Then it's something wrong with your MD.
Yes, leaving NiMH batteries charged for a longer period, than recharge and leave them again (i.e. standby-use) will severly shorten their life time. They perform best when slowly charged (with up to 0.1C current), (slowly) discharged and so on, keeping this cycle going threwout the entire battery life time, few yares of service could be expected. NiCd are much less sensitive to stendby-use regime (and much better batteries), but are difficult to get since using of toxic Cd is more or less discontinued. Wach out NiMHs for PI devices, if the RC filter is inadequate (high current pulse discharge regime) may last some 3 cycles only.
regards,
d.