From 2GreenEnergy Intern Fabio Porcu: Lead Acid Batteries

Lead cells were the first type of secondary batteries, invented by French physicist Gaston Plante in 1859. In the following 150+ years they have been refined considerably, and are currently used in many applications including, most notably “SLI” (starting, lighting and ignition) in automobiles, but also telecommunications, traction, and energy storage. They occupy about 70% of the world accumulator market. In addition, the recent rapid development of lithium ion batteries is causing further technological development and improved manufacturing processes for lead-acid batteries.

The positive electrode is composed of lead peroxide PbO₂, where the negative electrode is pure lead, and the electrolyte is sulfuric acid H₂SO₄. During discharge, the anode undergoes an oxidation reaction of the metal lead by Pb₂⁺, which combines with ions of the electrolytic solution and becomes PbSO4 as seen from the following half-reaction:

Pb + SO₄^—⇄ PbSO₄ + 2e^– with ⇄ : charge and discharge

On the positive electrode there is the reduction of lead peroxide with lead in the state Pb₄⁺⁺ which becomes PbSO₄. The reaction is the following:

PbO₂ + 2e⁻ + H₂SO₄ + 2H⁺ ⇄ PbSO₄ + 2H₂O

The global reaction is:

Pb + PbO₂ + 2H₂SO₄ ⇄ 2PbSO₄ + 2H₂O

During the discharge process in the electrode, both in the positive and in the negative there is a production of PbSO₄. In the charging process there is then the opposite, turning the Pb in the electrode negative and in PbO₂ in the positive. During the discharge there is a consumption of the electrolyte, which leads to a reduction of its density. When the battery discharges, the lead sulfate tends to aggregate in crystals. If the battery is not recharged as soon as possible, crystals are formed within it and the battery loses capacity.

The nominal voltage of the cell, generated by the normal electrochemical reaction described above, is about 2.2V. Normally batteries are used, composed by 3 or 6 cells in series. In this way we create voltages of 6V or 12V respectively. The tension in the discharge can fall to as low as 1.6V per cell and during charging may rise above 2.4V per cell. The charging process must be performed carefully, in fact, since above 2.2V there is electrolysis of water with undesirable development of hydrogen and oxygen.

There are several problems relating to lead-acid batteries, the most obvious is that lead is a toxic material, which caused the world to make regulations related to this technology. Today the path for disposal is very good, in that car batteries are the most recycled product on the planet.

However, as mentioned above, they are the most common storage batteries, for the following main reasons:

Good energy efficiency, more than 70%
Good performance when used with high levels of discharge
Wide range in operating temperatures ranging from -40◦C to 60◦C;
High single-cell voltage
Different options for indicating the state of charge;
Easy to produce in large quantities;
Many varieties have low or no maintenance requirements;
Recyclability;
Low cost

The disadvantages of this type of accumulator are:

Useful life is relatively short, typically about 500 cycles;
Low specific energy, typically 30-40 Wh / kg;
Risk of irreversible damage due to sulfation;
The batteries, in addition to the acid, contain arsenic and antimony, which are quite toxic;
Significant difficulties in the production of batteries of small capacity;
High short circuit currents that can irreversibly damage the battery.

Traction batteries:

This type of battery is used in electric vehicles where there are requirements to manage deep discharge cycles and to have good relations of specific energy. In this application it is necessary to keep the weight of the battery under control, because if the battery is too heavy the cars will need much more energy to move. For this reason the battery used in the cars has a high specific energy relative to weight.

SLI battery:

The purpose of this type of battery is to provide a large quantity of current in a short time, which is what is required to start the engine of a car. In these batteries the level of self-discharge ranges from 20% to 30% of their capacity per month.

Stationary battery:

These batteries are designed to have a long useful life and are usually kept charged constantly by a trickle charge. Their level of self-discharge varies from 1% to 3% per month. Among the applications are different UPS systems, telecommunications systems and energy storage.

VRLA battery:

The term VRLA means ‘valve regulated lead acid’; these are batteries for which there will be no maintenance. In fact, normally, the lead-acid batteries must be constantly kept under control, for example, the requirement to “top” the water.

There are several variants, and this type can therefore be applied to all the categories set out above, though they are mostly for industrial and stationary use. The level of self-discharge rate varies from 2% to 3% of its charge per month.

Lead acid batteries have reached the maximum of their potential development and for this reason most research has shifted to other technologies such as NaS batteries and the lithium polymer. Yet, despite the future prospect of these newer battery chemistries, lead acid will remain for a long while because of the characteristics mentioned above.

2GreenEnergy Intern Fabio Porcu, Lead Acid Batteries, starting lighting and ignition, traction batteries