not all knockoff batteries are the same. even from the same vendor. they are using cells that are who knows. sometimes good cells they got for cheap, sometime crappy cells they paid regular price for.
A buddy runs a home building company and supplies all his guys with power tools. its not a giant crew. for a while he tried to save money and use cheap batteries. he found that 75% of them had a pretty early failure rate. in the winter in the cold it was worse. he said some of them were just fine.
he ended up going back to OEM.