Why packs drift out of balance

In a healthy pack, every module sits at nearly the same voltage. Over time, they drift apart. The main causes are below.

No two cells are identical

Tiny differences from the factory mean some cells hold charge a little better, or leak it away a little faster, than others.

Self-discharge

Every battery slowly loses charge just sitting there. When a car sits for months or years, each module loses charge at its own rate, so the gaps grow and voltages can fall too low overall.

Heat

Modules in warmer spots in the pack age faster than those in cooler spots.

Series wiring

Because modules are in series, the same current flows through all of them. The weakest module hits its limits first and holds the whole pack back. The BMS has to protect that weakest link, so usable capacity shrinks, or the car refuses to charge at all.