Car question. When I turn on my windshield wipers, the energy for those two step motors comes from the battery. And it comes to the battery from the gas tank via the alternator. This means that if I drive with my wipers on, I will run out of gas sooner. But doesn’t the alternator constantly attempt to charge the battery? Where is the “switch” that allows the alternator to suck less energy out of the tank when I turn off my wipers? I imagine something like a bicycle dynamo that can be either on the wheel, imposing drag, or off the wheel.

Comments

  1. #1 D. C. Sessions
    January 23, 2012

    The alternator is managed based on the battery charge state. If you don’t run the wipers, the battery will require charging less of the time and thus the engine will run slightly less, using less fuel.

  2. #2 Martin R
    January 23, 2012

    Where is the mechanical switch for this alternator on/off and how does it work?

  3. #3 D. C. Sessions
    January 23, 2012

    It’s not mechanical any more. Long ago it was a little sealed box in under the bonnet that had what was in effect a relay that opened a switch when the battery voltage was above a set threshold.

    For the last several decades, it’s an electronic switch in one of the engine control modules. The nature of automotive electronics is such that the exact location can be almost anywhere, so it varies by make and model.

    By the way, the alternator gets controlled by varying the field coil current. That’s a lot less than the output current so it doesn’t take a huge switch or wiring. You might be able to trace it out by looking for the small wires going to the alternator, but they usually end up in a large harness and after that they could go anywhere.

    Your best bet if you want to track this down would be the service manual for that model.

  4. #4 Martin R
    January 23, 2012

    Thank you! So when the alternator releases its drag on the engine, what is the physical thing that is no longer applied to an axis?

  5. #5 D. C. Sessions
    January 23, 2012

    When the alternator is generating electrical power, it exerts a reverse torque (drag) on the belt that drives it, which in turn applies a drag on the engine. The force you’re looking for is transmitted through the belt, which may or may not be shared with other devices (e.g. water pump or power steering.)

  6. #6 Martin R
    January 23, 2012

    Aha, and when the alternator is not generating, its wheel just rotates freely on its bearings against the belt?

  7. #7 D. C. Sessions
    January 23, 2012

    Except for the inevitable losses from those bearings etc. that’s exactly it. When the alternator is generating power, the drag is electromagnetic.

  8. #8 Martin R
    January 23, 2012

    Many thanks! I’ve been wondering for years.

  9. #9 george.w
    January 23, 2012

    The wipers do not get energy from the battery – its only essential function is to provide energy for the starter motor. Once the engine is running, everything runs off the alternator. (In modern, electronics-laden cars, it also serves as a current stabilizer).

    There is a voltage regulator, but its function is in its name. Originally it was a complex electromechanical relay but today they’re solid state.

    Try this: turn a hand generator and have someone put a substantial load (like a heating coil) in the circuit. You will feel the increased drag from the energy being used by the load. Switch off the load and the generator will turn freely.

    A partially-charged battery acts as a load; a fully-charged one less so.

  10. #10 Martin R
    January 23, 2012

    Excellent explanations. You guys rock! I have no engineering training.

  11. #11 Lassi Hippeläinen
    January 23, 2012

    Think about electric motors/generatos as levers: you put in electricity and it rotates; and if you rotate it, it puts out electricity. A generator is just like a motor, but run in reverse. And just like with a lever, the force you need to apply at one end depends on the load at the other end.

    And those motors that move the wipers aren’t step motors.

  12. #12 Dave X
    January 23, 2012

    Another way to think of the constant attempt of the alternator to charge the battery is the way a toilet fill valve fills a leaky toilet: The farther the level in the tank is from the setpoint, the wider it opens the valve, and faster it fills the tank. As it gets close to the setpoint, the valve need not be open so much. In the car, the voltage regulator acts like the tank lever & valve, sensing low voltage in the electrical system, and increasing the current (and therefore the strength of the magnetic field) in the rotor part of the alternator. At a constant speed of the engine & belts, the stronger the rotor’s magnetic field is makes the output of the dynamo more.

    Maybe think of the switch as a bicycle dynamo with an variable electromagnet inside instead of strong permanent magnets.

  13. #13 Energy man
    January 23, 2012

    If we would take Japan’s model and use water to produce hydrogen to fuel the car, then we would not have to use gasoline and the damaging ethanol that we unforutantely use these days.

    We couldn;t do that. That would be self sufficient and government doesn’t like people who are self sufficient.

    Speaking of alternators, mine quite about three weeks ago. That sucked. Expensive repair. I changed one myself one time on another vehicle I had. Never again. The next upgrade if such a repair occurs is to go with a high output alternator and a high capacity battery like Optima and perhaps a 4 farad capacitor as well.

    Changing from incandescent lights to LEDS in the tail, signal and brake lights might help some. The batter wil not drain as fast.

    If only there were a place made for people who made money from destroying other people’s engines with ethanol. Oh yeah, there is a place. It’s called prison. I hope these congress tycoons invest everything they have and own into corn for ethanol and then the farmers chnage their mind to raise pigs and cows instead. One little loss in income might chnage their tune.

  14. #14 Birger Johansson
    January 24, 2012

    Regarding electricity and waste energy: In an electric car the heat needed for heating the car is drawn from the battery and in severe cold this drains the battery significantly faster.
    With an internal combustion engine you merely use the existing waste heat for warmth -an important detail if you live far north.*

    Re ethanol -it dissolves water, rusting out fuel tanks. Butanol is far better, but requires more specialised tech for manufacture.
    — — — —
    * Or maybe you can find one of those soviet era isotope batteries. If you do not mind a lot of shielding.

  15. #15 dustbubble
    January 26, 2012

    @13: “a place made for people who made money from destroying other people’s engines with ethanol. Oh yeah, there is a place. It’s called prison. I hope these congress tycoons invest everything they have and own into corn for ethanol”

    Round here, farmers used to turn corn into ethanol (on the grounds that it was too costly to dry the grain to stop it sprouting, works even if it doesn’t get properly ripe due to the endless rain, wind, cold etc.; at least some sort of cash-crop return, even if inedible (moulds, fungi, rot)).
    Nearly all of them have given up, after about 200 years of being chased up and down the glens by English Redcoats with muskets.
    Who wanted to put them in prison for doing it.

The site is undergoing maintenance presently. Commenting has been disabled. Please check back later!