№ 72 · electrical

Size buys hours, chemistry buys volts

A battery is a chemical reaction with a wall down the middle. One electrode gives up electrons, the other takes them, and the material between them is a poor road for electrons. They leave by the wire instead, and that detour is the whole device.

A reaction held apart

Take a reaction that hands electrons from one substance to another and split it. One half runs at one electrode, the other half at the other, an electrolyte between them. Electrons freed on the giving side cannot cross that gap, so they travel out through the metal, around the outside circuit doing work, and back in at the other electrode. Section 17.5 draws the two half-cells and the bridge between them; the electron-blocking picture is standard cell chemistry rather than its wording. The Daniell cell, among the first successful batteries, bridged its two halves with a porous clay pot in the 1904 design the source shows.

For a dry cell, chemistry buys volts, material buys hours

The dry cell shows the split cleanly. The same components built at four sizes — D, C, AA and AAA — give the same voltage in every size. Size changes something else: a larger cell holds more of the reacting material, so it can transfer a correspondingly greater amount of charge. For a dry cell of one chemistry, the volts come from it and the hours from how much you packed in. A small high-voltage pack and a fat low-voltage cell can both be the right answer to the same job.

Interactive Grow the cell material slider and watch the cell get bigger while its voltage stays put: what grows is how long the lamp burns. Then stack cells in series and watch only the volts move — the chemistry in each cell never changes.

cell material 1.00× cells in series 1 chemistry

Above, one cell. The can is the anode and holds the reacting material, drawn as the dots inside it, so the slider packs more of them in and the can grows. The voltage written beside it does not move, because the voltage comes from the chemistry and not from the size — the source states this for dry cells of every size, and the widget applies the same rule to whichever chemistry you pick. The drawing is schematic: a lithium-ion cell is not built as a can with a graphite rod, and only the voltage rule is borrowed for it. That 40 s holds whatever the chemistry you pick, which is a convention of this figure rather than a claim that chemistry buys no energy — the alkaline cell below buys three to five times as much at the same size. The bar on the right is the charge left in the cell while it runs: a cell holding one unit of material lights this lamp for 40 seconds (a stated convention for the figure, not a measurement), and five times the material lights it five times as long. The drain is drawn at eight times speed, so you do not have to wait. Below, cells in series: each cell keeps its own chemistry and its own voltage, and the stack simply adds them, which is how a stack of cells reaches a voltage that no single cell in it has. Reduced-motion readers get a step button that advances the discharge a fixed chunk per press instead of running it.

Inside a dry cell

The dry cell uses a zinc can that is both container and anode, the “–” terminal, and a graphite rod as the cathode, the “+” terminal. The can is packed with an electrolyte paste of manganese(IV) oxide, zinc(II) chloride, ammonium chloride and water, and the rod sits in the paste to complete the cell. Zinc is oxidised and manganese(IV) is reduced. The reaction does not conveniently run backwards, so the cell is a primary one: spent, then replaced.

The one that bites back

Alkaline batteries were developed in the 1950s as improved replacements, built around the same redox couples with an alkaline electrolyte, often potassium hydroxide, and can deliver about three to five times the energy of a zinc-carbon dry cell of similar size. Most are not rechargeable, and trying to recharge one that is not often ruptures it and leaks that electrolyte; it also leaks in storage, so cells are worth removing from anything left idle long. Where a cell must be recharged, lithium ion is among the most popular types, used in many portable devices, and its voltage varies with the cell reaction, sitting near 3.7 V in typical conditions.

Stacking cells in a row

When one cell's voltage is not enough, cells go in series: connected end to end, their voltages add, and every cell keeps the chemistry it started with. The lead-acid battery in a car does exactly this — each cell produces 2 V, and six in a row give 12 V; what a starter motor needs is high current. As a worked example, four dry cells in series give 4 × 1.5 V ≈ 6 V, and none of the four has changed its chemistry.

In short

A battery is a redox reaction held apart, so its electrons must take the long way round. For one chemistry the reaction fixes the voltage, whatever the cell's size, and the material fixes how long it lasts; and chemistry buys more than volts, as the alkaline cell above shows. Stacking cells in series lifts the voltage without touching the chemistry, and a secondary cell runs its reaction backwards when an outside source pushes current the other way.

Where this comes from

Chemistry 2e, §17.5 Batteries and Fuel Cells linked only, not reproduced
Flowers, Theopold, Langley & Robinson · 2019
openstax.org/books/chemistry-2e/pages/17-5-batteries-and-fuel-cells