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A battery that you can charge, store and use again seems ordinary today. In 1860 Gaston Planté showed the Paris Academy of Sciences one made of lead sheets and acid. This post covers what he presented in 1860, using his own report, what happens inside a lead-acid cell, a worked example with Faraday's laws, and what a student can take from the way he worked.
What was Planté working on?
Planté's notes appear in the Paris Academy's Comptes rendus for 1860, volume 50, which you can read on the Internet Archive. At the session of 19 March 1860 he sent a note proposing lead electrodes instead of platinum ones for "secondary currents", which a Monsieur Jacobi had recently proposed for electric telegraphs. A secondary current is the reverse current that flows from a cell after it has been charged by another battery.
A week later, on 26 March 1860, he presented "Nouvelle pile secondaire d'une grande puissance", a new secondary battery of great power. He reported that lead electrodes in acidified water gave a reverse electromotive force of close to 1.5 times that of a Bunsen cell, a standard cell of the time. He put this down to the strong affinity of lead peroxide, PbO₂, for hydrogen.
Little reliable detail about Planté's life is easy to find in authoritative sources, so this post stays with his work, which he described carefully.
What did he show the Academy?
His battery had nine cells with a total surface of 10 square metres. Each cell was two long, wide sheets of lead, rolled into a spiral, separated by coarse cloth and dipped in water acidified with sulfuric acid. Rolling thin sheets let a very large surface fit in a small space: the whole set sat in a box 36 centimetres on each side.
Charged for a few minutes by five small Bunsen cells, it gave a spark "of extraordinary intensity" when the circuit was closed. In his words, translated from the French, the apparatus "plays exactly the role of a condenser, since it lets one collect, in an instant, the work done by the battery over a certain time." To get a similar discharge directly, he estimated, you would need more than 300 ordinary Bunsen cells. Sealed jars, he added, could be kept charged in a physics room, ready to use.
What happens inside a lead-acid cell?
The NCERT Class 12 chapter on electrochemistry describes the modern lead storage battery, used in cars and inverters. It has a lead anode and a lead grid packed with lead dioxide as the cathode, in a 38% solution of sulfuric acid. During use:
Anode: Pb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻
Cathode: PbO₂(s) + SO₄²⁻(aq) + 4H⁺(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l)
Overall: Pb(s) + PbO₂(s) + 2H₂SO₄(aq) → 2PbSO₄(s) + 2H₂O(l)
On charging, the reaction runs backwards: lead sulfate on the two plates turns back into Pb and PbO₂. In Planté's design both plates start as plain lead; charging coats one of them with the lead peroxide, PbO₂, that his note refers to. Our post on batteries and fuel cells compares this with other cells.
Worked example: how much lead does one ampere-hour use?
Use F = 96487 C mol⁻¹, the value NCERT gives, with Pb = 207.2 g/mol, PbO₂ = 239.2 g/mol and H₂SO₄ = 98.1 g/mol.
Charge in 1 ampere-hour: Q = I × t = 1 A × 3600 s = 3600 C.
Moles of electrons: 3600 ÷ 96487 = 0.0373 mol.
Each Pb atom gives 2 electrons, so Pb used = 0.0373 ÷ 2 = 0.0187 mol, or 0.0187 × 207.2 = 3.87 g.
The same 0.0187 mol of PbO₂ reacts at the cathode: 0.0187 × 239.2 = 4.46 g.
The overall equation uses 2 mol H₂SO₄ per 2 electrons, that is 1 mol per mole of electrons: 0.0373 × 98.1 = 3.66 g of acid.
For a sample battery rated at 40 ampere-hours, each cell would turn about 40 × 3.87 = 155 g of lead into lead sulfate on a full discharge.
Common mistakes:
Using n = 1. Two electrons move for every Pb atom, so n = 2.
Forgetting the acid is used up. Water forms and sulfuric acid is consumed, so the acid gets weaker as the battery runs down.
Swapping anode and cathode on charging. During charging the cell is electrolytic, and the electrode roles reverse.
For more practice like this, see our guide to Faraday's laws of electrolysis. The Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer, which helps when you are unsure which mole ratio to use.
Where does Planté fit in battery history?
Volta's pile of 1800, told in our post on Volta and the first battery, made a current from two metals and a liquid. Planté's cell was built to be charged from another source and used again. The next big change in rechargeable batteries came much later with lithium, covered in our post on John Goodenough.
What can a student take from his way of working?
Swap one material and measure. Replacing platinum with lead, and comparing the result against a standard cell, was the whole discovery.
Think about geometry. Rolling the sheets packed 10 square metres of surface into a small box.
Describe what it is for. He explained his battery as a store that collects slow work and releases it fast.
Try it now: work out how many grams of lead a 150 ampere-hour inverter battery cell converts on a full discharge.
