On 04/12/2019 16:32, Scott wrote:
I found a few rechargeable batteries that had not been used for at
least three years. I charged them all and found they do not seem to
be holding their charge (unable to power a cordless mouse for more
than five minutes).

They may never have been able to power a cordless mouse even when new.
Rechargeable batteries have a lower terminal voltage than conventional
single use alkaline cells. At low currents this can matter some devices
shutdown when the cell reaches a certain voltage to help prevent leaks.

My desk phone display will not run at all on rechargables as the LCDs
cannot produce black with the limited voltage. Current requirement is
almost nil but without enough voltage the display just doesn't work.

Is this type of battery destroyed by being left discharged for a long
period or could it recover with a few more charges?

Leaving them flat for a long time or overcharging them will both
seriously shorten battery life. Check them on something with a more
realistic load like a torch and see how long they can power that for.

Some battery technologies handle abuse better than others and in general
the higher the energy density the less happy it is about being abused.
It is worth giving them a charge discharge cycle to see if they wake up.

--
Regards,
Martin Brown