The Medway Handyman wrote on 24/11/2009 :
Do NiMh batteries self discharge if left unused for a period of time, or is
this just a myth?

Over days, weeks or months - yes. How well they retain the charge seems
to depend in part on luck, rather than how much you paid for them.
Keeping them in a plastic bag in the fridge, is supposed to help.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk