We are doing a Fire Alarm Design using a manufacturer's Battery Calculation software and it is asking for us to input a Battery Derating value. Can anyone explain what this is?
Hi Bruce - "Derating" is the term the manufacturer is using to describe the safety margin that must be added to the standby battery calculation. In NFPA 72 (2013 Edition), 10.6.7.2.1(1) states "Battery calculations shall include a 20 percent safety margin to the calculated amp-hour rating." This "derating" is no doubt to cover for variations in battery performance over their lifetime. I understand that under some low temperature conditions 20% derating may not be enough, so the manufacturer allows us to adjust this value. See also 1.2.3 in NFPA 72. I hope that this helps.
Per IEEE Stationary Battery Committee (see IEEE 1188) 80% capacity is considered the end of battery service life. Consequently 20% is added to offset this natural degradation of battery capacity. The multiplication factor is actually 1.25 not 1.2 as published in a lot of the fire panel manufacturer's instructions (e.g. 100 % capacity / 1.2 = 83.33% capacity but 100 % capacity 1.25 = 80 % capacity). Battery manufacturers conform to IEEE 1188. Additionally IEEE 485 speaks to battery sizing including adjustments for temperature.
Right on Herbert.
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