The Unlucky Batch

The bogus batch can be identified in two weighings as follows.

First put one bracelet from each batch on the scales.  The difference between the weight measured and the expected total weight of thirteen bracelets gives the difference in weight between a fake bracelet and a real one.

Then number the batches from 1 to 13.  Put on the scales one bracelet from the first batch, two from the second batch and so on.  The difference in weight from that of 91 real bracelets, divided by the weight difference determined in the first weighing, gives the number of the batch of counterfeits.