Homemade batteries

Make a battery

by Joe Miller

Ten batteries you can build from fruit, coins, salt water, mud and the ground itself, and everything you need to know about the batteries you already own: which kind to buy, how to make them last, and which ones can hurt you.

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.

How a battery works

How every battery worksLOAD← electrons flow through the wire+++ions move through the liquidNegative electrode (anode)The "fuel" metal, like zinc. Itslowly dissolves and gives upelectrons.ElectrolyteSalt water, acid, juice or pastethat lets ions move between theelectrodes.Positive electrode (cathode)Copper, carbon or air. It takes inthe electrons that flowed throughthe wire.VoltageSet by which two materials you pair.Size sets how long it lasts and howmuch current.Two different metals in an electrolyte make a cell. Wire cells together to make a battery.
Every battery, from a lemon to a Tesla pack, has these same parts.

Ten batteries you can make

Ten batteries you can make, from easiest to most involvedBatteryVolts per cellCurrentEffort1Lemon0.9 V2Potato0.8 V3Coin stack0.7 V/pair4Salt-water jar0.9 V5Aluminum-air1.0 V6Earth battery0.6 V7Vinegar jar0.5 V8Daniell cell1.1 V9Magnesium cell1.5 V10Mud (microbial)0.4 VCurrent bars: more bars, more power. Effort dots: more dots, more materials or care needed.
From a 15-minute lemon to a living mud battery.

1. Lemon battery

0.9 V · Easy, 15 minutes

2. Potato battery

0.8 to 0.9 V · Easy, 15 minutes

3. Coin stack (voltaic pile)

about 0.7 V per coin pair · Easy, 30 minutes

4. Salt-water jar cell

0.9 to 1.0 V · Easy, 20 minutes

5. Aluminum-air cell

about 1.0 V · Medium, 30 minutes

6. Earth battery

0.5 to 0.9 V · Medium, an afternoon

7. Vinegar jar ("Baghdad battery")

about 0.5 V (iron) or 0.9 V (zinc) · Easy, 20 minutes

8. Daniell cell (gravity cell)

1.1 V, steady · Harder: uses a poisonous chemical

9. Magnesium salt-water cell

1.4 to 1.6 V · Medium, 20 minutes

10. Mud battery (microbial fuel cell)

0.3 to 0.7 V · Easy to build, a few days to start

How long each battery lasts

Homemade batteries make very small amounts of power, measured in thousandths of an amp (mA). They run an LED, a clock or a small sensor, not a phone or a light bulb. The less you draw from them, the longer they last.

BatteryLasts aboutWhat ends it
1. Lemon battery2 to 4 days, until the lemon dries outThe lemon dries out
2. Potato batteryAbout a week; boiled potatoes give more power but spoil soonerThe potato spoils (boiled ones give more power but spoil sooner)
3. Coin stack (voltaic pile)A few hours, until the paper dries; re-wet it to restartThe soaked paper dries; re-wet it to restart
4. Salt-water jar cellWeeks to years, depending on the load (see the bucket battery)Depends on the load; the zinc is slowly used up
5. Aluminum-air cellHours to a day, until the foil is eaten throughThe foil is eaten through
6. Earth batteryMonths; it keeps going as long as the soil stays dampKeeps going while the soil stays damp
7. Vinegar jar ("Baghdad battery")Days to weeksThe vinegar weakens and the metals corrode
8. Daniell cell (gravity cell)Weeks with careThe chemicals need topping up
9. Magnesium salt-water cellWeeks; replace the magnesium as it wears awayThe magnesium wears away; replace it
10. Mud battery (microbial fuel cell)Weeks to months, fed by bacteriaRuns as long as the bacteria in the mud stay alive
5-gallon bucket batteryUp to decades: about 35 years at 8.6 mA, about 300 years at 1 mAThe 4 kg zinc plate is slowly used up; needs water topped up and sludge scraped

Getting enough voltage

Series adds volts; parallel adds current and lifeSeries: + to −1 V+1 V+1 V+1 V + 1 V + 1 V = 3 VSame current. Use this to reach an LED's 2 to 3 volts.Parallel: + to +, − to −1 V+1 V+1 V+Still 1 V, three times the currentand three times as long at the same load.Homemade cells give about 0.5 to 1.6 volts each and only a few milliamps: plenty for an LED, clock or calculator, not a phone.
Wire homemade cells in series to reach the 2 to 3 volts an LED needs.

Know your batteries

The 5-gallon bucket battery

Can it really last 35 years? The math, the upkeep and the catch.

Battery types

Alkaline, lithium, NiMH, lead-acid, LiFePO4: what each is for.

Maintenance and storage

Make every battery last longer: charging, storage, terminals, water.

Which batteries are dangerous

Lithium fires, button cells and children, acid and gas.