I was always told never to discharge nicads past .7v per cell, due to cell reversal (polarity flips-creates a short). Nimh (up to GP3700) were able to be stored with a dead short prior to the IB cells that showed up. When the IB Cells took over people started to dead short and were not able to get batteries back to charge. Follow this link to learn more about batteries.

Hope this helps,
Cesar