Nimh vacuum?

I just bought a hand vacuum which has nimh batteries. Out of the box it seems to be charged and running quite well but the manual states to charge it first for 16 hours. Should I go by the manual or drain it first. I don't recall much of nimh but from my rc racing days I seem to recall needing to fully... show more I just bought a hand vacuum which has nimh batteries.

Out of the box it seems to be charged and running quite well but the manual states to charge it first for 16 hours.

Should I go by the manual or drain it first. I don't recall much of nimh but from my rc racing days I seem to recall needing to fully drain them before charging. It could be different for appliances with a charging base though. Also I read somewhere newer nimh batteries are not subjected to the memory effect.
Update: In case the base does not automatically stop charging after 16 hours, would it be ok to keep it plugged in all the time or should I only plug it in when I need charging?
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