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How much energy do batteries pass through an electric car

  1. Sep 16, 2013 #1
    I’m trying to figure out how to track the energy production from a battery to a motor as in an electric car. For example, the basic and primary components of an electric car are as follows:

    battery > controller > electric motor > driveline ====> car moves

    What I’m trying to figure out is how to measure how much energy the battery is really providing to make the car move and track that energy use through all the components to the end. It doesn't seem correct to use the voltage or the amps to do that, so I assume you should use kilowatts (volts x amps). Plus some of the energy is in the form of electricity and some is in the form of mechanical. I don’t know if I've got my terminology correct. But… Assume the battery is 120 volt and 10 amps. Then that means the battery is able to provide 1.2 kW of power (or is it energy? I know there’s a difference but can’t always remember which term to use).

    Therefore, is it right to say this?

    1. Battery produces 1.2 kW of power which it then passes to the Controller >
    2. Controller then passes the 1.2 kW of power to the motor >
    3. Motor then passes the 1.2 kW of power to driveline >
    4. Driveline then uses the 1.2 kW of power to move the car

    Now I kept it simple, but in reality some power get’s lost along the way through friction, heat, or whatever. But wasn’t sure how to show that.

    Am even close to describing this right? Should I use some other measure instead of kW?
  2. jcsd
  3. Sep 16, 2013 #2
    Hello PT -- In general kW is a fine unit of measurement ( well it is an EE forum...)

    The battery is storage ( really no different then a gas tank), your model:
    "battery > controller > electric motor > driveline ====> car moves"
    - is also OK, and for each step you are converting the energy from one form to another.

    As for the Power vs energy - power is the ability to do work, and energy is the total work that can be done.

    OK Battery then can output 1.2kW - but the total energy will be kWH ( hours) or may be rated in Amp-Hours, as a measure of total energy it can store. Batteries often will have a A-H specification, and this is without dropping below a certain voltage. So a single lead acid car battery 12V and 300 Amps peak = 3.6kW - but only for a few seconds.
    The controller then controls the power to the motor. Usually by controlling the voltage to the motor.
    The motor then converts the electrical power/energy into mechanical.

    Each step has an efficiency ( power out / power in)

    So - the best way to evaluate a battery - well - is to measure voltage and current, but you need to track this over time. There are a number of ways to do this. Ideally - the vehicle has a controller that can be used, not necessarily the motor controller, but at 1.2kW pretty small, you may want that function to be performed at that level.
  4. Sep 16, 2013 #3
    Thank you for the response. Couple follow-up questions/clarifications.

    Not really trying to measure or evaluate just the battery (may have worded my question wrong). I'm trying to measure and/or understand how power/energy is being used throughout the system from beginning to end.

    Using 12V and 300 amp battery (your example), that means (if I'm understanding this):

    1. Battery is putting out 3.6kW of power to start which is passed to the Controller
    2. Then the Controller receives (and starts with) the 3.6kW of power it got from the Battery, and then passes the 3.6kW of power to the Motor.
    3. Then the Motor receives (and starts with) the 3.6kW of power it got from the Controller, and then passes the 3.6kW of power to the Driveline.
    4. Then the Driveline receives (and starts with) the 3.6kW of power it got from the Controller, and then uses the 3.6kW of power to move the car.

    I'm imagining that 3.6kW of power is passed along from component to component until it gets to the end to move the car. Now, to be more technical/accurate, as it passes from component to component and gets converted from electrical to mechanical and it losses some power along the way. So, really what may be left of the 3.6kW of power at the end to move the car is maybe 3.0kW.

    Is the correct way of looking at it? Or should I look at it as 3.6kWh of energy being passed over time (vs 3.6kW of power being passed in an instant (?) I guess...)?
  5. Sep 16, 2013 #4
    The battery does not "put out" power, but it has the capacity to do so. For example, a car with a 250HP gas engine is (NOT) "putting out" 250HP all the time, only when it is requested to by the driver, and even then only in a small region of it's range of RPM.
    For an electric vehicle - you can get max power out of the system for maximum acceleration - but so steady state driving ( constant speed) only a portion of that is used.
    This power is instantaneous (in an instant) - if you record this over time you will see the total energy used - at that point in the system where the measurements are being taken. For electric vehicles the limits are total battery capacity ( energy) and efficiency. But the total system efficiency is pretty good, the battery capacity is the limiting factor.

    Conversely for a internal combustion / gas powered car the gas has a very high energy density (kWh/ KG), but the internal combustion engine is very inefficient (~14 to 20%)
    Last edited: Sep 17, 2013
  6. Sep 16, 2013 #5


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  7. Sep 16, 2013 #6


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    I think you meant to say that 'a car with a 250 HP gas engine is NOT "putting out" 250 HP all the time.'
  8. Sep 17, 2013 #7
    Thanks to everyone for your replies. I guess what I'm getting from this, is to follow the power used from component to component I should be using kWh (vs kW). In that case, I'm really following the energy used.
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