V2L Test. 7kW output...

[wattmatters]

My main usage will be to power a heat pump, where the main demand is in the wintertime when the power I get from my 3kW solar panels is often <<1kW. Re-charging the car batteries will be overnight, when cheaper grid power, which will also be powering the heat pump. I might expect to use 8-10 kWh for the heat pump on a cold day (a COP of 3-4 provides 25-40 kWh of heat energy). I tend to use the car only 1-2 short trips a day, so most of the time it can be supplying the home.

Thanks for clarifying. Yeah, a small solar PV array isn't enough to cover consumption. Makes sense.

I have 11 kW of grid-tied PV and 2.2 kW of off-grid PV and hoping to add several more kW to my off-grid system before long (I have it there, it's just a matter of installing).

With regard the car, it is at least designed to charge the EV batteries for up to 9 hours at 7kW. For someone who drives their car to work, say 100 miles return trip Mon-Fri, they would be charging for 4-5 hours every night. I think it extremely rare that I need to draw as much as 7kW for more than a couple of hours in any period. I think the vehicle battery system is well able to cope with the demands of my home requirements.

There are three separate parts to the system:
i. the battery and BMS
ii. the on-board AC charge controller 7 / 11 kW (single phase / 3-phase)
iii. the on-board V2L inverter. Note that MG4 specifications rate at 2.2 kW.

i. & ii are going to be fit for the purpose of everyday use.

iii. however may have been designed for occasional use as a handy tool but I have to wonder about deploying it for constant use and at higher rates of power draw.

There must be a reason why MG have limited the V2L output specification at 2.2 kW and why their own V2L adapter is limited to that.

My ideal setup would allow me to use the V2L output in an on-grid configuration, but to address both earthing problems and lack of synchronisation with the grid frequency by using a dc input of a hybrid inverter to generate grid-synchronised AC output. To do this the car's V2L output needs to be converted to DC - slightly wasteful in efficiency terms, but available immediately.

I can use use the V2L output to power an AC charge controller to charge my home battery at a modest rate suitable for the on board V2L inverter.

As you say, this introduced efficiency losses but then my dedicated off-grid home inverter, which is designed for this duty, can supply the home power as needed. It's got an 8 kW continual output capacity.

If I really thought the car's V2L could cope with regular higher power outputs (I have my doubts) then having it dedicated to running the ducted AC system at night would be very helpful. We need the Summer cooling and occasional Winter warming. It's the major load I do not support with the home battery.

Then I can recharge from solar PV in the daytime. For instance, this day last week:

I have loads, including the water heater and the EV charge station, which automatically vary their power draw so as to remain under the solar production curve (also the off-grid battery if it needs supplemental charge). You can see this was a pretty tricky day with clouds coming and going.

But that red blob of energy in the evening is the ducted AC system drawing from the grid, while during the day the solar PV covers it.

I would love to be able to offset that evening consumption. But I would need a second inverter for that.

 

[Coulomb]

There are three separate parts to the system:
i. the battery and BMS
ii. the on-board AC charge controller 7 / 11 kW (single phase / 3-phase)
iii. the on-board V2L inverter. Note that MG4 specifications rate at 2.2 kW.

My understanding is that items ii and iii are actually the same piece of hardware; V2L is essentially running the On Board Charger (an AC to DC converter) in reverse (thus turning it into a DC-AC converter). Essentially, instead of a bridge rectifier and boost converter at the AC end, which is unidirectional, they use a full bridge, which is bidirectional. The full bridge isn't much more expensive than the bridge rectifier and boost converter. It's essentially 5 diodes and 1 transistor versus 4 transistors, with the transistors costing more than the diodes, and you need 4 times the gate drivers. Of course the control firmware has to be a bit more clever.

Thinking about this a bit more, you probably need to do something similar at the DC end, to make that bidirectional as well. So you don't get V2L "for free" from a cleverly designed On Board Charger, but if I'm right it's not too much extra effort and cost.

But that means that the V2L system should be as powerful as the on-board charger; the power dissipation should be much the same in either direction. That's why I wasn't surprised that someone had been able to extract over 7 kW from the V2L system.

And of course, I could be wrong about this, and/or MG chose to provide a separate, lower power module to perform the V2L function. But that seems unlikely to me. The On Board Charger already has all the connections, the cooling, and so on; it just seems irresistible to make it bidirectional.

 

[wattmatters]

My understanding is that items ii and iii are actually the same piece of hardware;

Interesting.

But that means that the V2L system should be as powerful as the on-board charger

Yet MG have put a limit at 1/3rd the capacity of the charger. There must be good reason for that.

 

[timE]

I agree with Coulomb - there is a simple mod to tell the MG4 how much power it can deliver when discharging (470 ohm resistor in the plug), which really is just to indicate that the discharge cable is capable of conducting 30amps. The MG4 V2L cable is provided primarily to power AC mains devices away from home and hence does not need to use a hefty cable.

I wouldn't to presume understand MG's rationale for not advertising the full capability of it's V2L, but it clearly can be used in this way, as has been demonstrated by a number of MG4 users.

 

[wattmatters]

I wouldn't to presume understand MG's rationale for not advertising the full capability of it's V2L, but it clearly can be used in this way, as has been demonstrated by a number of MG4 users.

Short duration tests are one thing. Ongoing daily use at higher power draw for hours on end is something else.

What happens if the on-board unit fails as a result of consistent use outside of MG's specified limits?

I can't imagine it's a cheap fix.

 

[timE]

Whilst MG advertise that their V2L is capable of delivering 2.2kW I can't find anywhere in their documentation where they specify that this is the maximum power that can be draw from the car, and that exceeding this limit might void their warranty.

 

[wattmatters]

Whilst MG advertise that their V2L is capable of delivering 2.2kW I can't find anywhere in their documentation where they specify that this is the maximum power that can be draw from the car, and that exceeding this limit might void their warranty.

While vehicle warranty claims are a minefield at the best of times, the warranty documentation in Australia states:

The following uses, normal or natural occurrences and aspects of the vehicle and its ownership are not covered by this Warranty:

• any fault as a result of using the vehicle for a purpose other than that for which it is designed

I think given a stated V2L capability of 2.2 kW and that their own V2L adapter is limited to that, then using it beyond that may well be argued as being for a purpose other than for which it was designed.

While no warranty can override Australia Consumer Law, I think one would have a hard time claiming ACL for a V2L failure if the use was beyond what MG4 advertised as being offered.

 

[timE]

It can't be coincidental that the MG shuts down discharging when the load drawn exceeds 7kW (see MG4 rocking 7kW V2L Real full home use possible.). If 2.2kW was the peak output available for discharge then surely they would have set this as the cutout power level?

 

[wattmatters]

If 2.2kW was the peak output available for discharge then surely they would have set this as the cutout power level?

Many power systems have a peak capacity / shut off beyond the recommended use level. That they are allowing a wide margin is probably a good thing and significantly reduces the chance of a costly failure.

The primary issue for me is the lack of actual publicly documented specifications for their V2L.

 

[halogen]

My logic says if the onboard can accept max AC at 7kW... I'm sure the car can max discharge 7kW...

It can't be coincidental that the MG shuts down discharging when the load drawn exceeds 7kW (see MG4 rocking 7Kw V2L Real full home use possible.). If 2.2kW was the peak output available for discharge then surely they would have set this as the cutout power level?

 

[Ayoull]

V2L is limited to 2.2kW as that is the maximum constant load a BS1363 can take for a sustained period.

Any more is only permissible as a leak load for short durations.

And as the system is designed to power single appliances not houses the 2.2kW limit makes sense.

 

[wattmatters]

My logic says if the onboard can accept max AC at 7kW... I'm sure the car can max discharge 7kW...

Without seeing a circuit diagram, schematic of specifications for the charge and discharge systems, can we make that assumption?

V2L is limited to 2.2kW as that is the maximum constant load a BS1363 can take for a sustained period.

That is a limitation of the adapter plugged into the car's charge port, not the car's V2L system.

Truth is, we don't actually know what the V2L circuit has been designed to do. We know what some have done to draw a lot more power out (circa 7 kW using a custom built adapter), however that is a long way from knowing the actual specifications, capabilities and limitations as designed and built by MG.

Which is why I consider constraining to 2.2 kW to be prudent in the absence of any confirmation from MG on whether the V2L can be used at higher rates of discharge without concern (with an appropriate adapter and downstream wiring of course).