What do cell ratings actually tell us? Part 6B: Temperature

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Mooch

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May 13, 2015
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This post is a continuation of Part 6A.

75°C / 167°F
At roughly this temperature there are compounds inside the cell that start breaking down and creating gas. If this happens for long enough the cell’s venting disk could pop open to relieve the pressure (ruining the cell) or the CID (current interrupt device) inside the cell could trigger to stop the current flow (also ruining the cell).

Some of the reactions that break down the cell internally are exothermic, they produce heat. If this heat cannot escape or is being generated too quickly this is the start of what can eventually lead to thermal runaway and the destruction of the cell.

When using a cell very hard you get two things working against you. Part of the internal resistance of the cell causes it to heat up. This can cause the cell to get hot enough to start the exothermic reactions which causes the cell to get even hotter. This causes more exothermic reactions to start and so on. All these reactions start to feed each other and “run away”, leading to the dreaded thermal runaway event.

100°C / 212°F and higher
This where it gets murky. It’s the lowest thermal runaway threshold temperature I’ve seen listed in a research paper for a li-ion cell but that threshold can also be over 250°C.

So much depends on the chemistry of the cell, how fast the heat is being created, and how fast that heat can escape. A cell that is well insulated will hold on to its heat, forcing it into thermal runaway faster than a cell in an actively cooled environment.

The entire cell does not have to reach the thermal runaway threshold for it to go into runaway. Only one small point inside the cell does. If this happens, and the cell can’t move that heat away quickly enough, then that point will go into runaway. This creates more heat which spreads to adjoining parts of the cell which now also go into runaway.

This can very quickly spread throughout a cell even though the outside of a cell has barely increased temperature. It typically take a short-circuit to do this though, either internal or external.

Up to several hundred degrees-C/F
This is the range of temperatures for a cell in thermal runaway. It can be hot enough to melt the aluminum inside the cell and there has even been research showing beads of copper in the debris, which melts at 1085°C/1984°F! Thin foil sheets of aluminum and copper are what the battery “goop” is spread on and which is then rolled up and put in the metal can.

I am pretty sure temperatures high enough to melt copper are a localized and rare occurrence but it does demonstrate just how powerful these reactions can be in some cells.

For some chemistries, like that used in “LiPo” cells (LCO chemistry), the reaction temperatures are higher than for the standard li-ion round cells we typically use and a lot higher than LiFePO4-chemistry cells. This higher temperature is what ignites the flammable electrolyte (liquid inside the cell) and makes thermal runaway in LiPo cells potentially more dangerous as those flames can set other things on fire. LiFePO4 cells almost never catch on fire, usually just lots of smoke.

Cell Ratings
Most cells have three temperature range ratings:
  • Storage Temperature
  • Discharging Temperature
  • Charging Temperature
The Storage Temperature rating covers both the high and low end. For example, the Molicel P42A datasheet says to stay below 35°C but that it’s better to stay below 23°C for long term storage. There’s no explicit storage low temperature limit listed so we can just use the -40°C discharge limit for storage. Other cells might list the lowest and highest temperatures in one set of ratings.

The Discharging Temperature rating typically covers the ambient temperature the cell will be used in but might refer to the cell’s surface temperature. You need to carefully read the entire datasheet to know for sure. There could be one range for ambient temperature (for the P42A it is -40°C to 60°C) and then a surface temperature limit mentioned somewhere else (75°C for the P42A).

The Charging Temperature rating is often just one ambient temperature range (0°C to 45°C for the P42A), which the cell is assumed to be within at the start (like all the ratings).

But it can also be divided up. One range of temperatures at which you can fast charge the cell (often around room temperature) and other range you can only charge slowly at. For example, for the Molicel P42A you could use the Fast charge rate if at 0°C to 45°C but only at the slower Standard rate if at 45°C to 60°C. No charging should be done at above 60°C for the Molicel P42A and the limit could be a lot lower for some cells.

It doesn’t have to be complicated though, just stay near room temperature. But if you are getting the cells hot or operating in a cold environment then check the datasheet for the cell’s ratings. This is particularly important for pack builders so they can rate their packs to avoid having them used or charged at too high or low temperatures.

There is a huge range of temperatures a cell can be at, most of which are very bad. But if we keep within that narrow range around room temperature we can get the best performance and longest life out of our cells without adding a lot of risk.
 
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