I understand why this is wise, but what are the actual risks with LiPo batteries?
Almost every cell phone, Android or iPhone alike, contains a LiPo battery, which most people, including myself, charge unattended—often while left on a flammable or conductive surface. However, you never hear of someone who caught fire due to their cell phone exploding. Yes, I know there are weird accidents, but how dangerous are modern LiPo batteries? Why do so many online commentators treat standalone LiPo batteries like bombs waiting to explode, but don’t think twice about the LiPo in their pocket?
Every cell phone (as well as a laptop and just about everything with a rechargeable battery) uses LiIon/LiPo (basically equivalent for the purposes of this discussion). And you’re right: In terms of actual accidents, lithium-ion and lithium polymer are the most widely used safe battery chemical, except for anything.
And the only reason this omnipresent chemistry hasn’t killed you and/or your family multiple times is because these cells aren’t loaded unattended. You may not bring them in person, but each one of these lithium-ion batteries has a great deal of protection and monitoring circuits permanently built into the package. Works as a gatekeeper. It monitors every cell in the battery.
It separates the output terminals and prevents them from being overcharged.
It disconnects the output if it is discharged with too high a current.
The output disconnects if it is charged with a very high current.
If any of the cells are damaged, the output is separated.
If any cell gets too hot, it trips the output.
If any of the cells are excessively discharged, it disconnects the output (and permanently – if you forget to charge a lithium-ion battery for too long, you will find that it will not charge anymore. It is effectively destroyed, and the protection circuit will not allow you to charge the cells).
In fact, every single phone battery, laptop battery, *whatever rechargeable lithium battery is the most monitored, audited, and effectively managed, as opposed to the diagonal of “unattended” as one can get for the battery.
The reason there are so many additional problems is that lithium-ion batteries are very dangerous. They need protection circuits to be safe, and they are not safe even remotely without them. Other chemistry such as NiMH or NiCad can be used relatively safely as bare cells, without any monitoring. If it’s too hot it can vent (which has happened to me personally), and it can be pretty amazing, but it won’t burn down your house or land you for long in a burning unit. Lithium-ion batteries will do both, and that’s pretty much the only result. Ironically, lithium-ion batteries have become the safest packaged battery by being the most dangerous battery chemistry.
Lithium-ion batteries are fundamentally different. They store energy like a spring. This is not a metaphor. Well, like springs. Lithium ions are pushed between the atoms of the covalently bonded anode material, pushing them apart and “tightening” the bonds, resulting in energy storage. This process is called intercalation. Upon discharge, the lithium ions move from the positive electrode to the negative electrode. This is largely electromechanical, and both the anode and cathode are subjected to significant mechanical stress from this.
In fact, the anode and cathode both increase or decrease in physical size depending on the state of charge of the battery. However, this change in volume is uneven, so a fully charged lithium-ion battery actually exerts insignificant amounts of pressure on its container or other parts of itself. Lithium-ion batteries are generally subject to significant internal pressure, unlike other chemistry.
The other problem is that their electrolyte is a volatile and highly flammable solvent that burns strongly and easily.
The complex chemistry of lithium-ion cells is not fully understood, and there are a few different chemists with different levels of reactivity and inherent danger, but those with high energy density can all experience thermal runaway. Basically, if it gets too hot, the lithium ions will start reacting with the oxygen stored as metal oxides at the cathode and releasing more heat, speeding up the reaction even more.
The inevitable result is a battery that self-ignites, spraying the highly flammable electrolyte off itself, and igniting that too instantly, after a fresh supply of oxygen becomes available. This is just an extra fire, however, there is still tons of fire from oxidized lithium metal with an ample supply of oxygen inside.
If they get hotter that happens. If overcharged, it becomes unstable and mechanical shock can make it explode like a grenade. If it is excessively discharged, some of the metal in the cathode will undergo an irreversible chemical reaction and will form metallurgical conversions. These transformers will be invisible, until charging expands part of the battery enough that the separating membrane is punctured by one of these transformers, resulting in a dead short, which of course leads to a fire, etc.: the lithium-ion failure mode we know and love .
So, just to be clear, not only is overcharging dangerous but also over-discharging, and the battery will wait for you to pump a lot of power into it before it fails amazingly on you, with no measurable warning or signs.
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