Is my car actually charging at 10A ?

[GrahamW42]

At home, running off the 13A ring main, my car has always told me that it's charging at 10A and ~2.4kW.

The car shows a rounded 2kW on is screens and my Alexa skill reports 2.4kW.

I've just acquired a power meter to keep track of my car's charging. It claims an accuracy of +/-2% ... yet it reports 9A and 2.1kW. A discrepancy of way more than 2%.

Which should I believe ?

FWIW I was expecting losses in the charging cable to result in a higher measurement "at the wall socket" than actually arriving in the car ... not the opposite.

 

[MrBadger]

13A 3 pin sockets are not really designed for continuous high current load.

So the UMC sets a 10A limit when using the 13A 3 pin plug.

kW obviously depends on voltage. So theoretically, single phase it can range from 2160W to 2530W (nominally 230v -6%, +10%, typically still around 240v) less losses.

 

[GrahamW42]

13A 3 pin sockets are not really designed for continuous high current load.

So the UMC sets a 10A limit when using the 13A 3 pin plug.

Sorry everyone - I've just realised that my question was worded badly.

I know that our cars charge at 10A on a 13A ring main.

My issue is that my car says 10A@235V (meaning 2.35kW) whilst my new power meter says 9A and 2.1kW.

Which is most likely to be correct ?

 

[Glan gluaisne]

Sorry everyone - I've just realised that my question was worded badly.

I know that our cars charge at 10A on a 13A ring main.

My issue is that my car says 10A@235V (meaning 2.35kW) whilst my new power meter says 9A and 2.1kW.

Which is most likely to be correct ?

Take your pick. Neither are that likely to be very accurate, I suspect. in-house Smart meter displays seem to be less than perfect, and often don't seem to reflect the true power usage. I can understand this, as the in-house display I built uses a meter energy measurement chip to measure true power at the supply, in both directions, but has to average that over a few seconds to make the display meaningful. In practice this means that the display has an error of about +/- 10W, and always lags about 10 seconds behind.

Although the car onboard charger may be drawing 10 A, the charge power into the battery pack will always be a fair bit less than the product of voltage and current being indicated (or the true power being indicated) from the supply as the OBC probably loses ~10%, plus there's almost certainly some power being drawn by the car whilst charging, as some of the systems will be up and running.

 

[GrahamW42]

an error of about +/- 10W

I could live with that.

... and I know that there are losses in the system.

I'm trying to get an idea of how much "my house" is pumping into the car.

The car claims that it is drawing ~2.35kW but the power meter claims that it is only providing ~2.1kW. That's a big difference - and only one can be right.

After an overnight charge that can be several kWh.

 

[Glan gluaisne]

The only way to be sure would be to use a known to be accurate power/energy meter in line with the supply to the UMC and log the true power. My guess is that it will be a different number to that being reported by either the car or the smart meter...

 

[arg]

You should expect the voltage numbers to be different, slightly lower at the car (I assume your power meter has a button to push to show voltage).

The car's display rounds the amps and kW to nearest whole number.

So most likely the current is something like 9.5A that the car has rounded up.

There might also be an error due to power factor, though the Model S charger at least claimed >99% power factor at full load; possibly Model 3 is worse, especially at low load.

 

[vitesse]

UMC may well be less than 80% efficient (power consumed vs power received by the battery). Charging at 32A is around 90%. Could this be a factor in your observation?

 

[tsh2]

My DMM is rated for 10A, and I think it does power measurements too. Not sure I want to risk it though...

Looking at a 9h24 charge, TeslaFi reports 19.89 kWh, average voltage 225V. That makes for 2.16 kW, 9.4A.

Energy used was 21.2 kWh, so add a bit to the numbers above...

 

[Glan gluaisne]

Measuring current and then voltage with a DMM may not give you the true power, as that depends on the power factor (PF). If Tesla have done a reasonable job with PF correction within the OBC, then the PF may be close to unity, so just measuring current and voltage might be OK, but personally I'd always try to use a clamp meter to measure current, rather than a normal DMM, as it's a fair bit safer to use.

I've installed a few separate energy meters around our house, so I can keep track of the consumption of some individual circuits, and some of them have the ability to display total energy used, voltage, current, PF and power. I have one of these built in to my charge points, so I can both check the charge power and keep track of the energy used for charging. The meters I've used aren't that expensive, and fit to a DIN rail, so can be installed within an enclosure like a consumer unit, if there is enough space.

 

[GrahamW42]

TeslaFi reports .... 2.16 kW, 9.4A.

That's very similar to my power meter findings. I'm inclined to believe 2.1kW and an ampage closer to 9A.

I think that my car is telling porkies when it displays 10A and it has recruited Alexa to fib for it when she tells me 2.4kW charging is taking place !

 

[vitesse]

When I had to use the UMC at home while our charge point cable/plug was faulty the current, according to Teslafi, never dropped below 9.8 and was occasionally at 10.

 

[Banksy888]

We have a dedicated meter on our Tesla circuit. When I set Tesla to 10 A, over the course of one hour the car drew 2.35 kWh. The voltage at the car (according to the car) was 235 V, so 2350 W / 235 V = 10 A.

So yes your car draws 10A from the house. Hopefully most of that goes in the battery too!

 

[vitesse]

We have a dedicated meter on our Tesla circuit. When I set Tesla to 10 A, over the course of one hour the car drew 2.35 kWh. The voltage at the car (according to the car) was 235 V, so 2350 W / 235 V = 10 A.

So yes your car draws 10A from the house. Hopefully most of that goes in the battery too!

Only about 70% actually.