A Quick Illustration Of Why Perpetual Motion With EVs Doesn’t Work

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There’s a funny picture that keeps popping up on social media: a Chevy Bolt EV that someone attached an alternator to the back bumper of. What turns the alternator? A belt driven by the rear wheel. We don’t know for sure why they did this, as there could be some good reason we can’t think of, but it sure looks like they were trying to make a perpetual motion machine. After all, if the car can turn a generator (or alternator) to charge the battery up as it goes, the EV should be able to go forever, right? Or, at the very least, it should go further, shouldn’t it?

It turns out that people have been trying to make machines like this for hundreds of years. The US Patent and Trademark office got so sick of having crackpots apply for patents for perpetual motion machines that they hired wacky “inventor” Rube Goldberg to show why they don’t accept such patents anymore.

The problem? You just can’t get something for nothing. The laws of physics keep you from being able to do it. When a machine gets energy from somewhere, it can’t run forever without getting more energy to keep it resisting things like friction and the work you’re trying to get the machine to do.

But what about electrical devices? There’s no friction in them, right? So I decided to run a little test to see if I could come up with a perpetual motion machine.

I started with the Oukitel P501 power station I recently reviewed. It has a battery that powers plugs on the front, including a 120-volt US electrical outlet that can put out up to 500 watts. It also has a power input port that you can use to charge the power station using something like a solar panel or the included AC plug to charge it from a standard US electrical outlet. So, I decided to see if I could charge the power station using its own power from the plug! I mean, if it could charge itself up, I could power other things from it!

To my shock and dismay (not really, because I knew this wouldn’t work, but play along with me for a bit), I found that the charger for the power station was drawing 125 watts from the power station, but only putting 118 watts back in! Shouldn’t it be putting in at least what it took out?

But then I thought about what all is going on here. To get power from the DC batteries and make AC power, the power station has a power inverter, and it loses a little bit of power doing that. Then, the charging adapter turns that power back into DC power, which then is used to charge the batteries in the unit, which converts the electrical energy into chemical energy. Then, the batteries convert the chemical energy into electrical energy to run the inverter, and so on and so forth as long as I’m stupid and keep this plugged in this way.

Every time the power is converted or changed in any way, some of it is lost as heat energy, or waste heat. I’m surprised that this whole circle of stacking inefficiencies only wasted 7 watts, to be honest. The display is probably not factoring in all that was lost in this transaction.

This is just like the EV we keep seeing in the funny social media posts. It’s converting chemical energy from the battery into electrical energy, which gets turned into mechanical energy. Then, it’s converting mechanical energy into electrical energy, and then back into chemical energy. At every step of this process, energy is lost, so the whole contraption only wastes energy and reduces range.

The same is true for anything else that’s similar. Regenerative braking can’t capture all of the energy back and put it into the battery, because some is lost, for example. But some EV owners will argue until doomsday that regenerative braking is 100% efficient and is somehow a form of perpetual motion.

However, the truth remains — you can’t get somethin’ for nothin’.

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Jennifer Sensiba

Jennifer Sensiba is a long time efficient vehicle enthusiast, writer, and photographer. She grew up around a transmission shop, and has been experimenting with vehicle efficiency since she was 16 and drove a Pontiac Fiero. She likes to get off the beaten path in her "Bolt EAV" and any other EVs she can get behind the wheel or handlebars of with her wife and kids. You can find her on Twitter here, Facebook here, and YouTube here.

Jennifer Sensiba has 1868 posts and counting. See all posts by Jennifer Sensiba