In American and Canadian English, the verb smell makes smelled in the past tense and as a past participle. Outside North America, English speakers use smelled and smelt interchangeably, and neither form is significantly more common than the other.
For North Americans, smelt usually means (1) to melt or fuse ores, and (2) any of several small, silvery fishes of the family Osmeridae found in fresh waters of the northern hemisphere. Smelt as a form of smell is not unheard of in North America, but it is rare (see the Ngram below), appearing mainly in the rhyming jocular expression whoever smelt it dealt it (and its variants).
This ngram suggests that smelled may now be more common than smelt in British English (though this does not mean that smelt is wrong):
And this one suggests that smelled has been preferred in American English since the early 20th century:
Outside North America
In the film, Zuckerberg, having smelt success, quits college and throws everything at Facebook. [Telegraph]
And if voters did not know what it meant to prorogue Parliament, they certainly smelt a rat. [Sydney Morning Herald]
The moment they smelt weakness, they were all over South Africa like a cheap rash. [Guardian]