Usually a group of cells are welded together using a conductor. If they are in series, you need to balance the cells using balance leads. If they are in parallel though, they are balanced ahead of time to prevent too much current between the cells and thereafter they will balance themselves once they are wired in parallel.

In a parallel bank, a single cell going bad can bring down the rest to the same voltage. Even worse, if the bank is directly connected to other banks it can take out them as well. Also, if there is an internal short in one battery, the rest will pump current through it very effectively lighting it on fire. Individual battery protection circuits, smart switches, and internal short detection can help with this.