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How does an alternator charge a car battery?

I want to make a windmill to charge a car battery. How does an alternator in a car do it? How does the car take the AC coming out of the alternator and make it dc at the appropriate voltage and current to charge it and what is the appropriate voltage and current to charge a car battery? Any help would be appreciated.

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24 Responses to “How does an alternator charge a car battery?”

  1. Dan S said :

    Alternator = generator = motor that is being powered.

    There is a law of physics that if you pass a conducting wire through a rotating field you create electricity. The crankshaft of the car rotates the fan belt which turns the rotor in the alternator which is a large coil of copper wire inside of a permanent magnet field so it acts just like a generator.

    Because the rotor run through the north and south poles of the magnetic field it generates alternating current (hence the name alternator and why we use AC current from generators for our homes). The voltage regulator blocks out one phase (half) of the current to make it into DC current.

    According to Wikipedia:
    “In electricity generation, an electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The reverse conversion of electrical energy into mechanical energy is done by a motor, and motors and generators have many similarities. A generator forces electric charges to move through an external electrical circuit, but it does not create electricity or charge, which is already present in the wire of its windings. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water inside. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, the sun or solar energy, compressed air or any other source of mechanical energy.”

    The alternator has a voltage regulator to keep the current steady and low enough for the batter to handle. If you run a fan belt to the alternator that is spun by a windmill then you will be creating 12 volts DC current. The voltage regulator evens out the spikes caused by going at different speeds, and cuts out the negative half of the electrical phase making it DC current, so it takes off some of the energy, keeping the output at 12 volts DC. Most alternators have their voltage regulators built in although on older cars (pre 1980 it was a separate part).

    According to Wikipedia:
    “An alternator is an electromechanical device that converts mechanical energy to alternating current electrical energy. Most alternators use a rotating magnetic field but linear alternators are occasionally used. In principle, any AC electrical generator can be called an alternator, but usually the word refers to small rotating machines driven by automotive and other internal combustion engines.”

    According to Wikipedia:
    “A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level.

    It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.”

    Connecting DC to AC is a very bad idea, you need to remove that voltage regulator or get an older alternator that doesn’t have one in it or use an alternator from a different source or use a rectifier to convert the DC current back into AC current. Solar cells create DC power so need to be run through a rectifer before being tied into a house hold grid. However a solar cell can be tied directly to a battery.

    Running a windmill powered car alternator to charge a car battery is an excellent idea since that is EXACTLY what it is designed to do. You are only replacing the mechanical power of the engine with the mechanical power of the wind. You don’t need any extra circuits or to make any changes the power will be conditioned to be just right for the 12 volt battery. You want to use a modern car alternator since it has the proper size voltage regulator built in. All you need to do is to make the windmill; by either putting wind blades on the alternator or connecting the alternator’s rotor to a windmill rotor through a fan belt. Then run wires directly from the alteranator to the battery (make sure to get the poles right, but then you problaby know that). There is no other wiring or mechanical system needed except something high to hold the windmill up and into the wind. A tail on the end of it and swivel will let it turn to always point into the wind.

    According to Wikipedia:
    “A windmill is a machine that is powered by the energy of the wind. It is designed to convert the energy of the wind into more useful forms using rotating blades or sails. The term also refers to the structure it is commonly built on. In much of Europe, windmills served originally to grind grain, though later applications included pumping water and, more recently, generation of electricity. Recent electricity generating versions are referred to as wind turbines.”

    Wind turbines are designed to make AC power so you are making a windmill. Wind turbines have other differences and are more efficent, but you don’t need that extra detail.

    To make your system run better you could hook a solar cell in line with the battery (in series) so that on sunny windless days the car will still charge.

  2. billrussell42 said :

    There are a set of high current rectifiers in the alternator that rectify the AC to DC. A regulator controls the voltage and current into the battery. The regulator takes into account the amount of charge in the battery, and the temperature, to adjust the charging current.

    Lead acid batteries are very sensitive to charge rate. They can be damaged easily.

    You probably want the regulator as well as the alternator from an auto. You will have to gear up the rotation rate. The alternator need to be turning at at least 1000 RPM to operate.


  3. Irv S said :

    Most modern automotive alternators have a voltage regulator/rectifier
    unit built in.
    If the one you have doesn’t, buy the unit that went with it in
    the vehicle it came from.
    It’s ‘off the shelf’ hardware.
    Hook it up as it was connected in the vehicle, and when the
    alternator is turned fast enough it will charge the battery.

  4. lee26loo said :

    You do not need the powerful car alternator to charge car battery,because car alternator is designed to power the whole car.It is a 3 phases AC generator,inside has 6 rectifiers to convert AC into DC for car usage. It can produce more than 30 amperes. To charge the car battery,you 3 amperes or less is good enough.
    Car alternator might built in with a voltage regulator to maintain output about 14 volts DC. Car battery stops charging by the alternator when car battery voltage reaches 14 volts.
    To turn the car alternator it requires quite a large torque,unless your windmill is quite big like the one uses in Holland.If your windmill is a small home make type,you may choose the bicycle light generator to produce electricity. Its output is 6 volts AC and if you are lucky,you might find some bigger one produces 12 volts AC. Using two 6 volts generators in series (observe polarity when you connect them) shall give output 12 volts. You need to buy a rectifier called 1N5404 to series the output of AC generator to produce DC. Then hook it to the battery for charging.

  5. more slack said :

    Hard way: lash up a car alternator and regulator, figure all the mechanical issues, gearing, etc. Don’t forget to make it weatherproof while you’re at it. But at least it will have the right charge rate for a single battery built-in.

    Easy way: buy a kit that is pre-made for the task. Check out a marine supply like West Marine (link below), they sell small wind turbines for sailboats, as well as charger controllers, inverters, solar panels and so forth. Larger systems are availble for fixed use (second link.)

  6. Retired EE said :

    An unmodified automotive alternator probably needs to run at least 1500 to 2500 RPM. When it runs in its normal speed range (about 2000 to 10000 RPM), the voltage regulator that is designed to work with it keeps the output at the proper voltage for charging a 12 volt automotive battery (about 14 or 15 volts). I believe that the voltage regulator is usually in the alternator housing, but it may be mounted externally.

    Some people have modified standard automotive alternators by replacing the field coils with permanent magnets. That makes it possible to operate at much lower speeds. Look at: With this approach, you can’t use the alternator’s normal voltage regulator. You can probably get an inverter to accept a fairly wide range of input voltages and provide a constant output voltage.

    Here are some more alternator links:

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