LI-ion batteries are dangerous goods. Just ask UPS and Asiana pilots. They must be declared. No ifs, ands or buts.
But that's the point: there
are ifs, ands
and buts.
99% or more of people boarding a plane – pax
and crew – are carrying at least one Li-ion battery, installed in phone and/or other device, in pocket/handbag or in hand baggage; sometimes people may carry a separate charged laptop battery too – I used to do that sometimes on long-haul flights if I had a lot of work to do. There was no requirement to declare the battery involved in yesterday's incident (
Incident: Indigo A320 near Chennai on Sep 23rd 2016, Galaxy 2 smoke).
And, as mentioned above, single Li-ion batteries are still permitted to be shipped, undeclared, in defined circumstances. In both the UPS (2010) and Asiana (2011) incidents, multiple lithium batteries were being shipped in bulk.