If they are old Ni-Cads, it is possible they have simply suffered 'black wire' corrosion. This happens to the nicad black negative wire,it degrades and turns black... hence the term, and will no longer conduct. Try checking this by removing some of the black insulation and if the wire shows this symptom, remove the battery leads, replace them and try recharging. You may be pleasantly surprised and thereby save yourself a few quid.