Shown on this slide is a real example of a power tool battery pack that did not have cell balancing. This was a 6-cell pack, so the normal pack voltage range could be roughly between 18 and 24 V. From the external terminals, this battery measured over 19 V, which is in the normal range. But looking inside, one cell was visibly driven into reversal from repeated over-discharging. And, of course, the leakage (corrosion) which is a sign of catastrophic failure can be seen on the outside.