This review examines the influence of the various parameters involved in the cast-on-strap (COS) process on the quality of the resulting lug–strap joints. In addition, it provides the findings of an investigation on the macroscopic and microscopic features of COS joints in commercial lead–acid batteries. Some examples of the most common defects that can affect the performance and life of batteries are presented. The nature of the fusion at the lug–strap interface for the various lugs and strap alloy compositions used in conventional, hybrid and valve-regulated lead–acid batteries are also discussed.