What is a mechanical battery? And why do we still need them over electric batteries?



Basically the title.

In: Engineering

Mechanical batteries are, strictly, any mechanical thing that stores potential energy. Springs, flywheels, etc. they are useful for lots of stuff. Mousetraps are an example of a mechanical battery (a spring in this case) storing the energy needed to swing that trap arm.

Flywheels, in particular, are useful as an energy storage system because you can spin them up and use them to power electrical generation. It’s a weighted and carefully balanced wheel that can be spun up to the required high speeds and, if efficient, can even run vehicles. Look up the Swedish “gyrobus” system some time! Electric vehicles, using the flywheels to power generators, and the flywheels can be spun up to speed by machines at the stops or stations, and power the buses from stop to stop.

I think that you’re referring to chemical batteries when you say “electric battery”. The rest of this post is written under that assumption.

There are lots of ways to store energy, then release it later as electricity. Chemical batteries are popular nowsdays for their portability and relatively high energy density, but they have some problems. The chemicals aren’t good for the environment (and recharging is only a partial solution). They wear out over time, becoming less and less effective. They’re actually relatively heavy for their size, which isn’t a problem when they’re very small but it limits how large you can make them.

*Most* mechanical batteries work at larger scales than chemical batteries. One of the classic examples involves large water tanks at the top of a hill overlooking the river. You charge the batteries by pumping water into the tanks when the river is running high and strong. When the flow ebbs (or at other times when power is scarce), you can release the water slowly, using it to spin electric turbines and generate electricity. You can store and release more energy with this system than you could without chemical batteries, and you can do it without the nasty chemicals. The drawback is that it has to be very large, but you can’t really make chemical batteries large enough for this purpose anyway.

But mechanical batteries *can* be made smaller, up and to a point. [The GravityLight was an example of this](https://deciwatt.global/gravitylight). In this case, you charged the battery by lifting a weight: as the weight slowly fell, it would spin a turbine to generate electricity to power the light for 20 minutes. The goal behind this design was cost and robustness: it didn’t become less effective over time, and it didn’t need new batteries or fuel periodically. The whole system is still larger than a chemical battery, but it could be made “small enough”.

These systems aren’t perfect, but neither are chemical batteries. All technology involves trade-offs of some kind, and depending on the situation, sometimes mechanical batteries beat chemical, and sometimes the reverse is true. It’s all just a matter of figuring out what works best for the job.

A spring is an example mechanical battery as is a flywheel.

In theory any sort of thing that you can store energy in counts as a battery. Lifting something heavy from the ground and putting it on a table counts as storing energy as does stuff like inflating a balloon.

In practice there are many sort of occasions where we might want to store energy for later use without bringing chemical batteries into the mix.

A garage door with a counterweight for example might store the energy that would be released from letting gravity pull it down when closing it, by raising up a counterweight. Opening the door again will go much easier since the energy stored in the counterweight assists you.

You could achieve the same effect by an electric motor that does regenerative braking on the way down, but that would be overkill for something that you can do with little more than a chunk of metal and some pully or levers.

In many places electric batteries have replaced situation where we used to use some arrangement of storing mechanical energy.

Mechanical watches can be charged by turning a small knob to coil a spring. But digital watches are much cheaper to make nowadays.

On a larger scale we are are moving towards mechanical batteries again.

One of the big problem with green energy is that the sun doesn’t shine at night and the wind doesn’t always blow. You need to find ways to store huge amounts of energy produced when there is sun or wind for later when there isn’t.

Our current electrical grid works basically in a way so that all electricity gets consumed as soon as it is produced.

Chemical batteries are expensive to make and not very environmentally friendly.

One way around this is to create ways to store energy for later use and that mostly takes the form of mechanical batteries.

The most common form is pumped storage where electricity is sued to pump water up a hill and later let the water run down again to drive a generator.

Another more short term solution are flywheels, which aren’t suited for storing energy for long times but can even things out in the short term.