As nickle based batteries age, they can grow dendrites (crystal formations) that can penetrate the thin separation between the plates of the cells which causes leakage. Once this happens, no permanent fix is reliable. You can burn the partial short out with a high current pulse but the fix is only temporary as the dendrites grow right back at the same puncture in the separation material. Many years back a highly touted fix (never was a reliable one) was to charge a very large capacitor to 12 volts or more and then put it across the battery terminals. The momentary high current would burn out the dendrite and every thing appeared to work well but; unfortunately for a very short time.