Aigua, foc i curtcircuit: com falla una bateria de liti

When it comes to lithium batteries there are 4 things you should never do. 1. Never let them get wet. 2. Never expose them to fire or any other open gas. 3. Never sure circuit them. And 4. Definitely never function or disassemble them. But let's be honest, do we even know what those safety warnings are really for? And do we actually know what happens when you ignore one or more of them? Because they might just look like boring little metal cylinders. But inside each of these boring metal cylinders is a tightly rolled sandwich of materials you really don't want to mess with. Because that tightly rolled sandwich consists of a positively charged lithium and aluminum electrode and a negatively charged graphite and copper electrode. And very simply put, when those two touch you get...

...yet inside the battery the two are tightly rolled up like a sandwich. And the only thing that keeps them from touching is a super thin plastic layer in between. As long as those two electrodes don't touch each other, everything is fine. The battery behaves just like you'd expect. But if however a battery gets damaged and the positive and negative electrodes are forced in such a way that they do connect, things can and will suddenly escalate very, very quickly. Today we're going to test one by one what will go wrong exactly when you ignore each one of the four most important safety warnings. And to do so I put together four different test tricks, all of which can be remotely controlled. Each one is designed to trigger one specific failure mode at a time while I can keep a safe distance.

Let's just start with the simplest one. But it's the one people argue about the most. There's a very common belief that says if a lithium battery touches water it'll explode. But is it actually true? Well time to find out I guess. Three, two, one. No, just kidding of course. What really happens is quite the opposite actually. No explosion, no flames, nothing. Not even the tiniest little spark. Most likely because a regular tap water is actually a pretty poor conductor. But watch what happens when we add some simple kitchen salt to it. Things suddenly change. The water instantly becomes more conductive. Which means there's now a path for current to flow externally around the battery. So in the salt water the battery will simply be leaking energy through the water from one side of the battery to the other.

It's basically just a slow and uncontrolled discharge. The big question is how long will it take for the water to ship in and start to cause internal failure? While we wait for that to happen let's just put the decides for now and get back to them when there are some significant changes. Meanwhile we can continue with the next test. I think if there's one rule everyone agrees on it does. Never expose lithium batteries to fire. But what we want to know today is what happens when you do expose them to fire. So let's exactly what we're going to do. We're going to heat a battery using a little gas burner. And to make sure the battery is heated evenly from all sides instead of creating a single hot pot I slightly modify the all RC servo that will slowly rotate the battery while it's being heated.

Okay so far so good. The plastic cleaf has pretty much melted away and the surface started to discover a bit. Which means the battery is heating up. And as the temperature rises the pressure inside the battery also starts to build up. Until this happens. And no this is not the thermal runaway explosion we're looking for yet. It is actually the batteries built in pressure relief safety mechanism doing what it's supposed to do. It's releasing the pressure from inside the battery to prevent any further damage. However in our case that's probably not going to help much because there's still heat getting pumped in from the outside by the gas burner of course. And just like expected it didn't take long before thermal runaway begins. The internal chemistry breaks down and starts to release heat and oxygen at the same time.

So from this point on the reaction no matter the heat spires from the outside the battery becomes the fire itself. Look even after the battery hits the ground it keeps ejecting burning material for several seconds. So lithium batteries don't necessarily explode because of fire. Fire just triggers a reaction that can then sustain itself. So the battery becomes the fire. Before we continue with test number 3 a quick shout out to our fellow engineers over at PCBWay. Whether you're working on an electronics project or need 3D printed or CNC machine parts, PCBWay makes it incredibly easy to turn your digital designs into real high quality parts. Simply go to PCBWay.com, pick the service you need for your project, upload your files if necessary, get an instant online quote, order and your part will be delivered in no time.

Right now they're running their Christmas big sale until December 31st with massive coupons, savings of up to 50%, free PCB color upgrades like purple, matte black and pink solar mask and discounts on selected 3D printing materials. Take advantage of these huge deals or find out more via the link in the description. Now back to the experiments. This next test will be even more violent than the last one. The battery is pretty much fixed in place and we're going to penetrate the steel casing of the battery with a hard steel drill at a almost perfect right angle. The moment the drill breaks through the outer shell everything changes. There will be no delay and no build-up, just a high pitched humming sound, a few little sparks and then a lot of fire.

Mark my word. Inside the battery the layers that were never meant to touch are suddenly fierce and connected together by the conductor's steel drill. Positive electrode meets negative electrode instantly which causes a localized hotspot to form that burns its way deep inside the battery in the blink of an eye, where cooling is simply impossible. That's why this punch failure looks so extreme. It's like forcing an internal short circuit right into the core of the battery. And just to put things in perspective, looks at the drill. It's a red hot and completely melted and the thing is that the drill is made from high-speed steel. That's a material designed to survive extreme heat, yet it's melted. And not from the drilling, from the energy released inside the battery. This is why crushed batteries or battery's damage in accidents are so dangerous.

Not just because they look damaged, but because inside the positive and negative electrodes could be one vibration away from touching each other and turning into this final explosion. But what if I told you that the same thing can happen without any drills, fire or water? Because theoretically all we need are some simple copper wires. For test number four, we're going to short circuit a fully charged cell by connecting the terminals directly using thick, low-resistant cables. And this DIY 3D printed servo-acted switch I designed for this test. So no water, no drill, and no gas burner. Just the copper wires. As soon as the switch closes, a current starts to flow that can encase of some batteries via as high as 300 amps. Now, you don't see much at first, but because of those high currents running through it, the battery starts to heat up fast.

The temperature climbs all the way up to 63 degrees in pretty much no time. But wait, then it stabilizes. That's weird. I at least expected the buildings safe to even pop or something. But no venting, no fire, no explosion, nothing. So why is that? Well, because this is just one single lithium ion cell and the amount of energy it contains is limited. So the peak amperage might start at those 300 amps I mentioned earlier, the moment the switch closes, but that number will rapidly decrease after even just a few tens of a second. And on top of that, the build-up heat can easily escape through the metal casing, especially here in this case, where the ambient temperature is around 7 degrees. So a single cell like this doesn't have enough energy density to push itself over the edge and cross that point of no return.

But now, imagine this exact same short circuit, but inside a battery pack, where there are multiple cells that are tightly packed together. So each one will heat up the next. Suddenly, the heat has nowhere to go. And once a single cell and the thermal runaway, it becomes the ignition charge for all the others. That's why short circuits are so dangerous. Not because one battery instantly explodes, but because packs allow failures to cascade. And trust me, you really don't want to be anywhere near that when it happens. So conclusion. After ignoring every major safety warning, a pattern starts to emerge. Water, heat, puncture, short circuits. All very different mistakes, but with the same underlying failure. An internal short circuit. Lithium batteries aren't dangerous because they're unpredictable. They're dangerous because once their internal structure fails, the reaction feeds itself and the battery doesn't need fire, it becomes the fire.

Now, are there any battery experiments you'd like to see in a part two, maybe, that we didn't test in this video? Let me know in the comments below.