I heard something today that makes me very skeptical. A dealership claimed that charging an EV with 11 kW has higher energy efficiency than charging with 6 kW. (And they didn’t mean 3-phase vs. 1-phase!)

Even if it is somehow related to battery temperature and receptiveness, that would still be heat loss. With higher currents, resistance increases, too, no? Or is it proportional and always the same percentage?

In any case, I find it foolish to follow such advice because there are other factors involved, mainly a battery suffering more regarding longevity if charged faster, and that is a lot more expensive of a loss, eventually, also because an older battery is generally less efficient in several ways. (internal resistance, self-discharge)

Plus more load spike on the grid, external and house. But I don’t want to rant too much about the lack of reason in the world that seems to be a self-amplifying spiral of doom. Apparently house charger impatience is a thing. Three hours of charging twice a month - absolutely intolerable to some.

  • hmnahmna1@alien.topBEnglish
    1·
    10 months ago

    As a first cut, power loss in the charger goes as I^2 R, where I is the current and R is the wire resistance.

    The wire resistance is not a function of current in general. The power the battery gets is IV, where I is the current and V is the voltage.

    For a given charging rate, higher voltage requires lower current. So if you’re charging at 11 kw/120 V, the current required is twice as much as 11 kw/240 V. So the I^2 R losses go down by a factor of 4.

    Without knowing voltages/currents on the 11 kw and 6 kw chargers, I couldn’t answer for certain. But it’s certainly possible for the 11 kw to be more efficient.

  • Streetwind@alien.topBEnglish
    1·
    10 months ago

    Different hardware might handle charging at different speeds. Given that I’ve personally never heard of a vehicle charging at 11 kW on a single phase, I’m all but certain the dealer was referring to the difference between single-phase and three-phase here.

    A rough rule of thumb for charging losses is: 15% for single phase level 1, 10% for three-phase level 2, 5% for DCFC level 3.

    It varies from car to car, obviously. But the hardware handling three-phase charging is always less lossy than the one handling single-phase.

  • SyboksBlowjobMLM@alien.topBEnglish
    1·
    10 months ago

    11kW is three phase, 16A surely? So you’d be running more complex rectification hardware (unless the three phase models always have this overhead) which could make it less efficient, I’d have thought.

  • sylvaing@alien.topBEnglish
    1·
    10 months ago

    It will be more efficient if you have to warm up or cool down the battery to charge since the time to charge will be less, so less energy wasted for that process

  • sverrebr@alien.topBEnglish
    1·
    10 months ago

    The cars power electronics will tend to have it’s peak efficiency at close to its maximum output. This is how most such power electronics work out.

    Additionally if it is cold the battery must be heated to be charged whic reduces efficiency more the slower a charge gets, but this is mostly an issue in freezing temperatures so it might not apply.

    Rest assured that any charge speed less than 0.2-0.1C will be neglible wear due to charging speed. Essentially any charging on AC will be slow charging for most cars.

  • psaux_grep@alien.topBEnglish
    1·
    10 months ago

    Bjørn Nyland has made several videos on the topic of AC charging, but titles are a bit vague so hard to tell which ones, but I believe this one touches on it:

    https://youtu.be/Zpp0iQC5xxc

    Obviously you might not live somewhere where it is cold during winter, peak efficiency of the on-board charger is likely to be around the 80% mark, but the more the battery heats up during AC charging the lower the internal resistance is.

    As for 11kW damaging the battery? Nope. Not even. Most EV’s to day fast charge at 100+kW, some at 200+. The battery doesn’t even get tickled at 11kW.

    At the end though it doesn’t really matter. 11kW isn’t a huge amount and your household could easily pull that around dinner time.

    What 11kW charging often enables is full off-peak (during the night) charging of the battery. So instead of taxing the grid at day you can fully charge an empty battery when the demand drops.

    Ideally chargers are smart and can help frequency balance the grid by charging when the load drops, or stop when it surges, but we have only seen the very beginning of this.

  • theotherharper@alien.topBEnglish
    1·
    10 months ago

    It’s a very small difference. An EV might have 200 watts of “overhead” needed simply for battery thermal management and the standby load of the onboard charge unit. So yeah, it’ll be maybe 1.5% more efficient at 11.5 kW vs 5.7 kW.

    But on the other hand when you double your speed, your circuit wires, socket, EV charge cable etc. are losing 4 times as much power to voltage drop. If all that adds to 1.5%, that wipes out that efficiency gain. Nevermind the fact that all those things running 4 times warmer increases the chance of equipment meltdown.

    And tell me again what you are trying to save? Pennies on electricity? Well, destroying a $400 EVSE or $800 charge port really wipes out all those pennies.

  • bobjr94@alien.topBEnglish
    1·
    10 months ago

    I know any L2 charging is far more efficient than 120V charging, after that the difference is said to be very small. Something like 72% efficient at 120V, but 82-85% at 240V.

    I read if you want the absolute lowest charging losses run your charger at 80% of it’s max capacity. Like the cables are designed to carry 11kw/48A maximum with acceptable voltage drop, running it at 40A will cut that voltage drop. But in the end that may only be a cent or 2 of power lost/saved and not something to worry about.