Silly question about a closed energy circuit

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Tharg

Joined Dec 8, 2016
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I'm certainly a novice in the area of circuits and physics, but somewhat interested. I was pondering the idea of a closed energy circuit. My thought would be applied to vehicles. Is it possible to have a car with two batteries. One battery powering the car's wheel, the other in reserve. A dynamo is attached to the wheel charging the reserve battery. When the battery that is powering the wheel is expended the reserve battery takes over, it is switched over to powering the wheel and the spent battery is now switched to the dynamo to be charged. The cycle would seemingly continue endlessly. The only fail to this idea is the efficiency of the system that would gradually diminish the cycle, no wasted energy; and the dynamo must charge at the same rate as the battery discharge to keep it balanced. I'm sure there are a few general laws physics this violates, hence my query :)
 

wayneh

Joined Sep 9, 2010
16,400
You're right, the laws of thermodynamics tell us that such a device – a perpetual motion machine – is never possible. You could build it and it would work, but it would exhaust both batteries fairly soon. A rule of thumb is that the charging and discharging a battery is only ~50% efficient, end-to-end. So by the time your first battery is depleted, at best only 50% is now stored in the second battery. (At best, because obviously a lot of that energy was used to move the vehicle and was not recovered.) When that is gone, there is only 25% put back into the first battery. And so on.
 

Kermit2

Joined Feb 5, 2010
4,162
The generator you imagine charging the second battery. Where is the "work" to drive that generator coming from?
From the other battery running the motor maybe?
So you pull energy out of the battery to run the motor, but you have to turn the generator too. That takes more energy than just running the electric motor.
There is NO free energy to charge a second battery, you have to take it from the one running the motor!
 

OBW0549

Joined Mar 2, 2015
3,566
Is it possible to...
No.

Even if the conversion efficiencies between electrical and mechanical energy were 100% in both directions, you wouldn't be accomplishing anything more than what you could accomplish simply by hard-wiring the two batteries together.

Just get a bigger battery.
 

Thread Starter

Tharg

Joined Dec 8, 2016
4
These replies make complete sense. I knew there was some sort of down-to-earth variables involved.

So to be clear, it's not possible to have a dynamo produce enough power to charge a battery as fast as it can be drained? Conversely, two batteries cannot discharge and charge at the same rate?
 

wayneh

Joined Sep 9, 2010
16,400
Correct.

Modern electric vehicles DO capture a portion of the rolling energy when they brake. It's called regenerative braking and uses a generator to convert rolling energy into stored battery energy. You only need one battery to use this. In stop and go, city driving, this can provide a substantial boost to efficiency compared to traditional braking which just generates heat at the brake pads.

This is utterly useless on the highway, when you're rarely using your brakes. All the energy from the battery is expended in fighting air drag and rolling resistance losses. There's nothing "extra" to recover.
 

Thread Starter

Tharg

Joined Dec 8, 2016
4
No.

Even if the conversion efficiencies between electrical and mechanical energy were 100% in both directions, you wouldn't be accomplishing anything more than what you could accomplish simply by hard-wiring the two batteries together.

Just get a bigger battery.
So, a single battery being drained and charged at the same time would not be any different than one battery in use, the other in reserve being charged with a switch/toggling the batteries back and forth?
 

OBW0549

Joined Mar 2, 2015
3,566
So, a single battery being drained and charged at the same time would not be any different than one battery in use, the other in reserve being charged with a switch/toggling the batteries back and forth?
I'll be more clear: the scheme you proposed at the start of this thread DOES NOT WORK.
 

Thread Starter

Tharg

Joined Dec 8, 2016
4
Correct.

Modern electric vehicles DO capture a portion of the rolling energy when they brake. It's called regenerative braking and uses a generator to convert rolling energy into stored battery energy. You only need one battery to use this. In stop and go, city driving, this can provide a substantial boost to efficiency compared to traditional braking which just generates heat at the brake pads.

This is utterly useless on the highway, when you're rarely using your brakes. All the energy from the battery is expended in fighting air drag and rolling resistance losses. There's nothing "extra" to recover.

The dynamo I was thinking of was similar to the type you would attached to the side of a bicycle tire, the rotation of the car's wheel would run the dynamo which in turn would resupply the power source, instead of powering a lamp on the bicycle.

I'll be more clear: the scheme you proposed at the start of this thread DOES NOT WORK.
I understand that, I am just considering the why, that's all. No need to be testy, if I wanted to be yelled at I would go to a political website.

Thanks "wayneh" and "Kermit2" I appreciate your feedback, it was very informative, sorry for any interruption.
 
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wayneh

Joined Sep 9, 2010
16,400
The why is because of the myriad ways that energy escapes and cannot be recovered. You have to move air out of the way, deform the elastic tires as they roll, release heat in the bearings, and on and on. The dynamo places a drag on the wheel and requires maybe 12W extra from the bike rider to produce only 6W of lighting. Every time you do anything with energy, some of it will escape.
 

MrSoftware

Joined Oct 29, 2013
1,935
The setup that you're describing with 2 batteries, a motor and a generator, would travel significantly less distance than 2 batteries and a motor, minutes the generator, if both setups started with the same battery capacity. All the generator is going to do is lose energy to heat, it's not going to get anything back.

Regenerative braking, i.e. using the generator to stop the vehicle, simply recovers kinetic energy that would have been otherwise lost to heat in the brakes, and sends it to the battery. You do recover some energy, but you recover a lot less energy than it took to initially get the car up to speed in the first place.
 

BR-549

Joined Sep 22, 2013
4,938
Put a weal small wheel on the generator...........that way, we can get more out than we put in.

Super-perpetual energy. Think Yuge.
 

WBahn

Joined Mar 31, 2012
26,301
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