Li-Ion Battery Chemistries - What are the differences in their safety?
I wanted to outline some of the differences in the safety of the chemistries used in our batteries. The Battery Bro website has a fantastic explanation of the different chemistries themselves: Battery chemistry FINALLY explained.
Before we go into the chemistry differences lets define the two different events that can occur if the battery is abused too hard; venting and thermal runaway.
-- Venting --
Venting is a purely physical process that releases excess pressure that forms inside a battery if it is discharged too quickly or charged at too high a voltage. Both of these situations cause excess gas to be created and that increases the pressure inside the battery.
Each battery has a pre-weakened area of metal underneath the top contact. At a certain pressure level the weakened metal splits open and allows the pressure to escape. The solvent for the battery's electrolyte often oozes or sprays out too. This can be a problem because not only is it toxic but venting typically happens at around 130°C-160°C, which means the liquid is very hot.
Venting can be a rather gentle event or it can be a pretty energetic spurting and spraying of gas and liquid. Be careful, that liquid is toxic and flammable! The battery does not burst and there are no sparks or flames. The amount of gas produced is relatively small and can usually be handled quite easily by the venting holes we see in mods.
Once a battery vents, even just a little, it is ruined and should never be used again.
-- Thermal Runaway --
This is a catastrophic failure due to uncontrolled chemical reactions inside the battery. It always results in the bursting of the battery, sometimes quite violently, and can be accompanied by sparks and flames.
As the temperature of the battery rises during a discharge, certain exothermic chemical reactions can start as the temperature goes above about 75°C. This is the beginning of the process that can lead to thermal runaway if these reactions are not stopped.
If not being discharged too quickly, or the battery is being cooled a bit by ambient air flow or a metal mechanical mod tube, these new reactions can stabilize at a certain rate and not continue to increase the battery temperature. If the discharge current level is too high, or there's no cooling, then these reactions keep increasing the temperature of the battery. This causes more exothermic reactions to begin, which heats up the battery even more, which causes even more reactions to begin, and so on.
As the battery reaches about 125°C the plastic sheet (the "separator") between the two sides of the battery, positive and negative, begins to melt. This can lead to small short circuits forming at different points in the battery. These short circuits increase the temperature at those points, further increasing the rate that the battery temperature rises.
As the temperature continues to rise certain compounds start decomposing and releasing large amounts of gas. This increases the pressure inside the battery and, hopefully, leads to venting of the battery to release the pressure. But if the temperature and pressure buildup happens quickly enough, the battery won't vent in time.
At about 230°C - 270°C the thermal runaway threshold temperature is reached. This is where there the materials inside the battery are decomposing incredibly fast. There is a huge buildup of gas and the battery bursts open, often ejecting its contents and throwing pieces of battery a long distance. Depending on the threshold temperature the solvent can also ignite, resulting in a fireball to accompany the shrapnel.
While it can be quite violent, this isn't the explosion seen in a few videos that have made their way through the vaping groups and forums. Those explosions happen when a device doesn't have a pop off side panel or large open areas for the pressure to escape. The device holds back the gases for a bit but eventually it can't withstand the increasing pressure and it explodes.
It is quite difficult, but not impossible, to bring the temperature of a battery up quickly enough to go into thermal runaway without it venting first. About the only way to do it is with a short circuit.
Both venting and thermal runaway can take hours to occur or they can happen very quickly. You will typically be able to feel a battery getting hot before it vents but do not assume the same for preventing thermal runaway. That depends on a very, very fast rise in temperature, happening before the battery can vent. You might not feel the battery get hot first.
How do we prevent thermal runaway then? Never allow our batteries to be short circuited! Keep your battery wraps and top insulating rings in perfect condition, replacing them when necessary. Never use an atomizer with a press-fit or spring-loaded 510 pin on a hybrid top mechanical mod. Always make sure that the 510 pin sticks out past the threaded stem of the atomizer.
-- Differences in Battery Chemistry Safety --
Each chemistry has different characteristics that make it safer or less safe than another. The attached table makes some basic comparisons between their safety.
As the table outlines, ICR is the least safe, INR is safer, and IMR is the safest of the three. IFR batteries (lithium-ferrous-phosphate), like the ones from A123 , are the safest Li-Ion chemistry. Their lower nominal voltage, 3.3V versus 3.6V-3.7V, causes problems for regulated devices though. The "low battery" warning comes on much sooner than when using the higher voltage chemistries.
The term "IMR" is being used by some battery companies as a generic term for any of their batteries that aren't ICR. These "IMR" batteries can be true IMR chemistry or one of the hybrid INR chemistries. While not accurate this isn't a safety issue as both IMR and INR are the safer chemistries.
Not safe...safer. Any battery can be unsafe if abused enough.
ICR batteries are not recommended unless you are very familiar with your device, Ohm's Law, and battery safety. The consequences of abusing these batteries is much, much worse than with IMR and IMR.
-- Conclusion --
This can all sound quite ominous, making every battery sound like a bomb waiting to blow. While Li-Ion batteries can be dangerous if abused, we shouldn't fear them. A little knowledge and respect goes a long way towards making sure we never have problems.
Know your batteries and vape safe!
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