Chemistry and sulfation of lead acid batteries

The simplest way to explain the chemistry of lead acid batteries is by referring to the change in the specific gravity of the electrolyte. The specific gravity of the electrolyte is also the most reliable parameter for determining the state of charge of a battery.

An electrolyte is a mixture of distilled water and sulphuric acid, and the specific gravity of the electrolyte is the ratio between sulphuric acid and distilled water in the electrolyte.

In the absence of external impact on the specific gravity of the electrolyte, it changes depending on the state of charge of a battery, that is, the battery charge level.

The specific gravity in a completely charged battery is 1.285 kg/dm3, whereas in an empty battery it is 1.150 kg/m3 (at +200C)

This means that the share of sulphuric acid in the electrolyte varies according to the state of charge of a battery.

In practice, the specific gravity of the electrolyte is simply determined using a barometer, refractometer, etc.

When a battery discharges, the sulphuric acid from the electrolyte binds with the lead from the electrodes to form lead sulphate molecules (PbSO4) which appear as a layer of crystals on the electrodes. This creates a lack of sulphuric acid in the electrolyte manifested as reduced specific gravity.

The creation of a lead sulphate layer on the electrodes is called primary or daily sulfation.

During charging, the PbSO4 molecule decomposes into the SO4 group (so-called acid residue) and lead. This process is called primary desulfation.

When acid residue binds with 2 atoms of hydrogen to form again a sulphuric acid molecule H2SO, an increase in the specific gravity of the electrolyte is observed. A lead atom from the lead sulphate molecule falls into the sedimentation tank and builds up a so-called brown sludge or conductive sediment at the bottom of the battery.

Therefore, sulfation is a completely natural occurrence which is reversed when the battery is charged again.


Supplementary charging of the battery, SULPHATE STRATIFICATION

For a number of reasons, in everyday practice, you can rarely see a battery being completely charged or discharged in regular time intervals.

Battery users are often forced to resort to supplementary charging. Moreover, there are problems with self-discharge as well as charger issues.

In practice, charging almost never results in a completely charged battery. The duration of such charging often depends on the duration of the break or another interruption at work. This is in fact incomplete charging.

The chemical-physical aspect of this condition can be described as follows:

Part of the SO4 is on the plates in the form of PbSO4, and part of it is in the electrolyte in the form of sulphuric acid.

If we start the charging process now, we actually started the process of increasing the specific gravity of the electrolyte, that is, the process of decomposition of PbSOinto the SO4 group and lead.

If we were to charge a battery to 1.285 kg/dm3, we would actually transform all the PbSOfrom the electrodes into its constituents.

However, in practice, this is almost never the case.

By charging the battery for a short time, we actually transform only part of the PbSO4 into its constituents, leaving most of it on the electrodes.

Now, we restart the battery discharging process, that is, the process of reducing the specific gravity, that is, the process of crystallizing of the PbSO4 on the electrodes. The problem here is that this new layer actually builds on the previous layer thus increasing the sulphate layer. This is called LEAD SULPHATE STRATIFICATION.

This occurrence is harmful for a number of reasons:

  • it decreases the active electrode surface which makes it impossible to charge the battery to its nominal capacity.
  • it increases the internal resistance (impedance) inside the battery which increases the amount of heat lost during the charging of the battery.

Bojan Fort Mlakar, BSc (El. Eng.)