[Sergio]

Those batteries may be gone, draining the batteries down to 2v sometimes causes irreversible damage.

If You have a regular 12v car battery charger go ahead and charge 2 batteries at the time, the car charger has a lower charging current and may bring them up to voltage in a more gentle way.

You don't have to disconnect the cables, just start at the pack negative and charge the first 2 batteries, then move the charger leads to the next 2 batteries until You get them all charged.