Gravity Fall: a Mechanical Battery That Converts Gravity Into Electricity

What if you could store energy by lifting a weight?

Gravity Fall is a gravitational energy storage system — a mechanical battery. Lift a 15.65 kg mass to 1.80 meters, release it, and it generates electricity as it descends through a chain-driven transmission.

What this project achieves:

1. 13W peak electrical output
2. 58% energy conversion efficiency
3. Stable, controlled descent in 9.5 seconds
4. 5V USB output for device charging

What this project does NOT do:

1. It takes 394 cycles to fully charge an iPhone 16. This is a proof of concept that validates the physics, not a replacement for your wall charger.

Why build it? Gravity is free, constant, and doesn't degrade. Unlike lithium batteries that lose capacity over time, a mass can be lifted and dropped thousands of times with zero degradation. This project explores whether that principle can be turned into usable electricity at small scale. The answer: yes, at 58% efficiency.

All design files (FreeCAD), experimental data, and documentation are open source on GitHub: [INSERT LINK]

Cost: ~$7,000 MXN (~$400 USD) for DIY replication Time: 2-3 weekends Difficulty: Intermediate

To see more files and learn more about the project click here https://github.com/valeriamayara22-eng/Gravity-Fall?tab=MIT-1-ov-file

MECHANICAL:

1. Wooden beams for the frame (or commission a carpenter — we did)
2. Bicycle chain
3. Large sprocket (~48 teeth)
4. Small sprocket (~5-6 teeth) — for the 1:9 ratio
5. Steel axles (12mm diameter)
6. Bearings (6201-2RS, 12mm bore) x4
7. Mass container (bag, bucket, or custom)
8. Weights to reach 15.65 kg
9. Mounting hardware (bolts, nuts, brackets)

ELECTRICAL:

1. Hub motor (36V, multi-pole)
2. Neodymium magnets (for magnetic brake)
3. MPPT charge controller
4. LiFePO4 battery (12V, 7Ah — we used Howell Energy)
5. DC-DC buck converter (12V to 5V)
6. USB-C breakout board
7. Wiring (14-16 AWG)

TOOLS:

1. Wrench set
2. Screwdriver
3. Soldering iron
4. Multimeter
5. Drill

ESTIMATED DIY COST: ~$7,000 MXN (~$400 USD)

Note: Our prototype cost $15,000-$20,000 MXN because we commissioned professional carpentry and CNC machining for the frame and mounting discs. The FreeCAD files are on GitHub if you want to get them fabricated, or you can build the frame yourself and 3D print the mounting parts.

Before building, you need to understand the energy path:

1. GRAVITATIONAL POTENTIAL ENERGY — A mass at height stores energy (Ug = mgh). Our mass: 15.65 kg at 1.80 m = 276.3 J per cycle.
2. MECHANICAL TRANSMISSION — As the mass descends, it pulls a chain connected to a sprocket system with a 1:9 gear ratio. This multiplies the slow descent speed into faster rotation at the motor.
3. ELECTRICAL GENERATION — A multi-pole hub motor converts the rotational energy into AC electricity. A magnetic braking system controls the descent speed to prevent acceleration.
4. ENERGY MANAGEMENT — An MPPT charge controller regulates the motor's variable output into stable charging current for a 12V LiFePO4 battery.
5. OUTPUT — A DC-DC buck converter steps the 12V battery output down to 5V USB for device charging.

The key challenge isn't generating electricity from gravity — that's basic physics. The challenge is controlling the descent speed while maximizing power extraction. That's what the magnetic brake and MPPT controller solve.

The frame needs to be:

1. Tall enough for a meaningful drop (we used 1.80 m total height)
2. Rigid enough to handle the mass without vibration
3. Perfectly vertical — any tilt causes the mass to swing

Build a rectangular tower using wooden beams. Use bolts, not nails — you need structural rigidity. The mass will be pulling on the top of the frame during descent, so the joints need to hold.

CRITICAL DIMENSION: Not all of your frame height is usable. The top section holds the transmission, and the bottom needs clearance. Our effective drop was 1.39 m out of 1.80 m — that's 41 cm of dead space. Minimize this. Every centimeter of height is gravitational potential energy you're leaving on the table.

If you have access to the FreeCAD files from our GitHub, the frame_structure.FCStd file has the exact dimensions we used.

This is the heart of the mechanical system.

Mount the large sprocket on the descent axle (where the mass pulls). Mount the small sprocket on the motor axle. Connect them with the chain.

The gear ratio matters enormously. We started with 1:5 — it wasn't enough. The motor needs higher RPM to generate useful voltage, and a falling mass moves slowly. The jump to 1:9 was the single most impactful design change we made.

To verify your ratio: rotate the large sprocket one full turn by hand. Count how many times the small sprocket spins. It should be approximately 9.

Chain tension is important. Too loose and it skips teeth. Too tight and you add friction. Chain friction accounted for about 10% of our energy losses — replacing chains with timing belts is our top recommended improvement.

The hub motor is what converts rotation into electricity. We used a 36V multi-pole hub motor — the kind used in electric bicycles. The multi-pole design is critical because it generates meaningful voltage even at low RPM. A standard DC motor wouldn't work here.

The magnetic brake is what makes this system controllable. Without it, gravity accelerates the mass and the motor spins too fast, producing voltage spikes that damage electronics.

Position neodymium magnets near the motor's rotating element. The magnets create eddy currents that oppose the rotation — braking without physical contact. Adjust the distance between magnets and rotor to control braking force:

1. Closer = more braking = slower descent
2. Farther = less braking = faster descent

TARGET: The mass should descend smoothly in approximately 9-10 seconds. If it drops in 3 seconds, add braking. If it barely moves, reduce braking.

The fact that our angular velocity stabilized at 15.65 kg (varying only 0.01 rad/s) proves this braking system works. It's the result we're most proud of.

The wiring path:

Hub Motor (AC output) → MPPT Controller → LiFePO4 Battery (12V) → DC-DC Converter → USB-C (5V)

1. Connect the hub motor output wires to the MPPT controller input.
2. Connect the MPPT output to the LiFePO4 battery (watch polarity).
3. Connect battery output to the DC-DC buck converter.
4. Set the DC-DC output to 5V using the adjustment potentiometer. Verify with a multimeter.
5. Wire the 5V output to a USB-C breakout board.

SAFETY:

1. Always include a fuse between the battery and the DC-DC converter
2. Verify all connections with a multimeter before first test
3. LiFePO4 is safe chemistry but never short-circuit the terminals
4. Never exceed 14.6V charging voltage on the battery

The MPPT controller is what makes the system stable. "Maximum Power Point Tracking" continuously adjusts the electrical load to extract maximum power from the motor at any speed. Without it, you get inconsistent voltage and poor efficiency.

This is where the project goes from "cool build" to "engineering."

WHAT TO MEASURE:

1. Voltage at motor output, battery, and USB (multimeter)
2. Fall time for each test (stopwatch or phone timer)
3. Mass of the weight (scale)

WHAT TO CALCULATE:

1. Gravitational PE: Ug = m × g × h (Joules)
2. Electrical energy: Power × time (Joules)
3. Efficiency: (Electrical out / Gravitational PE in) × 100%

TEST PROTOCOL:

1. Start with your lightest mass. Record voltage, fall time.
2. Add 2-3 kg. Record again.
3. Repeat with at least 3 different masses.
4. Plot your results.

OUR RESULTS: Second image.

The most surprising finding: mass increased 56% but power jumped 155%. At low loads, friction dominates. At higher loads, the motor reaches its optimal efficiency range. This non-linear relationship is the most valuable data point from the entire project.

WHAT WORKS:

1. Gravitational PE can be converted to usable electricity at 58% efficiency
2. Magnetic braking successfully controls descent speed
3. MPPT regulation provides stable energy output
4. The system optimizes under load — heavier is better (within structural limits)

WHAT DOESN'T (YET):

1. 394 cycles to charge a phone is not practical for daily use
2. 41 cm of dead space wastes ~23% of potential height
3. Chain friction costs 10% of generated energy
4. Manual mass lifting is not scalable

WHAT'S NEXT:

1. Reduce dead space by compacting the transmission design
2. Replace chains with timing belts for lower friction
3. Automate mass return with a winch or pulley system
4. Test with masses beyond 15.65 kg to map the full efficiency curve
5. Explore multi-mass systems for near-continuous power output

If you build your own version, share your results. I want to see if someone can beat 58% efficiency.

https://github.com/valeriamayara22-eng/Gravity-Fall?tab=MIT-1-ov-file