I didn't say 40% of its voltage. I said 40% of it's full charge. For a typical 10 cell 12v NiCad that would be around 9.5v.
First off AA cells for Walkmans are probably NiMh cells. NiCads are different. I'l quote a statement on NiCad battery care for a Skil cordless drill...............
"If you anticipate long periods of (i.e. a month of more) of non use of your tool, it is best to run your tool down until its fully discharged before storing your battery pack." "