Posted 13 January 2013 - 22:48
there's a lot of battery myth around. for lithium ion/poly batteries in computers/phones/etc. there are three major factors affecting battery wear, only one of which you can really make a big difference in.
1. temperature: you obviously don't want the battery to be too hot or extremely cold. since the battery sits so close to the rest of the components which all generate heat, that arrangement isn't exactly ideal. however, there's not much you can do about it. you can't run the computer without power so you kind of need the battery to be in its slot. you can take it out and run only on AC when you're near an outlet, but there's a convenience/reliability trade-off in that if the AC is unplugged you lose power immediately. Considering that in well designed systems most of the time the temperatures won't exceed the battery's safe range, this isn't really worth the effort to save a tiny amount of battery wear.
2. charge level: ideal storage charge is ~40% full capacity. at either extreme (0% or 100%) it stresses the cells and wears them down more quickly. from anecdotal observation the near 0% end is a lot more damaging than the near 100% end. you can't really do much to optimize this either since it's not really practical to run around with a half-charged machine, although some manufactures do give you an option to artificially cap the maximum amount to charge your battery in order to lower wear (e.g. only charge to 80% to avoid the stresses of a full charge). that's dependent on the machine you have and battery time you require. in general, try to charge up your computer at around 20% and don't let it fall below 5-10% unless absolutely necessary. not much else you can do about this.
3. charge cycle: these batteries are usually rated to between 300-500 cycles (newer ones seem to be 300 cycles more often). each cycle is one full charge and discharge. so say if you only used 50% and charged it back to full, that counts as half a cycle. this is critical to your decision of whether to keep it topped up. since there's no memory like old alkaline batteries, the only concern is how you can minimize the number of cycles. so let's examine several scenarios.
3a. same usage pattern as "memoried" batteries (i.e. charge it up, use it down, charge it back up...): this is the worst result you can get. you're constantly charging and discharging the battery unnecessarily, using up precious cycles and repeatedly hitting the stresses of the high and low charges.
3b. manually keeping the capacity at ~40% charge: sounds good, except by manually doing it, what you're really doing is just continuously charging/discharging the computer by a couple of percentages each time at a higher frequency, since when you unplug the AC you're actually using the battery. this gives you basically the same cycle-count as 3a. not to mention the hassel of constantly plugging/unplugging and monitoring the percentage.
3c. have an application do it for you: this is much better. when the battery hits a designated percentage you don't switch to running on battery power, but simply stop charging until the natural discharge (loses some charge very slowly even when nothing's using it, happes to all batteries) reaches some level. gives you the same cycle-count as holding it at full but without the stress. con: you're only at around half-capacity when you need use it unexpectedly, and I don't know of any battery utility that actually lets you do this.
3d. run on AC with the battery pulled out at ~40%: this is even better for the battery, you avoid the heat too. but like I said above on temperature, the benefits don't really justify the convenience and reliability of having your battery in.
3e. keeping it at full charge at all times: this seems pretty bad on the face of it because you're constantly holding the battery at a stressed level at full charge. however, that's really the only downside. you have a fully charged battery ready to go. it gives you some flexibility, and you're not constantly charging and discharging it so you're not using cycles like crazy - at full charge it cuts off automatically, and since you're not actively using the battery, the natural discharge takes a while before capacity drops enough to engage a top-up charge, so it's not really constantly doing it. these top-up charges are eating into your cycle count but they're small enough that it takes a lot of them to eat away one cycle. it's not going to be a concern for most people at the rate with which these things are replaced with newer and shiner stuff. yea the 100% stress is bad, but it's far worse to keep using up precious cycles.
so looking at the options, keeping it plugged in whenever you conveniently could strikes the best balance between convenience and battery-preservation for the vast majority of use cases despite all the tips online about not keeping it at full charge all the time, because the alternatives either involve a lot more work on your part, or in fact wears down the battery quicker.