iPod classic battery life
Not a real answer, but a fair comparison...

I have a 10-year old iPod [iPod photo, I think] that lives in my car glove-box, hooked up to power &amp; connectivity on my car audio system. It only gets used when I'm driving. When I'm not out &amp; about it gets no power, sometimes up to 2 weeks at a time. I bring it into the house &amp; update it from iTunes every month or so.
Don't know how much longer it may last, nor have I ever tested how long the battery charge may last if used unpowered, but it's been no prob so far for my usage needs.

Based on current wisdom [google for refs, too many to list, idk if any of them are true or just copied the info from another site], it would appear to be best to keep the battery at somewhere between 50% &amp; 80% charge.
Leaving it plugged in shouldn't damage the battery, as the charger system is smart enough to not allow that - but I don't know how that would affect overall battery life.  

As mentioned above, mine gets charged when I'm in the car, sometimes goes weeks between charges, sometimes it's on 8 hours a day.
So far, all is still well.