Bucket battery
The 5-gallon bucket battery
by Joe Miller
A big copper-zinc salt-water cell can run a clock or a sensor for decades, but only at a very small draw and with a little upkeep. Here is how to build it and the honest math.
Safety first. Homemade cells are low-voltage and low-risk, but wear safety glasses, wash your hands, keep chemicals and small parts away from children and pets, and never eat the fruit or vegetables you used. Never try to build or rebuild lithium or lead-acid batteries at home, and never try to recharge a battery that isn't made to be recharged.
Build it
- Food-grade 5-gallon bucket with a loose lid (drill a small vent hole).
- Fill with water and about 1 1/4 cups of salt (roughly sea-water strength, 3%).
- Hang a thick zinc plate (about 4 kg or 9 pounds; zinc anodes are sold for boats) and a large copper plate on opposite sides, held apart by plastic spacers.
- Make the wire connections above the water line and coat them with grease.
- One bucket gives about 0.9 to 1.0 volts open and 0.5 to 0.8 volts under load. Most devices need two or three buckets in series, or a small step-up (boost) circuit.
Supplies, where to get them, and cost
| Supply | Where to get it | About |
|---|---|---|
| Food-grade 5-gallon bucket with lid | Hardware store (Home Depot, Lowe's) or Walmart | $5 to $8 |
| Salt, about 1 1/4 cups (plain salt or water-softener salt) | Grocery store, or hardware store water-softener aisle | $2 to $6 |
| Zinc plate or boat zinc anode, about 9 lb (4 kg) | Marine supply store (West Marine), boat dealers, or online (Amazon, eBay) | $40 to $100 |
| Copper sheet or roof flashing, about 12 x 24 inches | Hardware store roofing aisle, roofing supplier, or craft copper sheet | $30 to $60 |
| Plastic spacers, zip ties and bolts | Hardware store | $5 |
| Copper wire, ring terminals and dielectric grease | Hardware or auto parts store | $10 to $15 |
| Small step-up (boost) converter, 0.9 V to 3 to 5 V (optional) | Amazon or electronics stores | $5 to $10 |
| Digital multimeter (optional, reusable) | Harbor Freight, Walmart or hardware store | $10 to $25 |
Total for one bucket: about $95 to $200, mostly the zinc and copper. Two or three buckets in series multiply the cost. Prices are typical 2026 U.S. prices and vary by store and metal prices.
How long it lasts
| Average draw | 4 kg zinc lasts about | Notes |
|---|---|---|
| 1 mA | 300 years | Something else wears out first |
| 5 mA | 60 years | |
| 8.6 mA | 35 years | The 35-year mark |
| 50 mA | 6 years in theory | The copper can't keep up; voltage sags |
Why
- The zinc is the fuel. Each amp-hour dissolves about 1.2 grams of zinc. Zinc holds about 820 amp-hours per kilogram, about 85% usable, so 4 kg gives roughly 2,650 amp-hours.
- The copper limits the current. It isn't used up, but it needs dissolved oxygen from the water. In still water that holds the steady current to roughly 10 milliamps or less.
- Upkeep decides the real life: evaporation (top up with water), zinc sludge on the bottom (scrape every few years), and corroded connections (keep them dry and greased).
Want more power? Use more plate area (several copper sheets), stir or bubble air through the water, or use the magnesium cell for 1.5 volts. For real off-grid power, see a LiFePO4 battery and solar panel at solarcritical.com.
Sources
Checked November 11, 2026.