Required battery capacity from a DC duty cycle, with temperature, ageing and design margins applied. The plant DC system is what holds up protection, tripping and emergency lubrication when AC is gone.
Standard method — stated scope limits apply
Duty Cycle
Factors
Ageing factor is normally about 1.25, since batteries are sized to still perform at end of life (typically 80% of rated capacity).
Define the duty cycle and calculate.
About This Calculation
A DC battery system — for switchgear tripping/closing, emergency lighting, or UPS backup — must be sized for the actual DUTY CYCLE it will see during an outage, not just the peak or average load in isolation: a typical substation battery duty cycle includes a large momentary current spike (breaker closing/tripping) superimposed on a smaller continuous standing load (relay/DCS power, indication) sustained for the full required backup time. IEEE 485 provides the standard method for translating that duty cycle, plus aging and temperature derating, into the number of battery cells and their required ampere-hour capacity.
Frequently Asked Questions
Why does battery sizing need the full duty cycle, not just peak current?
A battery sized only for the momentary peak (breaker close/trip current) could be far smaller than what’s needed to sustain the continuous standing load for the full backup duration — and vice versa, sizing only for the steady load could leave insufficient capacity for the peak. IEEE 485’s duty-cycle method captures both correctly by working through each time period of the cycle in sequence.
Why derate for both aging and temperature separately?
They act through different, additive mechanisms: aging reduces a battery’s available capacity over its service life even under ideal conditions (a common design allowance is sizing for 100% capacity only needed at end-of-life, meaning it starts oversized), while low temperature reduces the electrochemical reaction rate and available capacity at any given point in the battery’s life, independent of age.
What is the difference between a VRLA and a vented (flooded) lead-acid battery for this application?
VRLA (valve-regulated, "sealed") batteries need less maintenance and no separate battery room ventilation for hydrogen, but typically have a shorter design life and lower tolerance for deep discharge or high temperature than vented lead-acid — vented batteries remain common for critical substation applications specifically for their longer life and more predictable aging.
Why size for the WORST-case duty cycle rather than a typical one?
The battery’s entire purpose is to be available during an actual outage, which by definition is not a routine event — sizing for anything less than the worst realistic combination of events (e.g. a fault requiring a trip near the end of the design backup time) risks the exact scenario the battery exists to cover.