...... and just make sure that the battery applies the current to the armature in the right direction, else you'll back to square one.This relies on some residual magnetism in the armature. If it hasn't been used for a while this may have faded away. I'm not an expert on this but I think you have to apply some current through the armature windings (by connecting a car battery) to re-magnetise.
I think it's the fields not the armature. In the old days we "flashed" the field terminal on the regulator when replacing a generator. Flashed meant to short between the battery and field connections. The fields are the 'magnet' and the armature is the power producer.This relies on some residual magnetism in the armature. If it hasn't been used for a while this may have faded away. I'm not an expert on this but I think you have to apply some current through the armature windings (by connecting a car battery) to re-magnetise.