What losing control means
A lithium battery cell can enter a chain reaction inside itself called thermal runaway, where heat builds faster than the cell can lose it until nothing stops it. A peer reviewed review of battery failure testing compiled results from a study that tested a nickel manganese cobalt cell and an iron phosphate cell side by side, the same cylindrical shape and size, charged to the same level, pushed to failure by the identical method, so the 2 results can be compared directly.
The numbers from a matched test
The nickel cell reached the point where its heating became unstoppable at 221 degrees Celsius, then peaked at 454 degrees. The iron phosphate cell needed 243 degrees before losing control, 22 degrees more, by MAOWCE arithmetic on those 2 source 1 figures. It then peaked at only 311 degrees, 143 degrees cooler than the nickel cell and about 46% lower, both again MAOWCE arithmetic on source 1. The same study found the iron phosphate cell lost control higher and peaked lower at every charge level it tested, half charge and quarter charge included.
All 4 cells are the same 18650 cylindrical format, tested with the same method inside 1 study cited by source 1. The nickel cell was tested only at full charge in that study.
Show the numbers
| Nickel cell, full charge | 221 to 454 degrees Celsius |
| Iron phosphate cell, full charge | 243 to 311 degrees Celsius |
| Iron phosphate cell, half charge | 237 to 282 degrees Celsius |
| Iron phosphate cell, quarter charge | 240 to 263 degrees Celsius |
The 3 stages every cell passes through
A separate peer reviewed review breaks the heating process into 3 stages that apply to any lithium battery chemistry. Initiation is when the protective layer on the negative electrode starts breaking down, at roughly 50 to 160 degrees Celsius. Acceleration follows, when the barrier between the 2 electrodes deforms and the cell starts generating gas, at roughly 120 to 250 degrees. The runaway reaction stage comes last, the strong heat producing reactions that follow, at roughly 180 to 350 degrees. The 2 cells above pass through this same 3 stage process at different temperatures.
These are general literature ranges for any lithium battery cell, not measured values for the 2 cells in chart 1. Source 2 states them to define what each stage means, not as a second temperature reading for iron phosphate specifically.
Show the numbers
| Initiation | 50 to 160 degrees Celsius |
| Acceleration | 120 to 250 degrees Celsius |
| Runaway reaction | 180 to 350 degrees Celsius |
Iron phosphate is not immune
Needing more heat to lose control is not the same as being unable to. In the matched test, the iron phosphate cell at full charge still went through a complete thermal runaway, just to a lower peak. A separate peer reviewed safety review found iron phosphate cells still release dangerous, flammable gas during a runaway, made mostly of carbon monoxide and hydrogen, and called the risk unexpectedly high given the chemistry reputation for staying stable. A third review names a route into runaway that skips the need for a crash or a puncture entirely, chemical crosstalk between the positive and negative electrodes that can generate enough heat on its own to trigger it.
What a real fire can cost to fight
What a real battery fire can cost to fight is documented by the National Transportation Safety Board, in a 2020 safety report built from 4 crash fires, all in nickel chemistry vehicles, not iron phosphate. The agency found 3 of the 4 crash damaged batteries reignited after firefighters believed the fire was out. 1 case needed more than 20,000 gallons of water over at least 2 hours to control a single damaged battery. The report names hydrogen, ethylene, ethane and propane, flammable gases, as the most significant part of the fire threat a damaged battery releases. None of this is specific to iron phosphate. It is what the report says a lithium battery fire in general can require.