What is the most efficient AC charging speed?

[HCassidy]

So I often charge at the minimum current of 5A with a line voltage of 240V, leading to a 1.2kW charging speed. My charger can output at 32A but wondering whether I am losing much in efficiency by charging at a lower current as the onboard AC/DC converter will inherently have losses.

If anyone's experimented with this it would be good to hear.

 

[GRiLLA]

I've no idea about the efficiency of the inverter, but there's about 300w of power to keep the car computer running to supervise the charging, so charging as quickly as possible then letting the car sleep is most likely to be the best option.

 

[WannabeOwner]

wondering whether I am losing much in efficiency by charging at a lower current

Its come up in discussion here in the past and the consensus was that it is definitely less efficient. (One reason to do it would be if Solar PV is exporting, but towards the end of the day when the sun is going down and there is only 5A available "for free"

Some reason why you would charge at 5A over a long period of time rather than, say 7kW / 32AMP ? (assuming you have a wall charger, otherwise max that Granny charger can do)

 

[Jason71]

So I often charge at the minimum current of 5A with a line voltage of 240V, leading to a 1.2kW charging speed. My charger can output at 32A but wondering whether I am losing much in efficiency by charging at a lower current as the onboard AC/DC converter will inherently have losses.

If anyone's experimented with this it would be good to hear.

It has been observed in the past that a 7kw charger is about 90% efficient when comparing kwh energy consumed as measured by the charger vs battery kwh increased. However a 2kw granny charger seems to be typically more like 85%. So on that basis yes I would expect it to be worse.
If you are doing it to protect the battery then don't bother . 7kW's charge rate on any typical EV battery counts as trickle charging.

 

[HCassidy]

I assumed that keeping the MCU active whilst charging would take up a fair chunk.

Let's just say certain energy tariffs give you increased off peak windows if you charge at a slower rate, thereby reducing cost of home energy usage

 

[Jason71]

I assumed that keeping the MCU active whilst charging would take up a fair chunk.

Let's just say certain energy tariffs give you increased off peak windows if you charge at a slower rate, thereby reducing cost of home energy usage

That may be part of it. The car uses 200w to 300w while awake so the longer you keep it charging the more you will use.

 

[jmaddr]

Another potential concern.
Not only is the MCU consuming energy being powered on while charging (BTW, this is not a problem if you are using sentry mode as sentry is keeping it on anyway), but if it’s cold where you live, the car will warm the battery prior to charging and potentially during, wasting even more energy.

 

[MrT3]

I am losing much in efficiency by charging at a lower current as the onboard AC/DC converter will inherently have losses.

AC/DC and DC/DC converters are most efficient near their maximum capacity (e.g. computer power supplies, UPSs etc.), so I don't see any reason why the converters in your vehicle are any different. So whilst you might be slightly more efficient with regards to heat loss in the charging cable I imagine you are worse off.

As others have stated, a large inefficiency is the car being powered up whilst charging and consuming up to 300 w so you want it to go back to sleep as quickly as possible.

Such a shame Tesla didn't take the opportunity with M3 Highland to use a power efficient ARM processor to manage charging and Sentry mode etc.