With any aircraft battery when the voltage drops below a certian level there is not enough current left to pull in the battery contactors so even if the GPU / APU can charge the batteries in situ you cannot get the batteries on line.
Those who apply an external voltage to the battery contactors to bring the batteries on line risk the scenario described by Brizeguy.
However sealed lead acid batteries are a bit more forgiving.
As per other posts a ni-cad nattery returned to the shop is completely discharged and each cell 'shorted out' so any cells that have reversed are corrected before any charge is applied.
The battery is then charged at the MM values and usually capacity checked by controlled discharge to ensure that no cells have failed and as long as the battery meets spec it is re-charged again and released for service.
Battery shop used to be a great place in the winter time!