The electrochemical reaction of the lead acid battery involves lead,
lead dioxide and an aqueous solution of sulfuric acid.

The electrode reactions are:

On the positive electrode	PbO2 + HSO-4+3H++ 2e-	discharge --> charge	PbSO4+ 2H2O
On the negative electrode	Pb + HSO-4	discharge --> charge	PbSO4+ H+ + 2e-
Overall cell reaction	Pb+PbO2+2H++2HSO-4	discharge --> charge	2PbSO4 + 2H2O

The free reaction enthalpy is (at 25 °C): The delta of G=-372.2 kJ (standard)
The reversible voltage is H₂SO₄ concentration/activity dependent.

Typically a H₂SO₄ concentration of 28 % (3.4 mol/l) to 42 % (5.6 mol/l) with a density of 1.20 to 1.32 g/cm3 (20 °C) is present in a fully charged battery. This electrolyte freezes between -30 °C to -70 °C. Dilute solutions start freezing from -0.5 °C.

The standard reversible voltage (a_H⁺; a _HS04^- = 1 mol/l) is:

U₀=- delta of G / n · F = 372.2 / 2 · 96500 = 1.928 Volt

The mass of reactants per unit reaction are:

207.2g Pb +239.2g PbO₂+2g H⁺ + 194g HSO₄^-=642.4g PbSO₄ +36 g H₂O

The theoretical specific power of the lead/lead dioxide/sulfuric acid couple is:

(53.61 Ah)(1.928V / 0.6424 kg )= 160.9 Wh / kg

The practical energy density is however only in the range of 15 to 45 Wh/kg due to limitations in active mass utilizations and the weight of electrochemically inert accessories (container, grids etc.) as shown below.

source: "Blei-Akkumulatoren", VHB

As a side reaction of lead acid battery operation water electrolysis occurs.
In this reaction a 1 Ah overcharging current decomposes 0.336 g of water forming 0.4 l of hydrogen and 0.2 l of oxygen gas. This mixture is explosive in the H₂ concentration range of 4 to 94 vol% and has to be disposed off safely.

VRLA type lead acid batteries emit, due to their particular operating mode, significantly less hydrogen than flooded, i.e., conventional battery designs.