EthosPowerAIA battery is bought at 100% and retired at 80%, and everything about sizing it correctly lives in the twenty points between. IEEE 485 handles those points with a single number, the 1.25 ageing factor, which says that the rated capacity of a lead-acid string must be at least 125% of the load it has to carry at the end of its life. The factor is easy to apply and easy to misunderstand, and the way capacity actually falls over a service life is the thing to understand first.
How lead-acid capacity falls
A flooded cell on a discharge of an hour or longer holds its capacity almost flat for the early years, declines gradually and predictably through mid-life, and then reaches the knee, at about 80% of rated capacity, where the decline accelerates and the cell is on its way out. That is the knee. The knee is the reason the 80% replacement threshold exists; IEEE 450 for flooded cells and IEEE 1188 for valve-regulated ones both recommend replacement there because the margin below it disappears quickly. A valve-regulated cell on a short, high-rate discharge behaves worse than the curve suggests, because its internal resistance rises faster than its capacity falls, its performance at the high rate can drop well below 80% while the long-rate capacity still reads acceptably, and the rate-specific knee arrives earlier than the one in the manufacturer's curve.
What the warranty promises
A typical warranty puts initial capacity at 90 to 100% on delivery and warrants 80% of the published rating for a period that runs from five to twenty years depending on the technology and the price paid. The gap between 90 and 100 at delivery is worth reading twice. A string that arrives at 90% has already spent half its allowance. Some Planté designs are guaranteed at 100% for the whole service life, and a string with that guarantee in writing can be sized at an ageing factor of 1.00 rather than 1.25, which is a quarter of the battery. Verify the guarantee before using it.
Lithium iron phosphate under a lead-acid method
Lithium iron phosphate strings are still sized with IEEE 485, a method written for lead-acid, and the result is conservative rather than wrong. Its capacity falls in a nearly straight line rather than to a knee, it varies less with temperature, it tolerates deeper discharge without ageing faster, and its cycle life runs to 5,000 and beyond where lead-acid runs to hundreds; the solid-electrolyte interphase grows more predictably than lead-acid grid corrosion does. The honest limit is data. Long-term field results for lithium iron phosphate in standby service are still thin, and until more installations reach end of life the 1.25 factor is the defensible number even where it is generous.
A worked case
For a critical load that needs 100 kWh at end of life, a standard or premium valve-regulated string is sized at 125 kWh, a lithium iron phosphate string at 125 kWh on the conservative factor, and a Planté string with a written 100% guarantee at 100 kWh. The same load, three chemistries, a 25% spread in what is bought. The sizing calculator on ebatt.ai carries the factor through the full IEEE 485 method, including the temperature and design margins that sit beside it.
Watch it fall
The sizing assumes a trajectory, and a capacity test each year for a critical installation is how the assumption gets checked: a baseline at commissioning, a trend that shows when 80% will arrive, impedance readings as the cheaper companion indicator between tests, and a record that supports both the warranty claim and the replacement budget. A string that is tested is replaced on schedule. A string that is not is replaced on the day it fails.
For sizing to IEEE 485 use ebatt.ai; for an assessment of a string already in service, start on the contact page.