Is it harmful to battery life to leave devices like iPods (Classic, Touch), tablets (Nexus), etc. plugged in (using factory provided wall chargers) for some number of hours (say up to overnight) after they have charged to 100%?
No.
Anything relatively new and fairly sophisticated like a smartphone or a tablet should have a circuit to limit the charging of the device, especially since they use Li-ion batteries which have a tendency to burst into flames if overcharged - very bad for business.
However to be safe, I have several autotimers like this that I use. Few li-ion devices require charging time of more than 6 hours in my experience, so I keep this plugged into the wall and the recharger plugged into the timer. When I need to recharge something (you can use a multi-outlet plug to have serve > 1 device), I just plug in the device and set the timer. Then I don’t worry about leaving it plugged in.
Incorrect. Leaving a full capacity lithium battery on a charger stresses the battery and will result in a shorter useful life.
Cite?
Specifically, that any smartphone or tablet plugged into a charger is actually feeding its battery any current when the battery is at full charge?
This may be true, and if so, I apologize to beowulff for my knee-jerk reaction. However, I don’t think it’s safe to assume that any specific device is so equipped and best practice is not to leave lithium batteries on a charger indefinitely.
Overcharging is a serious enough safety issue that I’d think that ANY device that uses Li-ion batteries has an “intelligent” charger circuit that at the very least senses the battery voltage to stop charging it. Heck, even NiMH batteries shouldn’t be trickle-charged (they won’t blow up, but will be damaged over time). The only issue that I can think of is that Li-ion batteries degrade faster if left at 100% charge all the time, but this is more of an issue for laptops which are kept plugged in all the time (beside the heat from the running laptop; best to remove the battery, when at around 80% charge, in this situation). Besides, cycling the battery degrades it as well, so no free lunch either way.
Actually it is safe to assume that. If you have older batteries like nickel cadmium then all bets are off because it is possible to use a simple charger on those. You can’t use a simple non-monitoring type of charger for lithium ion though. The charging algorithm for lithium ion is basically to monitor the voltage while applying a constant current. When the voltage reaches its peak value, then a constant voltage is applied until the current drops below a particular threshold value. Using a simple constant voltage or constant current current type charging algorithm like is done with nickel cadmium batteries will cause lithium ion batteries to do really bad things like catch fire or explode. So you can safely assume that a lithium ion battery charger is sophisticated enough to be safe because otherwise it would blow up batteries and catch fire under normal use, and that’s generally considered a bad enough thing that most folks won’t sell a device like that.
Note that this assumes that you are talking about lithium ion batteries. If you have older batteries like nickel cadmium, the charger just might be a voltage source with a current limiting resistor and that’s it with no smarts at all in it. You wouldn’t want to leave a battery in a charger like that indefinitely.
While it is generally safe to leave lithium ion batteries attached to their charger, the exception to this is if the charger generates a lot of heat. Lithium ion batteries tend to die an early death if they are kept too warm all the time. This affects lithium ion batteries left in laptops too, as many laptops by their compact nature generate a lot of heat. A lot of laptops will run hotter if you plug them in, because they run the screen brighter and may even run the CPU at a faster speed, allowing it to generate more heat (the dimmer screen and slower CPU speed are designed to prolong battery life when it isn’t plugged in). Some laptops also just have a bad design in that the battery is close to the stuff that gets hot and the battery is kept hot all the time as a result. Hot laptop batteries die an early death. It’s that simple.