When should a substation battery bank be replaced?

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.