For my understanding, the consequential clause would be a complete sentence until I faced the following conditional sentence.
使用できる時間が極端に短くなったら、バッテリーパックの寿命です。
Can a consequential clause after a 「たら」-clause be a noun clause? Or is this an exception?
バッテリーパックの寿命です fully qualifies as a main clause of this sentence. Its subject, which is omitted, is それ, vaguely referring to the situation previously mentioned. Subjects are omitted all the time in Japanese sentences, and there is nothing special in this case. Technically, the last half of this sentence is not a noun clause, because it has no nominalizer and ends with です, a copula (aka linking verb).
寿命 in this context is end-of-life rather than lifespan. 寿命だ／寿命です does not mean "It's a lifespan" here, but it means "is reaching / has reached the end of one's life," "is near one's end," "is dying a natural death," "is on one's last legs," etc.
In other words, 寿命 is sometimes used as a no-adjective, usually in combination with もう.
- その時計はもう寿命だ。
The watch has reached the end of its life (and thus not repairable).- また止まったの？ それは時計の寿命だよ。
It stopped again? That means the watch has reached the end of its life.- 彼はもう寿命です。
- もう寿命のスマートフォン a smartphone near the end of its life / on its last legs
See: 「バッテリが寿命です。」は英語で
たら is basically just a "if".
If A then B.
B can be pretty much anything depending on the context.
If you go -> I go too.(verb)
If it grows -> it's a plant!(noun)
