Why a pack loses capacity slowly
A car battery pack is not 1 large battery. It is built from hundreds to thousands of small units called cells, wired together to work as one. Battery University, a battery science reference published by Cadex Electronics, names 4 separate chemical and physical processes inside each cell that slowly reduce how much charge it can hold, building up gradually over many cycles rather than happening all at once. That gradual buildup, not a single event, is why the capacity of a pack typically falls a little every cycle.
Why one cell usually falls behind the rest
Cells wired into a pack are never perfectly identical. Battery University states that manufacturers build packs to a tolerance of plus or minus 2.5% between cells, because even a small mismatch snowballs. A cell with slightly less capacity than its neighbors drains and fills faster than the rest every cycle, so it is worked harder while the pack coasts. A cited test found that a pack mismatched by 12% between its cell groups lost far more usable capacity than packs mismatched by 5%, 6% or 7%. This is why a pack usually shows up as 1 weak module rather than an even decline everywhere.
The cited test found the pack mismatched by 12% lost far more usable capacity than the packs mismatched by 5%, 6% or 7%.
Show the numbers
| Pack tolerance | 2.5 |
| Test pack at 5% | 5 |
| Test pack at 6% | 6 |
| Test pack at 7% | 7 |
| Test pack at 12% | 12 |
When failure is sudden instead of slow
A different and much rarer kind of failure happens suddenly. Battery University describes microscopic metal particles left inside a cell during manufacturing that can converge on 1 spot and let a large current flow directly between the 2 electrodes there, heating that spot to as much as 500 degrees Celsius and causing a fire or a vent. A thin or uneven separator, the layer meant to keep the 2 electrodes apart, is a second route to the same failure. Battery University puts the failure rate at better than 1 in a million for a quality cell, and better than 1 in 10 million for the best cells, a figure for 1 cell, not 1 car, since a pack holds hundreds to thousands of cells.
Figures describe 1 lithium ion cell, drawn mostly from consumer electronics, not 1 car. A single pack holds hundreds to thousands of cells, so this is not an electric vehicle failure rate.
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| A quality lithium ion cell | 1 |
| The best lithium ion cells | 10 |
How rare failure is across real electric vehicles
Recurrent, a company that tracks its own community of electric vehicle owners, gives the real world version of that split. Across all years and models, Recurrent finds under 4% of electric vehicles ever need a battery replacement, once major recalls are set aside. Ordinary wear related replacement, outside a recall, typically takes 10 to 20 years. Recurrent excludes 2 recalls from that 4% figure, the Chevrolet Bolt EV and the Hyundai Kona EV, both tied to a battery defect from the same cell manufacturer.
What a real recall looked like
The Chevrolet Bolt EV shows what a sudden defect looks like once regulators catch it. The National Highway Traffic Safety Administration first opened a recall covering 50,932 vehicles, then widened it to a superseding campaign covering 57,414 vehicles from model years 2017 to 2019. The agency record states the battery could catch fire when charged to full or nearly full capacity, and that General Motors replaced the defective battery modules, smaller groups of cells bundled inside the pack, free of charge. Fixing specific modules rather than scrapping every pack matches the mechanism above, a fault concentrated in particular cells rather than spread through the car.
Both campaign numbers are National Highway Traffic Safety Administration identifiers. The later campaign superseded the earlier one and covered more vehicles.
Show the numbers
| First campaign, 20V701000 | 50,932 |
| Superseding campaign, 21V560000 | 57,414 |