Batteries are the main short term power storage of most UPS systems.
“How long do they last?” is a leading question, as it can refer to either:
Both, of course, are in the case of a utility mains failure.
First, let’s define the battery. In this instance we are talking about a VRLA (Valve Regulated Lead Acid) battery.
In the case of a UPS or emergency lighting unit the term battery relates to the complete battery string, which can consist of multiple battery cells. A collection of cells housed in an individual container is termed a battery block, or just block. The most common battery blocks consist of either 3 or 6 cells, and as each cell has a voltage of between 2 and 2.25 volts, depending upon its state of charge, we consequently have the 6V and 12V battery block.
The battery block can be safely (meaning that when the block is recharged it will not suffer damage) discharged to a value of 1.63V per cell, depending upon the time it is expected to supply current for. If it is required to continue to supply current for a long period of time, as in the case of an emergency lighting unit which needs to supply its load for either 1 or 3 hours, it is generally accepted that it is not wise to discharge the battery block beyond 1.75V per cell to avoid damaging the battery block and reducing its operational life.
Battery autonomy: defined as how long will the battery support the load for.
The capacity of a battery to deliver power is measured by the Ampere Hour (Ah) of the battery. This is the total number of Amperes (measure of current) that the battery can supply for one hour, at which point the cell voltage will be between 1.63 and 1.85V, depending upon the application. Therefore a 100Ah battery will discharge at 100A for 1 hour.
There is, of course, a sting in the tail. If a battery is fully discharged, its capacity to supply the rated current is diminished. Typically, a battery block is designed to be able to supply 100 full discharge cycles. In the case of a UPS or emergency lighting application this very rarely happens during the designed operating life of the battery.
The battery design-life is defined as the period of time until the battery can only deliver 80% of its rated capacity, typically 3 – 5 or 7 – 10 years.
Expect to replace 5-year design-life batteries after 3.5 – 4 years.
Expect to replace 10-year design-life batteries after 7.5 – 8 years.
NOTE: Never expect the design-life of a battery to be the ‘REAL’ life of a battery.
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