What type of battery is generally fitted in a UPS?
A battery is an electrochemical device that stores energy at one time for use at another. The energy is stored in chemical form and converted to electrical form during discharge. Batteries come in all sizes and shapes and are used to power everything from flashlights to space probes and satellites. There are only a few types used for UPS battery banks.
The UPS battery may furnish power to the inverter for a few seconds, many minutes, or hours. The battery capacity is determined by the amount of power the inverter and its load require during that time.
The cell / block is the basic building brick for a battery. Combined cells / blocks in series sums their individual output voltage and delivers the same current as a single cell/ block at the increased voltage. Paralleling cells / blocks delivers the sum of their currents at the voltage of a single cell / block. UPS system battery banks are parallel – series combinations that deliver from a few amperes to several thousand amperes.
A cell contains a positive element, a negative element and a liquid / gel solution called electrolyte. The cell may be sealed or vented. Sealed cells are commonly referred to as maintenance free if periodic addition of water is not required. There are basically two types of batteries – Nickel Cadmium and Lead Acid – which can be utilised in a UPS system. The type refers to the plate material.
NICAD – The nickel cadmium cell or NICAD battery is best used in low voltage applications and / or applications which need the ability to withstand hostile environments. The NICAD battery is considerably more expensive than the Lead Acid battery and, in an environmentally conscious world, is costly to dispose of after use.
LEAD ACID – Lead Acid Batteries are by far the most common type of battery used in today’s modern UPS system. Lead Acid batteries can be categorised under three separate headings.
Wet Cell – This is the most basic lead acid battery. This type of battery vents gases during charging and discharging and requires periodic addition of water to the electrolyte.
Maintenance Free – This is a wet cell lead acid battery designed to eliminate the need to add water during its useful life.
Sealed Maintenance Free / Valve Regulated – This is, by far, the most common type of lead acid battery utilised in UPS systems. The term sealed is somewhat misleading, as it means that the battery vents only if the cell is abused by overcharging or high temperature operation. The vents open only under excessive internal pressure. By nature of the design a valve regulated battery effectively controls generation of gases and allows gas recombination within the battery of over 99% of the gases generated in normal usage. The batteries are provided with a safe low pressure venting system designed to release excess gas and reseal automatically. Normally this sealing technique should ensure that no electrolyte leakage can occur from the terminals / posts or case of the battery.
For all types maintenance consists primarily of equalising charging and water replenishment. As the name implies Maintenance Free Batteries require no water be added during their operational life. However, today battery manufacturers prefer to describe this type of battery as Low Maintenance as to ensure battery design life is achieved batteries should be checked periodically to ensure that an equal charge is being applied across the battery pack and battery interconnects are tight.