In the design of battery back up systems for the protection and communication systems of bulk transmission substations, a large contributor to the capacity of the battery required and thus the cost is the requirement that the battery has to be able to not only keep the equipment powered up for the full duration of a loss of AC service, but also has to be able to supply a large momentary load near the end of its designed service time representing a major protection event such as a bus fault followed by a breaker failure during which time the battery must supply high currents to breaker trip coils. This big current draw near when the battery is discharged is really tough on all types of battery systems, and designing for it leads to larger, more expensive battery systems.
There is an alternative, however. This alternative is a combined battery and ultracapacitor UPS. Ultracapacitors can deliver large short term load currents at any time without damage to their longevity and without requiring any additional maintenance, and if working in parallel with a battery, they will reduce the peak current drawn from the battery (because the source impedance of the ultracaps is considerably less), and then will draw current from the battery to recharge over a longer period of time. But the Ultracaps need not be simply connected in parallel for substation protection systems. Rather, they can be charged when the system is at nominal voltage and switched in by protection relays at the time when protection events actually occur which provides the perfect power at the perfect moment. Such a system would therefore separate the continuous load required to keep IEDs and SCADA systems online from the momentary load of protection events, which means smaller, cheaper batteries.