When a battery sits too long the voltage goes below the voltage threshold that an automatic charger needs to detect voltage polarity. Jumping (paralleling) the battery with another similarly-rated voltage battery fools the charger and it can start charging. The jumper battery should be disconnected within a few minutes so the discharged battery takes in all the charge current it can.
Charging a severely discharged battery takes an extremely long time, sometimes weeks.
Severely discharged lead-acid batteries also have reduced capacity (Amp-Hr rating) because the lead plates become coated with insulating crystals of lead-sulphate. It can be removed by following a desulphation process. The easiest way to restore capacity is to connect the battery to a special battery charger that sends high-frequency electrical pulses to the battery plates. The special frequency of the pulses causes lead-sulphate crystals to resonate and break. Once broken, the crystal pieces dissolve into the electrolyte solution and increase the conductivity of the liquid electrolyte. Concurrently, the plates are cleaned of insulating crystals and battery capacity is restored.
Bringing back severely discharged batteries takes a very long time. Most people don't realize that keeping any battery charged maintains its usability and extends life.
Really neat stuff isn't it?!
And, just think, lead-acid batteries were developed more than 160 years ago. Cheap, easy, and 100% recyclable.