Nobody chose 2 volts. No committee sat down and decided a lead-acid cell should read 2.04 volts at equilibrium, or a lithium iron phosphate cell about 3.2. Those numbers were fixed when the universe set the energies of chemical bonds; battery engineers merely discovered them. This article is the short version of that discovery — the electrochemistry every battery datasheet silently rests on. It asks a little arithmetic of you and repays it with something valuable: the ability to smell impossible claims instantly.
A battery is a controlled redox reaction
Every galvanic cell earns its living the same way: one electrode is oxidised — it gives up electrons — while the other is reduced, accepting them. Separate the two half-reactions physically, connect them with an electrolyte that carries ions inside and a circuit that carries electrons outside, and the chemistry is forced to pay its energy through your motor on the way to completion. Discharge runs the reaction forward; in a rechargeable cell, driving current backwards runs the chemistry backwards — a battery on charge is, strictly, an electrolytic cell. In the lead-acid cell, spongy lead is oxidised at the negative while lead dioxide is reduced at the positive — lead in the +4 state accepting electrons down to +2 — and both products converge on lead sulphate — the double-sulphate reaction, proposed by Gladstone and Tribe back in 1881 and defended by them against a decade of rivals until the evidence settled it: PbO₂ + Pb + 2H₂SO₄ ⇄ 2PbSO₄ + 2H₂O. The full mechanism, including the acid’s unusual double life as reactant and electrolyte, lives in the chemical-reaction guide.
Each electrode brings a potential; the cell is their difference
Every half-reaction has a characteristic electrode potential — its electron pressure, measured against the standard hydrogen electrode’s zero. Crucially, a potential is an INTENSIVE property: it does not care how much material is present. A flashlight lead-acid cell and a two-tonne substation cell read the same voltage, because voltage measures the chemistry’s intensity while capacity measures its quantity — the water-tank analogy holds: voltage is the water level’s height, current is the pipe’s bore. Cell voltage is simply the positive electrode’s potential minus the negative’s. For lead-acid: about +1.69 V at the lead-dioxide plate and −0.36 V at the lead plate, so E ≈ 1.69 − (−0.36) ≈ 2.05 V. Run the same subtraction for nickel-cadmium and you get its familiar ~1.3 V; for lithium iron phosphate, ~3.2 V; for the cobalt-oxide lithium chemistries, ~3.6–3.7 V. Series strings then multiply the cell voltage into working banks — the string rules take it from there.
Thermodynamics signs the certificate
The deeper law behind those potentials is Gibbs free energy — the maximum work a reaction can deliver. The bridge equation is ΔG = −nFE: free-energy change equals electrons-per-reaction times Faraday’s constant times voltage. Faraday’s constant — 96,485 coulombs, or a beautifully memorable 26.8 ampere-hours per mole of electrons — is the exchange rate between chemistry and electricity. Feed the tabulated formation energies of the lead-acid reactants into the ledger (the classic treatment is Bode’s Lead-Acid Batteries, 1977) and the arithmetic returns ΔG ≈ −394 kJ per mole; divide by two electrons and Faraday’s constant and out falls E° ≈ 2.04 V. The same bookkeeping predicts how much active material a cell must carry: one ampere-hour costs about 3.87 grams of sponge lead at the negative and 4.46 grams of lead dioxide at the positive — figures every plate designer in our plant works from, straight from the same thermodynamic ledger.
Theoretical energy — and the honest gap below it
Add up the masses the reaction equation demands — lead dioxide, lead, acid, 0.642 kg per two-electron mole — and the theoretical specific energy of the lead-acid couple computes to roughly 170 watt-hours per kilogram: 26.8 × (2 × 2.04 ÷ 0.642). Cross-checked by independent methods in the literature, the number holds. Real cells deliver a fraction of it — grids, connectors, separators, containers, water and unused acid all board the scale without contributing ampere-hours, and every chemistry pays this same packaging tax (a lithium pack gives up a meaningful share of its cell-level figure the same way). Where each chemistry honestly lands in practice is the business of the chemistry comparison. The theoretical number’s real use is as a ceiling: any claim that edges near it — let alone past it — has violated thermodynamics, and thermodynamics does not lose.
Why a battery buyer should care
Because the physics above is a fraud detector. A cell’s voltage is set by its couple — so a “12 V battery” is always six lead-acid cells, and a seller vague about cell count is vague about everything. Capacity scales with active-material mass — so two batteries of equal rating and very different weight cannot both be telling the truth. And energy per kilogram has a hard ceiling per chemistry — so a brochure outrunning its own chemistry’s ceiling is fiction in a nice font. The glossary for every term used here lives in the battery-terms guide; the working consequences fill the rest of our knowledge library. And if you’d rather hand the thermodynamics to people who do it daily, that is literally our job.