The phone was identified as an iPhone 6 Plus. It's a known fact: phone batteries can overheat at any time. This time was a bit different because it occurred in a store. In many cases, the company attributed the problem to an "aggressive" battery design. Currently, when it comes to older iPhones (including the 6 Plus), Apple is offering to replace the batteries for $29 not because they're unstable, but because some may be slowing the phones excessively. The same chemical properties that make batteries work also make them likely to catch fire. There are many common ways for rechargeable lithium-ion batteries to break.Generally speaking, batteries could be dangerous. Exploding batteries can be the consequence of overeager companies pushing technology to the limit.