Lead batteries

The lead battery was developed during the nineteenth century, and its operation is well understood. Two electrodes of lead and lead oxide (PbO2) are inserted into an electrolyte made up of dilute sulphuric acid. When the two electrodes are connected to an external appliance consuming current, they are converted to lead sulphate (PbSO4) and the acid is diluted, a property that enables the state of charge of the battery to be measured by checking the specific gravity of the electrolyte. When a reverse current is applied to the system, the acid re-concentrates and the two electrodes return to their initial states (Figure 5.1).

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Figure 5.1 Diagram of a lead-acid battery and chemical reactions (here during discharge)

Positive

electrode

Acid Negative

electrode

Positive

electrode

Water Negative

electrode

Charge
PbO2 2H2SO4 Pb PbSO4 2H2O PbSO4
Discharge

At the positive electrode:

PbO2 + H2SO4 + 2H+ + 2e – $ PbSO4 + 2H2O (5.1)

At the negative electrode:

Pb + H2 SO4 $ PbSO4 + 2H+ + 2e – (5.2)

The nominal equilibrium potential is the sum of the potentials at the two electrodes:

E + (PbSP4/PbO2) = 1.7 V and

E – (PbSO4/Pb) = -0.3 V; that is, E = 2.0 V (5.3)

We will not go further here into the details of the electrochemical reactions of electrodes and the origin of the 2 V potential, but will give some more general and practical information; the reader wishing to get more deeply into these questions can refer to the excellent article on lead-acid batteries by Dr D. Berndt.1

The positive electrode is made of brown lead dioxide and the negative elec­trode of spongiform grey lead. During charging, lead dioxide forms on the anode, while the cathode is transformed into pure lead and the sulphuric acid becomes more concentrated. During discharging, a part of the electrolyte binds to the lead and transforms it into PbSO4; this reaction produces water, which reduces the density of the electrolyte. This variation in density can be calculated as the ratio of the charge/discharge: for each Ah of discharge, 3.654 g of acid binds to lead and 0.672 g of water is produced, with the charge producing the same values in reverse. This variation in specific gravity provides an easy way of checking the state of charge of open batteries.

The variation in specific gravity of the electrolyte has another important effect that limits the use of batteries in low temperatures, because the freezing point rises as the battery discharges.

D. Berndt, ‘Valve-regulated lead-acid batteries’, Journal of Power Sources, 2001;100:29-46.

Each cell of a lead battery supplies an average voltage of 2 V, and the appropriate number of elements is assembled in series or parallel to produce the desired voltage and current. Small capacities are often met by using batteries of 6 or 12 V (three or six cells in series), while larger requirements use multiples of 2 V elements connected in series and in parallel, and may reach several thousand Ah.

Updated: March 18, 2020 — 7:33 pm