Responding to the first post. I'm going to reply without having read any of the other responses.
I had this exact question and several years ago and did a lot of research to find a definitive answer. I even read a few research papers. Unfortunately I don't have the reference to link for you anymore, this was 5-6 years back. I will recounta summery.
-Most device (laptop, cellphone, etc) work with lithium ion batteries. All lithium ion batteries are essentially the same, same characteristics and life span. (In fact they work very similar to SSD cells).
-A typical li-ion battery has a reliable lifespan of 500-1000 charges from null to full before most cells die out.
dying of all cells are gradual, not sudden. Every time a cell hold is charged it loses some of its sensitivity. It becomes less responsive to charge at the same voltage. After about 500-1000 it stops retaining electrons, retains less in amount, and for shorter time. Electrons can dissipate without use. To regain sensitivity you have to increase voltage, in which case the cell finds "life" again, for a while, before it becomes desensitized again and a higher voltage is needed again. This is why if you charge your device with the wrong adapter of higher voltage the device will charge fast and work nice, but if you then go back to older charger it will have lost sensitivity and wont charge at all. Higher voltage damages the battery, lower voltage fails to charge. Current systems to not play around with voltages.
- High or low current doesn't damage the battery. Low current slows the charging process, reduces stress on charge, prolongs life of battery a bit.
- The life span is strongly affected by temperature and by charge load (% charged).
- The study showed that charging and using the battery was best in temperature between 10c to 40c, the further you ventured outside of 0c and 40c, the battery lifespan became heavily adversely affected. you should avoid below freezing usage. You should avoid getting the phone over 50c. Will kill your batter very fast!
- When the battery was charged from 0 to 100% and then to 0 and then to 100% repeatedly in "room temperature" it lived about 500 cycles.
- li-ion batteries should not go down to 0%. This very negatively affects them and sometimes they die instantly as a result. Special protection is built into devices to prevent this. I can not confirm that when your phone says 0% whether this is "actual" 0 or "protected" 0. The article did not explicitly explain, but alluded that its "protected" 0, which means its actually not 0. The article also did not explain if the research results shown were done from actual 0 or from protected 0.
- When the charging was done from 20% to 100% and then to 20% and repeated, the life cycled double or tippled. (can't recall)
- 20% to 80% - 2000-3000 cycle lifespan
- 40% to 60% - 5000-6000 cycles lifespan. By this it did not mean 6000 charges from 40% to 60%. It meant an equivalent to 6000 "full" charge cycles of lifespan.
- As you can see keeping the batteries close to the middle section close to the 50% mark gives a massive boost to overall life cycles.
- It was shown that best way to preserve the battery for long storage was to charge the battery to 40 or 50% and keeping it in a cold environment 0c to 10c. (Can't recall if the number extended below zero, so I will recount safely)
- overcharging, trickle-charging, exerting power beyond the 100% damaged cells. New adapters apparently are smart nowadays and cut power automatically. But what if the device is left connected for a while? what happens then? I don't know.
I maybe off a few numbers here or there, but what I've explained is whats left in my brain from it.
I actually tried to implement this strategy for a few years and learned somethings. I will share the conclusion with you.
- I tried doing 20 to 80. In reality it became tedious and impractical. It was hard to enforce and I didn't have enough daily lifetime. I also tried 10 to 90, same story.
- As your battery ages, the "last few %" becomes harder and harder to charge and take longer and longer. And when you reach the 100% mark and start using the device those last few % are lost again pretty fast. This is due to cells dying (becoming insensitive). Insisting on charging those last few % actually ends up damaging your battery more, in my opinion. But I can't back it up with scientific data, and I could be wrong. But I think once you notice you have arrived at the last bit, just let it go and unplug.
- charging often in middle ranges doesn't harm your battery. it theoretically makes it survive longer, its just not very practical. So don't be afraid to middle charge and and don't be a 0 100% guy.
- Don't charge over night, unless if you have to. once you notice a full charge, unplug.
- Put all devices that have batteries that you are not using at 50% charge while they rest, and keep them out of the sun.
- Don't put your phone in the sun or on hot surfaces. Hot phones really kill your battery.
- In reality the lifetime you "increase" in your phone by being extra careful may not be that important because your phone may not live long enough in "other aspects" to justify the hardship you put yourself to save the battery, So make a balanced judgment call.
- you need to let your phone run really low and then charge it to 100% about once a month. This is both important for calibration, but also important to give all your battery cells a "jolt". It keeps them from dying off. Partial charging repeated can cause some cells never getting touched and that can be a problem as well. All cells need activity. and stimulation, or they die.
In the end I just came to the conclusion of just not worrying too much about it, just using my phone normally while observing the following.
- don't go down to 5% often.
- don't need to 100% all the time.
- don't overcharge.
- don't night charge.
- Temperature!
- dont charge with computer or other non-charger devices. Their voltage regulators are not very reliable and can overload and over charge. So computer connection.. very BAD! specially if you are already at 100%.