Low Speed charging rate variable

[Coulomb]

On a related note, how is the current limited? The only way I can think that the charger can do it on its own is to drop the voltage a lot, but that can't really be it. Does the charger tell the car to back off?

Yes, the "charger" tells the car to reduce its current draw. The on-board charger responds by reducing its output voltage very slightly, so that less power flows into the battery.

For AC charging, the maximum charge current is communicated to the vehicle over the control pilot. This is one of the thin pins in the type 2 (or type 1 J1772) connector. The EVSE (AC "charger" outputs a 1kHz ±12V square wave over this wire, and the duty cycle (percentage of the time it's high compared to the whole cycle) is adjusted from some 5% to I think over 90%, representing 6A to 80A for the current limit.

The original intent of this signal was I think to prevent overloading of cables, but it's also useful for current sharing, and even for roughly matching EV charge power to excess solar power. The current sharing is usually at one cabinet that has two plugs, but it can also be implemented on a whole site basis with a bit more complexity.

DC rapid chargers often current share as well, both at a cabinet level (or two cabinets in the case of Tesla V2 superchargers), or of course whole site throttling, as a site with say 12 dispensers capable of 250kW each could total three megawatts. Often the site transformer will be limited to 2-4 megawatts, and often the site might have other loads that can't be throttled (e.g. at a shopping centre or workplace).

The current limit for DC chargers is negotiated digitally and dynamically between the vehicle and the charger. This also involves the control pilot signal, still with the 1kHz square wave, but now with a special duty cycle, and using Power Line Communication on top of this. At least that's my guess, perhaps they remove the 1kHz square wave once digital communications have started; the two transitions every millisecond would be very disruptive to the comms.

 

[Checo]

Excellent, thanks @Coulomb. That's exactly the information I was looking for. I thought it must involve those thin pins and that the EVSE must have the ability to limit the current draw.

 

[Jonoffun]

I have a 10kWh battery, Growatt 6kW inverter, and an MG4. I am wondering whether the following is practical. Could I disconnect from the mains at the consumer unit, then plug the car into a socket in the garage and set the car to discharge. Would the car be seen by the growatt inverter as GRID, so it would continue to use the battery in the loft to supply the house with any excess above 3kW (the battery discharge limit) being pulled from the car. This would be limited to 2kW (as currently using a granny plug). Would that be a practical way of emulating car to house If I wanted to charge the loft battery in winter I would have to limit the car discharge to 2kW overall due to the limited capacity of the cable to the car and the plug which needs to not get warm.

 

[fshuk1]

I have a 10kwh battery Growatt 6kwh inverter and an MG4. I am wondering whether the following is practical. Could I disconnect from the mains at the consumer unit, then plug the car into a socket in the garage and set the car to discharge. Would the car be seen by the growatt inverter as GRID, so it would continue to use the battery in the loft to supply the house with any excess above 3kw (the battery discharge limit) being pulled from the car. This would be limited to 2kw (as currently using a granny plug). Would that be a practical way of emulating car to house If I wanted to charge the loft battery in winter I would have to limit the car discharge to 2kw overall due to the limited capacity of the cable to the car and the plug which needs to not get warm

Vehicle to Grid is the requirement you are looking for. As far as I know no car has VTG. Most cars have Vehicle to Load (VTL). VTG also requires a special home charger which is currently very expensive. The only car that is capable of this (with the proper home charger) is Leaf (ie CHAdeMO)
.

 

[ReintjeWA]

Vehicle to Grid is the requirement you are looking for. As far as I know no car has VTG. Most cars have Vehicle to Load (VTL). VTG also requires a special home charger which is currently very expensive. The only car that is capable of this (with the proper home charger) is Leaf (ie CHAdeMO)
.

I think all new VWs with a battery pack over 77 kWh (don’t quote me on that) now support V2G provided you have a specific charger from this one company. Like you said, all very specific and probably expensive, but it is starting to come. Not for our cars though.

 

[Coulomb]

Could I disconnect from the mains at the consumer unit, then plug the car into a socket in the garage and set the car to discharge.

Eek! Never use an outlet as a power inlet. It means that you have lethal voltage on a plug with exposed pins.

But you could arrange a caravan style inlet near your consumer unit (we'd call that a switchboard), and that would be safe.

But the inverter will want to blend AC-in and battery power to supply the loads. I would not like to attempt that, even if the inverter somehow didn't realise that it wasn't connecting to an actual grid.

Apart from that, you need to set a limit on the AC-in power, not on the battery power.

So for a number of reasons, I don't think it's practical, sorry.

 

[tsedge]

Vehicle to Grid is the requirement you are looking for. As far as I know no car has VTG. Most cars have Vehicle to Load (VTL). VTG also requires a special home charger which is currently very expensive. The only car that is capable of this (with the proper home charger) is Leaf (ie CHAdeMO)
.

New Renaults apparently come with V2G.

 

[Jonoffun]

Vehicle to Grid is the requirement you are looking for. As far as I know no car has VTG. Most cars have Vehicle to Load (VTL). VTG also requires a special home charger which is currently very expensive. The only car that is capable of this (with the proper home charger) is Leaf (ie CHAdeMO)
.

No I am not looking for VTG I am wondering if the MG4 can discharge into a house socket, with the mains switched off, and whether that will "fool" the Growatt inverter and house batteries into believing "they" are still connected to the grid. Then if there is a power cut, I can run the house from the growatt batteries, with any spike voltage above 3kwh being absorbed by the car, up to 2kwh. I am not an electrician and I have no idea if that is possible or if there are technical reasons (sin waves etc) it is impractical. If I can do the above I can charge the car up at night, switch off the mains and then run the whole house on the house batteries and car, maybe? although it would mean resetting the cooker clock everyday - which would be a real nuisance!