2012

Battery Options for Uninterruptible Power Supplies

The lead-acid battery was invented by the French physicist Gaston Planté in 1859, and is one of the oldest rechargeable battery technologies. For over 150 years, it has been the mainstay when a high-energy-capacity battery was required, and represents 70% of the secondary (rechargeable) batteries used worldwide. Even today, with the advent of higher-power-density and low-weight batteries, the lead-acid battery in all of its forms is the most commonly used battery type.

altIt is the mainstay of the Uninterruptible Power Supply (UPS) and automobile industries. This is primarily due to its high power capabilities and very low cost. Lead-acid batteries are available in various design topologies, such as flooded SLI, traction and stationary types, Valve Regulated Lead- Acid (VRLA or SLA), Absorbed Glass Mat (AGM), and Gel Cell recombination types.

The UPS industry uses flooded SLI, stationary types, and their sealed counterpart, the VRLA battery. Due to the dangers associated with spillage, and the hydrogen gas emissions associated with flooded batteries, they are primarily used with large, stationary UPS systems in permanent, fixed locations.

Charge and Discharge

The basic flooded cell design consists of a positive electrode made of lead dioxide, and a negative electrode constructed from a highly porous sponge lead. Both electrodes are continuously submerged on a solution of water and sulfuric acid. Antimony is added to the electrodes to increase the battery’s performance. The following details the charge and discharge electrochemistry of the battery.

The charging process is accomplished by the forcible removal of electrons from the negative electrode, and the forcible introduction of them to the positive electrode.

Negative electrode reaction:
PbSO4(s) + H+(aq) + 2-e → Pb(s) + HSO-4(aq)

Positive electrode reaction:
PbSO4(s) + 2H2O(l) → PbO2(s) + HSO-4(aq) + 3H+(aq) + 2-e

As the cell approaches a fully charged state, the majority of the PbSO4 has been converted to Pb or PbO2, and the cell voltage then increases to the gassing voltage of approximately 2.39 Volts. Beyond this state of charge, an overcharge reaction begins, resulting in the production of hydrogen and oxygen. Internal battery water is also lost due to it being converted into hydrogen and oxygen and expelled from the battery. If substantial overcharging occurs, positive electrode corrosion and even plate buckling can occur, resulting in permanent battery damage.

The discharge process is caused by the conduction of electrons from the positive electrode back into the cell at the negative electrode.

Negative electrode reaction:
PbSO4(s) + H+(aq) + 2-e → Pb(s) + HSO-4(aq)

Positive electrode reaction:
PbSO4(s) + 2H2O(l) → PbO2(s) + HSO-4(aq) + 3H+(aq) + 2-e

altThe nominal voltage of a single cell in a lead-acid battery is 2 Volts. For example, a 12-Volt car battery is constructed from six cells. When discharged below an 80% depth of discharge (1.7 Vdc per cell), cell damage can result. The resulting destructive process is referred to as excessive sulfation, or the crystallization of lead sulfate. Basically, lead is taken into the electrolyte solution, crystallized, and coats the battery’s plates (electrodes).

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