If a battery can explode regardless of vent holes it might be useful to know why. May be over-aged batteries are more vulnerable or may be fake products with defects are what we read about in the exploding mod stories. But if explosions are entirely random across all high output batteries then using them in mech mods is Russian Roulette.
There must be speific causes that can be controled. I've read that the Tesla sedan has a battery pack that consists of 7000 18650s. If one of those went bad it wouldn't be pretty.
Batteries never really burst with the energy that I would call an explosion. It's the trapping of the expanding gas in the mod that causes the mod to explode. A venting battery can do so quietly or it can spew hot toxic liquid and lots of gas several feet. If in thermal runaway there is an extraordinary amount of gas produced along with heat and, occasionally, flames. That's when things get serious.
A vent hole on the bottom could work to release the pressure before the mod explodes or it could be absolutely useless. If it's not big enough...trouble. If the battery case swells from the pressure (quite common) the path for the gasses exiting from the top of the battery can be blocked off...trouble. A single small hole in the bottom of a mech is completely inadequate in my opinion. I wonder if there's a single mech mod company that has brought batteries into runaway to test the adequacy of their mechs.
If a battery has an internal defect, this is very rare for quality cells, then an internal short circuit can occur at any time. The consequences of this short-circuiting can be anywhere from gentle venting to instant, violent, flaming, bursting of the battery. It depends on cell construction, location of the short, how charged up the battery is, and the battery chemistry. Typically, it's external abuse that causes problems though.
Just exceeding the battery's rating, even by quite a bit, can lead to venting but rarely to runaway. It's the short-circuits that cause these explosive events with mods in my opinion. The temperature and pressure builds up much, much faster diring a short circuit. Especially with the high-amp rated batteries we use for vaping.
Old batteries do have higher internal resistance. This causes increased heating during use. If the battery was being run at the ragged edge of stability before then this increased heating could push it into venting. Typically it takes a lot more than this to enter thermal runaway though, like a short.
All of the chemistries vent at about the same temperatures and internal pressures. But if the abuse is severe enough then the battery can go into thermal runaway before it can vent.
IMR chemistry batteries have the highest thermal runaway threshold temperature of the chemistries we use for vaping...this is good. They also have the lowest "temperature of reaction". This lower temperature means a less violent reaction and it almost never leads to ignition of the flammable liquid(s) in a battery.
INR/NMC/NCA "hybrid" chemistries, like the 25R and many others, has a lower thermal runaway threshold temperature and a higher temperature of reaction than IMR. The reaction can be more violent than IMR. But it is still pretty safe and only occasionally are there flames during runaway.
ICR chemistry batteries, which includes LiPo, is another whole different ball game. It has the lowest thermal runaway threshold temp and the highest temp of reaction. In runaway the reactions are extremely violent and usually accompanied by flames. This higher reaction temperature creates a lot more gas a lot faster. This can lead to very violent bursting of the battery case and can easily send any nearby batteries also into thermal runaway. If an ICR goes into runaway in a tube mech....ohhhhh boy.
The best way to prevent thermal runaway is to never short a battery....ever. Exceeding ratings, leaving it in a hot car, etc., usually just leads to venting at the worst. Tesla and other car companies severely derate their batteries to gain safety. They only charge them up to like 80% and only discharge them down to 30%, at the lowest. This reduces heating and significantly increases life. They also monitor the voltage of each battery (or small troup of batteries) and can isolate any particular group of batteries if there is a problem detected. Temperature is also monitored. Battery current is prevented from exceeding certain limits and active cooling is provided to give additional protection. About as different from the operating environment of a battery in a mech as you can get.
