Lithium-ion batteries have the tendency to go on fire.
I checked baggage policy of European low cost carriers (easyJet, Ryanair, WizzAir), and while spare batteries are banned from cargo hold, it seems that batteries in devices are allowed (also hand baggage could be placed in hold if there is no place in cabin).
If spare batteries are dangerous, aren't batteries in devices too?
A few incidents happended when lithium-ion battery exploded, and the fire has been extinguished by cabin crew.
Such fire could also happen in cargo hold. What's happens then? I've seen people question the ability of halon fire suppression systems to extinguish lithium battery fire, because it works by blocking oxygen from fire, while lithium-ion batteries do not need oxygen to burn:
Halon basically works by displacing the oxygen that is feeding the fire - and won't work when the material that is burning can provide its own oxidizer. Li-O batteries fall into that category, along with things like ammonium perchlorate (used to make solid rocket propellant), gunpowder, and oxygen generation canisters. Once ignited, they will continue to burn until the fuel is exhausted....
Also this: https://batterybro.com/blogs/18650-wholesale-battery-reviews/19673027-plane-safety-systems-are-confirmed-to-not-stop-lithium-ion-battery-fires
On the other hand in comments to this question: What safety differences exist between carrying laptops in checked versus carry-on baggage? people suggest that it will keep fire from spreading until battery extinguishes on its own.
Moreover checked baggage is not always handled gently by airport employees, which can damage electronic devices - and batteries in them, which makes them more likely to catch fire.
For me it seems like any airplane carrying electronic devices could go on fire at any moment, and each flight is as safe as the batteries it carries. On the other hand aviation has stringent regulation about using electronic devices on board out of fear of interfering with airplane systems. I guess that if lithium batteries were so dangerous as I think then they would be banned from cargo hold.
So is airplane crash due to lithium battery fire a real danger, or do I worry too much? (I guess I'm a little nervous - I'm going to fly a third time in my life so far.)