If you look this up, you will find that 'sweat' apparently has two past tense forms: an irregular one which is identical to the base form ( 'sweat') and a regular one ( 'sweated').
Some grammar sources say that the first should be used for the literal sense of the word and the second for figurative uses. According to this convention, you'd say "The day of the race was very hot and humid, so the runners sweat a lot", but "I sweated the onions before adding the other ingredients" or "I admired him because he never sweated the hard stuff".
Personally I would never use the irregular form. I know that both forms existed in the past but, for me, 'sweated' is the only past tense form which sounds correct in modern English. It would be interesting to hear from users of American English, as this might be a regional difference. 'Fit', for example, is regular in British English ( "He tried on the shoes and was surprised to find that they fitted") but irregular in American English ("He tried on the shoes and was surprised that that they fit"). 'Sweat' could be a similar case.