On the magical day that is Mardi Gras day, I performed the sacred ritual of giving the small child next to me a throw I caught. In this case, it was a Zulu spear from the truck parade. A few minutes later, his mother handed me a small box in return and said, “My kids don’t need this!”
Inspecting my spoils, I immediately thought: "jackpot!"
Inside the box was a small, round, “rainbow ring selfie light”, with a mirror inside. It connected to your phone via a Bluetooth connection. It fit perfectly with my previous blog post with Andrew Bellini’s guide on hacking small IoT devices. A cheap, obscure gadget with wireless connectivity is basically a hacker’s invitation, and I was ready to get my feet wet in trying to find out what language it had been coded in and API calls this thing would make when connected.
I opened the packaging later, set it on my desk, and planned to come back to it in a few weeks. When I showed it to my coding partner a week or two later, I noticed something strange: the mirror had popped loose from the ring on one side. When I opened the casing, the reason became obvious. The tiny lithium battery inside had begun to puff.
If you’ve never seen this before, it’s unsettling. There's a whole 'spicy pillows' subreddit. Lithium batteries that fail often swell as gas builds up inside the sealed pouch. Once that happens, the safest option is to throw the device away before it becomes a fire hazard or just explodes.
So my exciting little Mardi Gras hacking project ended its life in the trash. Super disappointing.
I tried to find another one online, but the throw was so low-quality and obscure that I couldn’t even locate a replacement. Apparently the world is not overflowing with Bluetooth selfie-light mirrors designed to be thrown from parade floats, especially colored ones. So for now, the hunt continues for the perfect low-security IoT device to take apart.
But the whole experience left me wondering something: why do lithium batteries fail like this? I knew they had a chemical reaction to work in the first place and this was probably something to do with how that was inturrupted, but I wasn't exactly clear why/how the whole process worked.
What’s Happening Inside a Lithium Battery
Lithium-ion batteries work through controlled chemical reactions. A battery has three main components: an anode, which is negatively charged and holds a large number of electrons; a cathode, which is positively charged and lacking those electrons; and an electrolyte, which allows lithium ions to move between them.
When a circuit is completed, electrons flow through the external device while lithium ions move internally through the electrolyte. That movement of charge is what powers the device.
All of this normally happens in a carefully balanced chemical system. When the balance is disrupted, however, the reactions can start producing heat faster than the battery can safely dissipate it.
Thermal Runaway
The most dangerous and typical failure mode for lithium batteries is something called thermal runaway.
Thermal runaway happens when a battery begins heating internally and the temperature increase triggers further chemical reactions that generate even more heat. Instead of stabilizing, the system begins to feed on itself. As the temperature rises, components inside the battery begin to break down, releasing gases and accelerating the reactions further. This feedback loop is what eventually leads to swelling, venting gas, fire, or even explosion.
Several different issues can trigger thermal runaway.
Mechanical damage is one of the most common causes. If a battery is crushed, punctured, or sharply bent, the thin internal separator that keeps the anode and cathode apart can fail, allowing the electrodes to touch and create an internal short circuit. This barrier is only about 15 microns thick, so even small impacts can cause microscopic damage that develops into a failure later.
Electrical stress can also destabilize a battery. Charging beyond the safe voltage range or failures in the charging electronics can cause lithium to deposit unevenly inside the cell, forming tiny metallic structures that pierce internal layers and create short circuits(and start the whole swelling process).
Heat damage, from the surrounding environment can push a battery past its safe operating limits as well. As the temperatures rise, the electrolyte and electrode materials begin to decompose, releasing gases and additional heat that further accelerates the reactions. So being transported in unventilated shipping containers, or being in the sun in trash backs on top of the truck awaiting the start of the parade.
Manufacturing defects are another important trigger that is being seen more and more, and really peaked my interest when I found several articles from this previous fall and winter about how these failures have become more common with the rise of inferior product design in the post-covid supply chain.
Lithium batteries are extremely precise, with internal separator layers thinner than a human hair(that 15 micron buffer). If electrode layers are misaligned, if microscopic contaminants are left inside the cell, or if insulating layers are imperfect, those internal flaws can eventually create internal short circuits and initiate thermal runaway.
In those articles from last year, researchers using industrial CT scanners to inspect batteries have found that a significant portion of low-cost cells contain internal defects such as electrode misalignment or structural irregularities that increase the risk of failure.
The Cheap Battery Problem
Once I couldn’t find another version of the device online, the possibility of poor manufacturing started to seem more likely. The swelling I saw in my selfie light was likely an early stage of this breakdown processes. Gas buildup inside the pouch battery pushed outward until it forced the plastic casing apart and dislodged the mirror.
Demand for lithium batteries has exploded over the past decade thanks to smartphones, laptops, e-bikes, scooters, and electric vehicles. That rapid growth has created a huge global supply chain, and not all batteries moving through it are made to the same standards.
Investigations into low-cost and counterfeit lithium batteries have shown that some cells lack safety features that are standard in legitimate products. Proper lithium cells typically include internal protection mechanisms designed to stop dangerous conditions before they escalate. These can include devices that interrupt current if internal pressure rises too high, or components that increase electrical resistance when the battery overheats.
In cheaper or counterfeit batteries, those protections are sometimes missing entirely.
In other words, when electronics are extremely cheap, the battery inside may literally be missing the safety features meant to prevent the exact kind of failure I encountered.
So What Happened to My Mardi Gras Throw?
I can’t know for certain.
Maybe the battery was damaged when it hit the ground after being thrown from the float. Maybe it was overheated somewhere along the way. Or maybe it simply contained a poorly manufactured battery cell from the beginning and was doomed from the get go.
Lithium-ion batteries pack a surprising amount of energy into very small spaces. When they’re built well, they’re incredibly reliable. When they’re built poorly, the chemistry can go sideways in dramatic ways.
So sadly, my would-be little IoT hacking project never got off the ground.
But the discarded selfie light still ended up teaching me something: even the cheapest gadgets contain surprisingly sophisticated, and occasionally dangerous, technology.
And somewhere out there, the perfect cheap IoT device is still waiting to be caught. Preferably one with a battery that stays un-puffed long enough for me to take it apart.
Thanks to these sources!
Thermal Runway and why you can't bring a lithium battery on a plane
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