EthosPowerAIA battery is bought at 100% of its capacity and retired at 80%. Sizing it correctly depends on what happens between those two figures. IEEE 485 handles this with a single number, the 1.25 ageing factor. It 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. To use it well, first understand how capacity falls over a service life.
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. It declines gradually and predictably through mid-life. At about 80% of rated capacity it reaches the knee, where the decline accelerates and the cell is near the end of its life. 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. The knee for that rate 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. It warrants 80% of the published rating for a period that runs from five to twenty years, depending on the technology and the price paid. Note the range at delivery: a string that arrives at 90% has already spent half its allowance. Some Planté designs are guaranteed at 100% for the whole service life. A string with that guarantee in writing can be sized at an ageing factor of 1.00 instead of 1.25, and the difference is a quarter of the battery. Verify the guarantee before you use 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. The result is conservative, not wrong. Lithium iron phosphate capacity falls in a nearly straight line, with no knee. It varies less with temperature, and it tolerates deeper discharge without ageing faster. Its cycle life runs to 5,000 and beyond, where lead-acid runs to hundreds. Its solid-electrolyte interphase grows more predictably than lead-acid grid corrosion does.
The limit is data. Long-term field results for lithium iron phosphate in standby service are still thin. Until more installations reach end of life, 1.25 is the defensible factor, even where it is generous.
A worked case
Take 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 is sized at 125 kWh on the conservative factor. A Planté string with a written 100% guarantee is sized at 100 kWh. The load is the same, and across the three chemistries there is 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 that capacity will fall in a certain way, and testing checks that assumption. For a critical installation, run a capacity test each year. Take a baseline at commissioning and keep a trend that shows when 80% will arrive. Use impedance readings as the cheaper indicator between tests. Keep 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 tested 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.