Eighty percent. That is the state-of-health figure at which high-performance electric vehicles declare a battery pack end-of-life — not because the cells stop working, but because they stop meeting the application's requirements. In battery energy storage systems, cells are typically rated EOL at 70% SoH and whole systems at 60%. The gap between those numbers, and the reasons the industry never runs packs down to 0% SoH, comes down to four degradation mechanisms that compound as a battery ages.

SoH is calculated simply: existing capacity in ampere-hours divided by nominal capacity, times 100%. End-of-life is the SoH value at which the battery is no longer fit for its intended application — a definition that varies by use case. A power-tool battery reaches EOL when it can no longer deliver the high discharge current the tool demands; the system simply cuts out on voltage drop or an overtemperature event. An EV battery reaches EOL when range loss, slower fast charging and weaker regenerative braking fall below what the vehicle and its driver expect.

Internal resistance sets the first hard limit. As cells age, internal resistance climbs. Depending on the application, EOL is declared when ACIR reaches 1.5x to 2x its original value. Higher resistance means more heat generation inside the pack, lower round-trip efficiency and increased auxiliary power draw for cooling. It also degrades C-rate capability — the capacity for fast charging and high-power discharge. This is why performance EVs draw the line at 80% SoH rather than squeezing out remaining capacity.

The voltage profile drops before capacity runs out. Aging cells sit at a lower voltage across their discharge curve. That drop can trigger the undervoltage cut-off mechanism prematurely, shutting the battery down even while the cells still hold usable ampere-hours. The energy is there; the system cannot reach it.

Uneven degradation is the decisive factor. Beyond a certain SoH, cells in a pack no longer degrade evenly. A cell that sits just 4% lower in SoH than its neighbours limits the entire pack: the other cells cannot fully charge or fully discharge around it. The lowest-capacity cell dictates the energy the system delivers. This, according to the analysis, is the key reason cell-level and system-level EOL thresholds are defined at all.

Cell EOL and system EOL are not the same. BESS projects illustrate the distinction: cells rated EOL at 70% SoH, systems at 60%. The battery management system can balance cells to a degree, and operators avoid running depth of discharge right up to 100%, which buys the system another 10 points of usable life beyond the cell rating.

That management strategy carries conditions. Running a large LFP storage system at close to 100% DoD has serious consequences once system SoH falls below 70%, particularly with a centralized power conversion system. SoC estimation error runs up to 5% in large-scale LFP systems and grows as the system approaches EOL. Rahul Bollini of Bollini Energy, who authored the analysis, recommends a dedicated string PCS for every cluster in multi-cluster installations operating near full DoD. A string PCS manages each cluster's discharge profile individually and maximizes energy output — but costs more than a centralized PCS, a trade-off that can affect project viability.

A pack retired from its first life still holds value, but only after reprocessing. Because cells degrade unevenly, an EOL pack must go to a repurposing or refurbishing facility, where cells are grouped by similar characteristics — with DCIR values playing a central role in matching — before assembly into second-life systems. These second-life batteries typically serve low-voltage, low-power and, critically, low-depth-of-discharge applications.

What to watch next: how second-life grading economics hold up as the first wave of large EV packs crosses the 70-80% SoH threshold, and whether string-PCS cost declines make high-DoD BESS operation viable without sacrificing cluster-level control.