Two different metals and a little vinegar make electricity. Not enough to charge a phone, but enough to light a lamp in the dark, and with nothing, that's already a lot.
UNDERSTAND
A battery is two different metals that don't touch, with an electrolyte between them: a liquid or paste that carries charge, salt water, vinegar, lemon juice. The metal most "eager" to dissolve (zinc, aluminium, magnesium) gives up electrons, which travel through the wire to the other side. See Batteries (single-use cells).
The voltage depends on the pair of metals, not on size:
| Pair | Voltage per cell, roughly |
|---|---|
| Zinc and copper | 0.8 to 1 V |
| Aluminium and charcoal (aluminium-air cell) | 0.7 to 1.2 V |
| Magnesium and copper | 1.5 to 2 V |
| Iron and copper | 0.3 to 0.5 V |
The current depends on the surface area of the metals and the quality of the electrolyte. The bigger, the more it delivers.
For more voltage, you put several cells in series: the copper of one joined to the zinc of the next. That's exactly what Volta did in 1800 with his "pile" of stacked discs, which gave its name to all the batteries after it.
The reality, to know before you start: a lemon gives about 0.9 volts and less than a milliamp. A phone needs 5 volts and about one amp to charge. You'd need thousands of lemons. A makeshift battery is for an LED, a calculator, a clock, a small signal, not for charging a device.
ACT
The coin battery (the most practical)
You need: copper or copper-coloured coins (1, 2 and 5 euro cent coins are copper-plated steel and work; so do old copper pennies), galvanised washers (grey, zinc-coated, from the hardware aisle), cardboard, salt or vinegar.
- Cut rounds of cardboard or kitchen paper slightly smaller than the coins.
- Soak them in very salty water or vinegar, then let them drip: damp, not dripping, otherwise the liquid bridges the cells and everything stops.
- Stack: coin, cardboard, washer, coin, cardboard, washer… Each trio gives about 0.5 to 0.8 V.
- Six to eight layers are enough for a red LED; ten to twelve for a white one.
- The bottom wire touches the first coin (plus), the top wire the last washer (minus). The LED goes the right way round: long leg towards the copper. See LEDs.
- Hold the stack together with a rubber band or inside a piece of tube.
The aluminium-air cell (the one that delivers most)
- A sheet of kitchen foil laid flat.
- On top, kitchen paper soaked in saturated salt water.
- On top, a layer of charcoal crushed to powder, moistened with the same salt water (activated carbon from a filter works best).
- Press a copper wire, a pencil lead or a metal plate onto the charcoal, without touching the foil: that's plus. The foil is minus.
A single hand-sized cell can give about one volt and a few tens of milliamps. Two or three in series light an LED for several hours, until the aluminium is eaten away.
The fruit or vegetable battery
A galvanised nail and a coin or copper wire pushed into a lemon, a potato or a jar of sauerkraut, two or three centimetres apart. Three or four in series make a red LED glow faintly. Boiling the potato first makes a clear difference.
ADAPT
You have no multimeter. Test each cell on your tongue, the way old electricians tested a 9 V battery: a slight tingle means there's voltage. Only on these small homemade cells, never on anything else. Better: see Le multimètre.
The LED won't light. Turn it round. Then check that no liquid runs from one cell to the next. Then add cells.
You really want energy. Homemade cells aren't the answer. Salvage battery cells from a laptop (Salvaging a laptop), a scooter or a cordless tool, and recharge them from the sun or a dynamo: see Powering a circuit without the grid.
You want to listen to the radio. A crystal set needs no battery at all: the energy comes from the radio wave itself. See Building a radio receiver.
Homemade cells run down when the zinc or aluminium is eaten away: replace the washer or the foil, and re-soak the cardboard. They can be rebuilt forever, as long as there's metal.