Both NiCad and NiMh have a rather rapid (relative to PB or LI based batteries) self discharge, NiMh twice as bad as NiCad. Both increase in self discharge rate with age. Initially NiMh about 3-5% day and NiCad 1-3% per day. Note: this is in % of present charge, like a capacitor, never discharges to zero but continues to discharge a percentage of remaining capacity each day.