Short: Batteries

Getting the most possible life out of your electronic devices' batteries is a lot easier than it used to be.  The old nickel-cadmium, or NiCad, batteries used to work best if they were periodically completely discharged and then charged back up to full.  That's not the case with modern battery architectures, like the Li-ion (lithium ion) batteries that are currently used almost exclusively in smart phones and laptops.  Those actually operate at their best when they are allowed to charge fully; they'll manage their own internal charge and cut over to AC power when full.

The one exception is when you have a device that you plan to turn off, but keep around for a very long time, like multiple years. In that case, the battery is best discharged to the halfway point and then removed from the electronic device.  Ideally, it should also be cold-stored in a fridge or freezer, too.  This is not the treatment to give your laptop or your smartphone, but if you are a collector or if you have a bunch of spare batteries this is the best way to treat them.