

There are plenty of Power Meters which will tell you how much you have used, just Google Power Meter - anything from £8 to fully certified at £100+. Lots of them will allow you to put in a price per kWh and more and more have an App, so you don't even have to leave the sofa!

The car itself will schedule charging - it is a built in feature. Set the start and stop times and read what it says on the power meter.

Job done?

 

[Bam Bam]

To get back to the original question(s)...

There are plenty of Power Meters which will tell you how much you have used, just Google Power Meter - anything from £8 to fully certified at £100+. Lots of them will allow you to put in a price per KW and more and more have an App, so you don't even have to leave the sofa!

The car itself will schedule charging - it is a built in feature. Set the start and stop times and read what it says on the power meter.

Job done?

Wasn't the issue that they also have a Puegot that doesn't have a scheduler, unlike the MG.

I have no idea about all this. Our Givenergy system gives us all the info in a very user friendly way.

I know that Octopus does a unit that records usage so I don't know if that would help build a picture, though if looking at total usage you'd have to deduct other uses.

 

[Everest]

There are plenty of Power Meters which will tell you how much you have used, just Google Power Meter - anything from £8 to fully certified at £100+. Lots of them will allow you to put in a price per KW and more and more have an App, so you don't even have to leave the sofa!

I guess it depends what accuracy the OP wants to measure - if the MG draws 10A on the granny charger, then the power drawn can be estimated to within about +/-5% (i.e. about the amount the grid voltage will fluctuate on average).

But, if a more accurate measurement is needed, I suspect that the cheaper (if any) power meters will not be equipped with a BS1363-2 EV compliant socket. It may be preferable and safer to add an external CT clamp-based power meter with clamp on the live wire for accurate power measurement.

 

[Everest]

But, if a more accurate measurement is needed, I suspect that the cheaper (if any) power meters will not be equipped with a BS1363-2 EV compliant socket. It may be preferable and safer to add an external CT clamp-based power meter with clamp on the live wire for accurate power measurement.

For anyone reading this thread later, @EvTek23 posted a link to a pre-made compliant socket and Eastron power meter here...
3 pin charger with Wi-Fi and usage calculator - recommendations?

 

[GaryMG4]

I've just checked some stats and in the last 12 months I've put 2122 kWh through my Tapo smart socket. For overnight charging I set the Taco to power up the charger 5 minutes before the car scheduler starts the charge and power it down 5 minutes after the car has ended the charge. Neither the Tapo nor wall socket are showing any sign of overheating. I know Taco state
"EV charging is not supported since it typically has a higher power and its charging current required exceeds the rated current of the plug" but in the UK the MG EVSE is limited to 2.3 kW well below the rated 3kW of the smart plug.

 

[jayD]

Thanks all,

I’ve seen one of these, do you think it’s a viable option???

https://www.screwfix.com/p/knightsbridge-13a-2-gang-sp-switched-smart-socket-white/824ty

Thanks all in advance

 

[Everest]

do you think it’s a viable option???

NO... the spec sort of gives it away + no dual gang socket is EV compliant!

 

[jayD]

NO... the spec sort of gives is away + no dual gang socket is EV compliant!

View attachment 34671

Doh! Didn’t see that thank you