[garaetjjte]

NiMH packs are usually used without balancing, because in principle they should equalize at each full charge. Maybe the charger circuit is too gentle and terminates charging too early with low ΔV when not every cell is fully charged yet.

I think methods used in that video are somewhat dodgy. When dealing with severely unbalanced packs care should be taken when discharging, as unbalanced cells could go below 0V and become damaged by reverse charging. And while cooling them during charging might keep the temperature in check, it probably doesn't completely prevent internal outgassing and pressure buildup. Very long charge at low current would likely be safer option.