Charging efficiency using different charging methods

Discussion in 'General' started by Yanni, Sep 24, 2018.

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  1. Yanni

    Yanni Member

    Hi! I can't seem to find this topic elsewhere. If I missed it, please redirect me!

    Haven't yet bought an EV (should be soon!) but in talking with our electric company today, they said that charging with a level-2 charger will "save me money" on electricity because it takes less kWh to charge up a battery when you're doing it faster. ?!?

    I'm not an EE so this doesn't make any sense...and the lady on the phone couldn't give me a "math sentence" so that I could compare the two delivery methods to better understand what she was saying.

    As an example, let's say your battery takes 20 kWh.

    The jist of what she was trying to explain is that if you want to charge your 20kWh battery, and it takes you 10 hours to fully charge your car (using a standard 100 plug) that method uses up MORE ELECTRICITY than if it takes only 4 hours to charge your 20 kWh" battery when using a faster charging system (like 240).

    This makes no sense to me because I'm comparing it to the gasoline paradigm, where #gallons = # gallons whether you are filling your tank with an an eye dropper or a gas pump. SURE it will take more TIME if you are using a smaller delivery method, but does it use more ELECTRICITY overall too?

    Is there THAT MUCH WASTE in using 110 vs 240?

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  3. bwilson4web

    bwilson4web Well-Known Member Subscriber

    There are two parts to the problem and both are in your EV of choice:
    Most built-in chargers are a switching architecture that includes an oscillator that switches current through an inductor. This oscillator has a fixed overhead. Draw a small amount of power and most of the energy is operating the oscillator. Increase the power passing to the battery and the oscillator overhead decreases as a percentage.

    The second part is the effect of voltage and current on the efficiency through wires. We can go into a technical discussion but the simple formula is:

    (I**2) * R :: the current squared time the resistance​

    As the voltage goes down, to get a kilowatt hour of power, the current goes up.
    • 120 VAC * 8 A ~= 1 kW
    • 240 VAC * 4 A ~= 1 kW
    The higher voltage cuts the current in half and this is the factor that is squared when calculating the wire loss. There are more technical details such as wire size but these are the primary effects.

    Bob Wilson
  4. Yanni

    Yanni Member

    Hello Bob-
    Thanks for the info!
    Hmmmm...I asked her something to that effect, but it sounded like she was reading off a script and couldn't understand my lay-lay-layperson question. I admit it wasn't as electrical and coherent as what you have written.

    So....with all this in mind, does it matter/how does this affect
    * the car I purchase
    * the charging apparatus I purchase
    I'm currently considering a used Leaf (2016).

    Any advice/warnings/confirmations are appreciated. I have a rudimentary understanding of all this.
  5. bwilson4web

    bwilson4web Well-Known Member Subscriber

    Uh, this is more difficult to address. Probably need to start with 'Goals' and 'Objectives.' Don't worry about the charger until you've got the car you want. But it makes sense to get a used one, especially if it is coming off lease, as the depreciation is much larger than the new car tax credit. A leased car will be in excellent shape and low mileage.

    I'm partial to plug-in hybrids because I want EV efficiency around town and gas for cross country. EV cross country still has challenges although it is getting 'less bad.'

    I also recommend going with liquid cooled instead of air cooled batteries. Heat is the enemy and air cooling remains simpler but marginal in some climates.

    Bob Wilson
    Yanni likes this.

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