When regular disposable or rechargeable batteries (alkaline, nickel-cadmium, or nickel-metal hydride) go bad and leak, you get a mess of variable severity and maybe a ruined device. If it happens, it’s obvious and, for the most part, not particularly hazardous. Of course, you don’t want to get the caustic residue on your skin or in your eyes, but basic safety precautions and common sense will protect you. You could say it’s perfectly normal for these batteries to leak.
Lithium batteries, while highly efficient and long-lasting, hence their popularity, can and do leak, and what comes out of them is far more dangerous than anything we’ve dealt with before. But it isn’t normal, per se. Quality batteries left in a device — but otherwise not abused — shouldn’t leak simply because they’re old. There usually must be some other cause.
At the top of the list are cheap batteries with poor design and/or manufacturing quality control. If the components aren’t high quality and properly assembled, or the sealing process wasn’t done carefully, you have built-in weak points that can easily fail. You’re starting out in a hole with these. Don’t do it. The quality mandate applies to chargers as well.
Next is physical damage. A dropped or roughly handled battery with visible damage to the casing likely has internal damage too, and that internal damage can lead to a leak.
Overheating is another way to simultaneously shorten battery lifespan while increasing the chances of a leak. Improper use or storage can cause overheating, and often the charger itself can be a source of overheating, damage, and fire risk.
Lithium batteries are, in short, highly flammable by nature. Anything that causes them to leak also raises the chances of a chain of chemical reactions leading to thermal runaway — and then fire.
If they do catch fire, the blaze may be nearly impossible to extinguish.
