Scheduled charging and 8A

[MGPHOAWR]

I use a granny charger for my MG4 (no wallbox needed as mileage is low).

I'm well versed in the safety considerations of doing so and one of the safety precautions I like to use is to reduce the power draw to 8A rather than 'AC current' (which I understand would be 10A in the UK rather than 13A).

However, I used scheduled charging for the first time now we're on an EV tariff. The setup and timings worked no problem, but I noticed that it ignored my instructions to draw at 8A and was instead charging at 10A (1.8-1.9kW AVG).

I know this is technically within the limits of safe usage but as I intend to exclusively granny charge (normally twice a week, 7 hours each time) I want to be extra cautious.

This was my setup:

1) Unlock car
2) Plug in charger and switch on (charging starts)
3) Get into car and manually finish charging
4) Set up scheduled charging in car
5) Change current draw to 8A
6) Leave car and lock
7) Car charged between scheduled hours BUT did so at the default AC current setting, not my setting

Am I doing something wrong? Or is this just a limitation of the software?

 

[decrep]

I suspect that once car is locked and restarted, the charging current goes back to default, max available.
I can't think of an easy way around this if the granny charger has no controls.

 

[Coulomb]

it ignored my instructions to draw at 8A and was instead charging at 10A (1.8-1.9kW AVG).

230V x 8A = 1840W or 1.84kW, right in the middle of your reported power.

Was this measured with a power meter that the Granny "charger" was plugged into? If so, it is drawing close to the requested 8A.

If this was charging power reported by the car, then that's 1.8-1.9kW into the high voltage battery, after losses and 12V power draw. This may be what experience tells you you usually see when the car is charging at 10A.

 

[MGPHOAWR]

230V x 8A = 1840W or 1.84kW, right in the middle of your reported power.

Was this measured with a power meter that the Granny "charger" was plugged into? If so, it is drawing close to the requested 8A.

If this was charging power reported by the car, then that's 1.8-1.9kW into the high voltage battery, after losses and 12V power draw. This may be what experience tells you you usually see when the car is charging at 10A.

This was the rate which was reported in the app whenever I checked it. Low electricity rates are 2AM-9AM and I woke up about half 7 and checked the app to see it was drawing at that sort of power.

Is that consistent with 8A or 10A?

I don't have any other means to check the power draw other than what the car and app tell me.

 

[scal]

IIRC, the set limit affects the value the car sees and reports, i.e. the value after charging losses. That actually makes sense, since it’s the only current reading the car appears to have.

The wall‑side draw will be higher. If that’s an issue, the simplest solution is to use a granny charger that can set its own current limit.

Edit:
I checked my Home Assistant stats: with my Ohme charger charging at 10 A, the car sees about 2 kW.
You could also try limiting the car to 6 A, that should keep the wall‑side draw under 8 A.

 

[Martinonline]

If you have a smart meter with an in-house display you can see the power draw and can adjust with your normal usage.

With my scheduled charging if I use the app after I've readied the car for the scheduled charging the car starts charging. Perhaps use of the app may reset the set current. (Shrödinger's cat?)

 

[EvilSeagull]

Granny chargers are super inefficient, you lose 20% or more compared to a standard 7.4kW unit. Proper charger will pay for itself rather quickly.
Unless you're renting and hoping to move in less than 12 months get yourself a proper charger bud.

 

[fnegroni]

I use a granny charger for my MG4 (no wallbox needed as mileage is low).

I'm well versed in the safety considerations of doing so and one of the safety precautions I like to use is to reduce the power draw to 8A rather than 'AC current' (which I understand would be 10A in the UK rather than 13A).

However, I used scheduled charging for the first time now we're on an EV tariff. The setup and timings worked no problem, but I noticed that it ignored my instructions to draw at 8A and was instead charging at 10A (1.8-1.9kW AVG).
<snip/>

Am I doing something wrong? Or is this just a limitation of the software?

it's a limitation of the original mg4 software.
The 2026 edition should retain the AC limit you set

 

[Coulomb]

This was the rate which was reported in the app whenever I checked it.

Is that consistent with 8A or 10A?

That is probably 10A AC. It's likely not saving that setting.

Granny chargers are super inefficient, you lose 20% or more compared to a standard 7.4kW unit. Proper charger will pay for itself rather quickly.

Granny "chargers" and 7.4kW wired "chargers" are both EVSEs, i.e. they both are very efficient, wasting only a few watts on the control and display electronics. There is no power conversion electronics in there, just a relay or contactor that is on or off.

However, charging at low power as a granny "charger" does means that a greater proportion of the AC power goes to "overheads" like the 12V systems and losses. But a wired in "charger" capable of 32A (~7.4kW) but set to the same current limit as a granny "charger" would be about the same efficiency as the granny.

So you are right that charging at 7.4kW is more efficient than granny charging, but it's because of the current limit, not the efficiency of the granny "charger". The main difference is the amount of time that the 12V system is in high power mode, including some fixed losses inside the On Board Charger. Charging at 32A will actually incur higher I²R losses.

 

[EvTek23]

As @Coulomb says the loss will be almost constant whether it's 2kW or 7kW input power. These are the losses in the CCU within the car, so as a percentage of the input power the 200W to 300W lost seems greater than at higher power where the loss is masked by the much larger charge rate.

 

[siteguru]

That's why (when talking about AC charging efficiency) I refer to fixed losses and variable losses .. variable losses increase as charge rate increases, but fixed losses are independent of charge rate. Ergo charging efficiency reduces as charge rate reduces, because the fixed losses become a greater proportion of overall losses.

i.e. I agree with the previous 2 posts.