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I understand some of these laws of thermodynamics. An alternative to this question, being inventive, is 'Can an inverter power a dynamo (which is powered by an electric motor) to charge the same inverter batteries?' It may seem redundant but I am pushing the limits with a beginner knowledge.

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    \$\begingroup\$ No. Current either comes out of the battery or goes into it. It can't do both at the same time. \$\endgroup\$
    – Finbarr
    Mar 9, 2021 at 12:12
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    \$\begingroup\$ Perpertum mobile? \$\endgroup\$
    – user263983
    Mar 9, 2021 at 12:12
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    \$\begingroup\$ It can, but the batteries will run down faster than they'll charge up \$\endgroup\$
    – user253751
    Mar 10, 2021 at 10:22
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    \$\begingroup\$ The inverter -> charger section is just a fairly expensive room heater. It's like trying to cool your kitchen with a fridge, or trying to oxygenate water with oxygen you just extracted from the same water. All processes are lossy; that has turned out to to be one of the most fundamental principles of the macrospcopic universe, from cars to black holes. Like evolution, it it follows directly from first principles. That's why it cannot be circumvented or tricked with clever ideas. It's common to all aggregates with large numbers of elements, independent of their makeup. \$\endgroup\$ Mar 10, 2021 at 15:40
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    \$\begingroup\$ In exactly the same way that you can save money by shopping \$\endgroup\$ Mar 11, 2021 at 3:40

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can an inverter power a dynamo

If the dynamo has field windings then, yes it could power the dynamo's stator windings and the main power would come from the electric motor and, that arrangement could top-up charge to the batteries. But, you might just as well not use the inverter and instead, directly attach the battery (via the appropriate circuit) to the dynamo field windings. The battery power into the dynamo field windings will be a small fraction of what could be extracted from the dynamo rotor winding to re-charge the battery.

But this then logically leads onto to the dynamo being excited by its own rotor output and, this is perfectly feasible so, a battery connection or an inverter connection is then redundant. The power from the dynamo that is left from it exciting its own windings can then charge the battery that feeds the inverter.

However, if you believe that the electric motor driving the dynamo can also be powered via the inverter from the same battery then that won't work. It can only work if there is a different power source for the motor.

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    \$\begingroup\$ If we're cutting out redundant parts, I'd keep the battery and lose the rest. Just try and charge a battery by connecting it to itself :) \$\endgroup\$
    – Robyn
    Mar 10, 2021 at 9:00
  • \$\begingroup\$ @Robyn I think you are missing the point of what I'm trying to say; if the motor that is spinning the dynamo is independently powered, then it can work. If it isn't independently powered then it won't work. No need to reply. \$\endgroup\$
    – Andy aka
    Mar 10, 2021 at 9:02
  • \$\begingroup\$ TBH for all the use you'd get out of it, the simplest thing is to connect the battery to a load resistor and use it as a heater. All the round-trip losses with motors/dynamos just end up as heat however you engineer the system. \$\endgroup\$
    – Chris H
    Mar 10, 2021 at 9:11
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    \$\begingroup\$ @ChrisH it seems that you are also missing the point of my explanation. \$\endgroup\$
    – Andy aka
    Mar 10, 2021 at 9:13
  • \$\begingroup\$ @Andyaka I still don't think so on re-reading; I follow your reasoning but I'm picking up on your "might just as well not use the inverter " in the context of reading the Q as a perpetual motion system (not the only way to read it I admit). That leads me to the view that you might just as well not use the inverter, motor, or dynamo. \$\endgroup\$
    – Chris H
    Mar 10, 2021 at 9:18
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Hmmm, that looks suspicious:

enter image description here

But this one looks totally legit:

enter image description here

All the "free energy" gimmicks, work the same way:

  1. Start with an impossible goal: extract unlimited energy from something finite.

  2. Add enough complexity to obfuscate the basic facts and make it believable.

This latter step is usually done with lots of magnets, lots of alternators, or marxist theory for example.

Most people understand by intuition that one can't lift off the ground by pulling on their bootstraps, but once you add enough bells and whistles, copper coils and big iron to the exact same thing, the magic happens and it becomes believable.

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    \$\begingroup\$ Yeah, the light wouldn't stay on for very long ;) \$\endgroup\$
    – bobflux
    Mar 9, 2021 at 14:12
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    \$\begingroup\$ @bobflux Depending on the number of loops in the conductor, it may stay on for a while! (See en.wikipedia.org/wiki/Superconducting_magnetic_energy_storage.) \$\endgroup\$ Mar 10, 2021 at 15:37
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    \$\begingroup\$ @Peter-ReinstateMonica Thanks! Yeah, if you add enough bells and whistles, like superconductors and a liquid helium cooler, everyone forgets about the electricity bill for the liquid helium cooler (unless you keep your superconducting loop in deep space, of course, in which case it's legit) \$\endgroup\$
    – bobflux
    Mar 10, 2021 at 15:46
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    \$\begingroup\$ Which reminds me of ITER, by the way. Enough magnetic energy in the superconducting magnets to turn the whole installation into rubble if the supercooled supraconductors get hot, next to gorillion degrees glowing plasma, tons of water-cooled lithium, and enough beryllium to turn the whole region into cancer wasteland. What could possibly go wrong? \$\endgroup\$
    – bobflux
    Mar 10, 2021 at 15:48
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    \$\begingroup\$ pshaw, Marxist theory elucidates where power comes from, not obfuscates \$\endgroup\$
    – llama
    Mar 10, 2021 at 17:40
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Yes it can but that is a pointless exercise. Why? Because you will never get as much output from the generator as you are putting into the motor.

This is an electrical version of a "perpetual motion" machine. It CANNOT work despite all the Youtube videos showing it.

The system will run for a while until the batteries run down and then it will stop and you will have gained nothing.

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    \$\begingroup\$ Worse than that, the charger will actually add additional load to the supply thus the system would have run much longer without the charger. \$\endgroup\$
    – slebetman
    Mar 10, 2021 at 6:50
  • \$\begingroup\$ Not nothing. Rather a noisy room heater whose moving parts wear out or break often. \$\endgroup\$
    – Vorac
    Mar 11, 2021 at 3:41
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Other people may be overthinking their answers to such a basic question.

Yes, you can do this. However, the battery will run down faster than it charges up, no matter what you do. Nobody would bother to actually build this device, because it's pretty useless. If you wanted to run down a battery, you could do it in a simpler way.

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I understand some of these laws of thermodynamics.

No. The understanding of those laws implies that you know that there can be no perpetual motion machine of the first kind - the kind that violates conservation of energy laws, as would need to happen in the contraption you describe. There can be some leeway given maybe for seeing how your circuit would have to waste energy, and other answers cover that. That's where that line of inquiry may well end. You can say that you understand the basics of thermodynamics when you can apply them to decide that a certain machine is a perpetuum mobile and won't work. That's how you can actually test your understanding, and that's how this understanding may be tested in an academic setting as well.

The laws of nature have no regard for inventiveness. If thermodynamically we know that there can be no perpetual motion machine - then it doesn't matter what shape you build it in. It won't work. That's all. Inventiveness is only a disguise used to trick investors who prefer their beliefs over the facts one can learn from a good enough high school physics textbook. It's entirely unnecessary when discussing perpetual motion - in obscures the matter at hand, and usually does so by design - the very intent of the confabulator. That's why perpetual motion machines are made to look complex. The aim is always the same: to get money from someone foolish enough to part with it.

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    \$\begingroup\$ None of this is wrong, but it could be said in a much nicer way. \$\endgroup\$
    – Philipp
    Mar 11, 2021 at 11:49
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It's possible and this is how regenerative breaking works in electric cars, or how flywheels store and release their energy. An electric car going up hill will use a lot of energy from it's batteries, but on the way down it will recharge a good chunk of it (wikipedia says up to 70% efficiency). However if you're in that car then you'll be on the other side of the hill, fulfilling the purpose of a car!

You could of course connect the motor directly to the dynamo (which is also a motor) and use that to power the battery but I'll explain why it's exactly like the car going up and down a hill. If you have an electric motor without anything attached and power it up with a certain voltage, that will make it spin at a certain speed until its magnets induce a voltage in the stator exactly opposite to the driving voltage. The design of each motor and the strength of it's magnets determine the speed and voltage relationship which is typically called "Kv" rating. This characteristic is the same when driving the motor as when using it as a generator. Therefore if you tie 2 identical motors together and drive one of them, you'll see the same voltage or lower in the generator motor (lower because of resistance and magnetic hysteresis losses). Since the driving voltage is given by the battery you won't be able to recharge it unless you boost the generator voltage up, using a "boost converter" for example. A boost converter works by toggling the current through an inductor on an off a few hundred times a second and uses the voltage spike when toggling off to charge the battery. Finally the net effect of this current passing thorough the generator motor is increased resistance, as if moving up a hill, and when the current is off the generator motor picks up speed as if moving down the hill. In practice it's like climbing and descending a hill every 10 microseconds, and of course for each of these cycles you lose some power to heat.

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  • \$\begingroup\$ This explanation is useful although it applies to Permanent Magnet motors only. \$\endgroup\$
    – jpinto3912
    Mar 17, 2021 at 10:50
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An idea that popped up in the comments is too good to not make it an answer that can be referenced in future discussions of perpetual motion:

Asking "can I charge a battery with a charger plugged into an inverter plugged in to the same battery?" is like asking "Can I make money buying and re-selling the same books on ebay over and over again?".

The books resemble the electrons, while ebay and the postal service are the lossy wires and electronics involved in transporting them.

Put in this most simple form, the idea is obvious nonsense; but it's also pretty obvious that obscuring the original lossy circulation by adding more middlemen, going through different addresses and credit cards etc. will, if anything, only make matters worse.

I think this is a very fine analogy. That it doesn't dive into the electronic details is on purpose: The laws of thermodynamics are an emergent property of complex systems and do not depend on the "substrate" they emerge from, which gives them their universality.

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Key to your understanding of this issue is that currents add at a node... so Ia from the charger and Ib to the motor will add (with different signals) at the battery. As charging will have <1 efficiency, it's easy to see why it's not smart to have both flows ON continuously.

But asides the net lower efficiency, if makes all the sense, if used at different times. I.e. in real world in makes no sense to attempt charging while you're taking current from the battery to propel. The smart way is to do it at the instances when you have mechanical energy to spare. That's EV Regenerative Braking.

The electrical machine type you are employing is also key to this question: A lot of electrical motors are easily turned into generators, which is referred to as Motor-Generator. Others are not practical (e.g. because you'd need a dedicated charger), or have very low efficiencies.

For Regenerative braking you avoid the "charger" functional block. You change the excitation biasing conditions of the machine so that you have current flowing back to the battery. EVs with regenerative braking do just that: out-flowing current when accelerating, change bias conditions to get in-flowing current when coasting/braking.

Trains, cars, etc, employ different techniques to change the excitation bias, but generally (series-wound, PM/reluctance & asynchronous) we play with stator/rotor potential, slip speed and angles. These same parameters are modulated across motor speed and load conditions, to optimize efficiency and power.

When you can't easily control the excitation bias would be where a "charger" might be required (adding cost and efficiency losses). Shaded-pole and DC PM-synchronous motors would be examples: Only the rotational speed imposed to the motor-generator governs the amount of Electro-Motive-Force developed, so when connected to a fixed Battery potential this will give you uncontrolled (albeit able to be calculated) currents... might even require DCDC boosting to turn it to a charging flow (more cost and losses). I.e. the charger would be required to modulate the input impedance of the battery pack, as seen from the GM, to attain controlled charging.

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It is not true the power only goes in or out of a battery not both at the same time unless I'm completely oblivious to how a vehicle works because the alternator which charges the battery as it's running goes directly to the battery the rest of the components of the vehicle run directly from the battery therefore the current coming out of the battery is running the objects in the vehicle while the alternator is charging the battery if this is completely wrong please let me know.

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  • \$\begingroup\$ As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center. \$\endgroup\$
    – Community Bot
    Feb 3 at 21:09
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"No. Current either comes out of the battery or goes into it. It can't do both at the same time. – Finbarr" This is the reality of current flow (Kirchoff's Laws or something) so this is the simplest answer. NO! (Amusing question and answers though.)

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  • \$\begingroup\$ of course, you may build it in order to prove the theory is correct \$\endgroup\$
    – user253751
    Mar 11, 2021 at 9:33
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    \$\begingroup\$ I know what you mean, but you assume constant state - if your charger circuit is a square-wave, then essentially it can, especially if you can do this 180 degrees out of phase, so that you draw current, THEN charge the battery, then draw current... Still pointless of course. \$\endgroup\$ Mar 11, 2021 at 13:17
  • \$\begingroup\$ Funny, the first sentence is obviously and trivially wrong (at least for more than a millisecond or so): All current that comes out of a battery goes back in, electron for electron. What you probably mean is energy (you charge a battery or you discharge it), but that is trivially true for all systems. Also, one edge state is that the power that goes in equals the power going out; neither energy nor charges prevent a perpetuum mobile of the third kind (a loss-free machine). For that, you need thermodynamics. \$\endgroup\$ Mar 11, 2021 at 19:57

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