Keep the network reliable, from the substation to the last SWER line

They supply the DC auxiliary system, protection relays, control and tripping circuits, SCADA and RTU, communications, alarms and indication, so that protection can still operate and the substation can still be controlled when the AC supply is lost. This is the “tripping battery” or “station battery”, and it is the reason a substation can clear a fault during a total AC failure. Loss of the DC supply means loss of protection and control of the high-voltage assets, which is why these banks are among the most closely managed low-value assets on a network.

Ten hours is the published Australian benchmark. AusNet’s asset management strategy states DC systems “have been designed to maintain DC supply to 10 hours after AC interruption for system black or ZSS black purpose.” For comparison across the utility sector, Sydney Water’s HV installation specification requires a minimum of four hours plus the capacity to complete two full operations of the installation within it, sized by the IEEE 485 method with a 10% design margin and a 25% ageing factor, which is the exact formulation Australian utility specifiers use.

Bank voltages across Australian distribution fleets span 6 V to 250 V DC, with 125 V DC the most common and capacities up to around 400 Ah, per AusNet’s published fleet data. Legacy 32 V and 48 V systems are common, and Sydney Water’s current specification nominates 48 V DC for new installations with 32 V and 110 V retained only for existing sites. Standardising is worth doing for spares, testing equipment and technician familiarity, but it is an asset-replacement programme rather than a retrofit, the DC distribution, protection settings and equipment input ranges all follow the bank voltage.

Two published Australian policies give you defensible numbers. AusNet: useful life is typically 12 years while capacity remains above 80% of rated, but “may be substantially less” where the environment is not temperature controlled, as low as 10 years. Over 40% of their battery population is 10 years or older, with the oldest recorded at 24 years. Powerlink: nominal replacement age of 12 years for 2 V cells and 8 years for 12 V monoblocs, based on impedance testing and historical fault data, with a six-monthly inspection and test regime. The monobloc figure being a third shorter is a genuinely useful input to a format decision.

This is the real problem, and AusNet states it plainly: full discharge testing and discharge-rate testing are “impractical on zone substations with a single battery bank”, which “prevent[s] accurate measurement of the DC system available capacity.” Their response is a new standard requiring individual battery cell monitoring of voltage and impedance plus SCADA alarm monitoring, with research directed at estimating remaining useful life. The honest position: impedance monitoring is a trend indicator, not a capacity measurement, it catches open cells, dry-out and connection degradation reliably, and correlates with capacity fade, but it does not replace a discharge test. A second bank, or a portable load bank programme, is the only way to get a direct measurement.

Yes, and in the direction most people do not expect. AusNet’s asset strategy notes that the absence of monitoring “may void the DC cell warranty as the manufacturer requires proof that the DC cell was operated in a controlled regime.” Monitoring data functions as the evidence file for a warranty claim. Related point worth checking on your charger fleet: AusNet reports around half of its chargers have temperature control functionality, while the current standard requires temperature-compensated charging, a charger without it may itself be the reason a claim is declined.