I have looked into this issue a
lot because I became a bit obsessed with the effects of inductive charging on my Palm Pre Plus a couple years ago. A good place to start reading is
Battery University. Believe it or not, the [URL="https://www.apple.com/batteries/#mn_p]Apple website[/URL] also has some good info, too. Though I have studied electrical engineering and physics extensively and done a lot of reading on the subject of batteries, I still don't feel like I have a 100% complete understanding of the best way to deal with battery charging, especially since the chemistry involved is always evolving. That said, I think there are five main enemies of the lifetime of current lithium ion battery technology:
- Heat: letting the battery heat up, and especially using the battery while it's hot is definitely detrimental to its longevity. My Pre Plus used to get fairly toasty while inductively charging sometimes, so it's something to watch when charging using that method. Playing a game or doing something else intensive while charging can also make the phone heat up.
- High voltage: yup, as far as I can tell, charging the battery fully is not in fact good for it. You obviously need to go for a full charge if you want to maximize battery life over a single discharge, but I think that doing so will lead to a more rapid degredation of the battery capacity over multiple charging cycles. I'm not sure if the battery manufacturers (or the makers of the charging circuit) take normal smartphone use into account when setting the charging parameters or not. Maybe they do something to compensate for the negative effects of prolonged high voltages.
- Deep discharge: using a lot of the battery's capacity is harmful to it. I don't know at what point in the discharge cycle it really starts to become a problem, but I just try to keep mine from going too low whenever possible.
- Usage of battery cycles: a Li-Ion battery is rated for a certain number of cycles before it loses a certain percentage of its capacity. This is a well known quantity, and you can look up graphs of capacity versus number of charge cycles for common batteries. A cycle is a full 100% discharge, but if you, say, use your battery repeatedly between 100%-95% (a hypothetical repetition of 5% capacity discharges with charging in between), you need 20 of those %5 usages to equal a full cycle. So in that sense it doesn't matter how often or how many times you charge.
- Time: Li-Ion batteries degrade over time, even if you don't use them, and that's just the way it is.
So, as far as I can tell, an ideal usage scenario would be something fairly unrealistic like never discharging the battery below 60% and never charging it above 90% or something along those lines. I think the electric cars like the Tesla that use Li-Ion batteries actual have a charging/discharge cycle something like that to make the batteries last as long as possible, though those batteries are being used in a different way and are expected to last longer than those in smartphones. With my Galaxy Nexus I just charge it whenever possible, so it spends a lot of time in the high voltage state. I haven't worried about it because I know I could always pop in a new battery without breaking the bank. Now that I will be getting a DNA tomorrow, though, I'm thinking more about my charging strategy since the battery can't be replaced without some major surgery.
I guess the important thing is to figure out which of the factors I listed above is the most detrimental and plan accordingly. As far as I cant tell, though, the general consensus seems to be that frequent charging is better than deep discharging, so I will probably keep trying to top off rather than letting the battery run down a lot.
Edit: Here's the relevant summary straight from Battery University on how to prolong Li-Ion battery life:
Of course this still leaves some question about the best way to treat the battery because we don't know exactly how the manufacturer of the device has chosen to report the percentages. If they are actually percentages of the full allowed voltage (either 3.6 or 3.7 V) then the 20-80% figure is accurate. However, if the percentages are scaled in some way then it won't be exactly right anymore. But the main point still stands: avoid 0% and 100% for best battery longevity. It would be nice if smartphones would come with the option to limit battery charging below 100% so that the phone can be left plugged in all night without harming the battery. Some laptops have that capability, as do some smartphones through custom kernels (Franco's kernel for the Galaxy Nexus has that option, I think).