Donating your computational power for scientific projects is, in some cases, available for Android. One of them, the World Community Grid, states that "Using the application will not significantly reduce your battery life or your recharge time." (source)
On the short term, I'm sure this is true. However, I was wondering about the long term. On my Nexus 5, I have experienced that, even when the device is fully charged and still plugged in, if I use power-hungry applications, battery level may drop to 99 or 98 percents. This indicates that the battery is still used when the device is plugged in.
For this reason, I was wondering if the battery can actually be bypassed at all, or if the device is just permanently charging. Notebooks with removable batteries can obviously do this as they still work without the battery, but as I can't remove the battery on my Nexus 5, I can't check this there.
I'd be interested in an answer regarding my specific phone, but general information about how most Android devices operate there would also interest me a lot.