I normally charge my iPhone all night to get 100% power. Does this harm my phone's battery?
Update [Oct 1, 2017]: Thanks for all your great answers. I noticed there is a related video which addressed my question well.
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Lithium Polymer batteries, such as those used in the iPhone, are a very well understood battery chemistry and use specific chargers to manage their charging.
While the latest charging ICs Apple uses are proprietary (they work with Dialog for their power management ICs), it is unlikely that Apple performs a significantly different charging process than every other battery charging IC manufacturer.
This process charges the battery as quickly as reasonable while protecting the battery from damage or increased wear and tear. Please note that the charging IC - or Power Management IC - is located inside the iPhone. So this applies regardless of the external USB powering device you use to charge it.
When the battery is full, the charger turns off the charging process. The battery is left alone without current going into or out of it. This is because most Lithium chemistry batteries don't like trickle charging (a process you may hear from others where a constant voltage and low current is applied to a battery all the time to keep it topped off).
Charging ICs therefore turn the current completely off and power the phone itself from the charger. This is the only time the battery isn't charging or discharging - when it's full and the phone is plugged in.
The battery isn't discharged and charged in some sort of cycle after charging is complete. I'm not sure where this myth stems from, because it would actually place additional wear and tear on the battery.
Every battery has a "self-discharge" rate. This is the battery's natural capacity loss when not connected to anything. For Lithium Polymer batteries, this is a low (compared to other rechargeable battery types) 5% per month.
The charging IC does check up on the battery and if it loses significant charge over time while plugged in, it will start another charging cycle. But this would occur perhaps once every two weeks after several percent of the battery charge has been lost - certainly not overnight. If you leave your phone plugged in for a month, it might start another charging cycle once or twice in that time.
The phone and its battery, therefore, will suffer absolutely no damage being plugged in for days at a time.
There is one caveat to this answer: Lithium chemistry batteries don't like to be stored with full charges. They degrade more quickly if they aren't used and have a full charge. If you are going to store your mobile device unused for more than a month, charge it up fully, then discharge it (watch videos over wifi or play video games) until it's 40-50% charged. Then turn the device fully off.
If you are using an older iPhone in a fixed installation (picture frame on the wall, data collection, baby monitor, etc) while turned on and in use with it plugged into a charger all the time, you should consider disconnecting the charger and discharging it fully once a month. Similar issue to the above - the battery doesn't like being "stored" for long periods of time at full charge, and the charger will keep it near 100% the entire time - degrading the battery a bit more quickly than desired. Going through a discharge cycle will exercise it and help it to last longer.
While I use a newer phone now, I charged my iPhone 3GS every day for 8+ hours a day (ie, overnight) regardless of how much charge was left - and more often than not, I had used less than 20% on a given day. I used it for 4 years that way, and never replaced the battery - it still lasted much longer than a day. Since then (3 years now), it's been essentially an iPod touch, being charged for days at a time when no one is using it, and being left in the car discharged for days (or weeks!) at a time when it's forgotten. Battery is still going strong, and this is certainly more than 900 charging cycles for this device.
My iPhone 5 and iPad (3rd gen) were used the same way. No negative impact from having them charge all night every night.
I'm now using the iPhone 6+ the same way. This is an expected use case for a mobile device, and it's running just fine. I expect that to continue to be the case. You can leave your iPhone plugged in all night and it will not be harmful to your iPhone.
No, it's not harmful.
Modern phones are smart enough to stop the electric current when fully charged. That may have been a problem before lithium-ion batteries, but nowadays you shouldn't have to worry about it.
However, if your iPhone has a case that doesn't allow heat to escape very well, it might be better to remove the case to avoid overheating. Lithium-ion batteries don't like drastic temperature changes or extreme temperatures.
No it does not. Nightly charging is the norm today for many people, and unlike older technology from a few years ago, today's lithium ion based batteries use newer technology and battery life is not damaged with nightly charging. All batteries wear out eventually, but unless you are charging multiple times per day, your battery is likely to outlast your phone or at least last a couple of years.
Leaving the iPhone to charge overnight doesn't affect your iPhone drastically. As phones are very smart, once it gets fully charged, it knows when to stop the current from coming in to protect the phone from overcharging. But there are also some reason which can affect your iPhone plugged in overnight, this can leads to slow down the lifespan of your iPhone. Lithium ion battery is used in iPhone, which can react poorly if your iPhone experience higher temperature.
Li-ion does not need to be fully charged, as is the case with lead acid, nor is it desirable to do so. In fact, it is better not to fully charge, because a high voltage stresses the battery. Choosing a lower voltage threshold, or eliminating the saturation charge altogether, prolongs battery life but this reduces the runtime. To satisfy maximum runtime, most chargers for consumer products go for maximum capacity; extended service life is perceived less important.
Technically the Li-ion charging concept is given below.
Lithium-ion operates safely within the designated operating voltages; however, the battery becomes unstable if inadvertently charged to a higher than specified voltage. Prolonged charging above 4.30V forms plating of metallic lithium on the anode, while the cathode material becomes an oxidizing agent, loses stability and produces carbon dioxide (CO2). The cell pressure rises, and if charging is allowed to continue the current interrupt device (CID) responsible for cell safety disconnects the current at 1,380kPa (200psi).
As someone who works for a battery specific retailer and is asked this question a lot, the answer is "it won't hurt it in the short term, but in the long term it will shorten its lifespan." Between your phone's charging system and the internal circuit board built into the battery itself, there are enough layers of protection that will prevent either your phone or the battery from being damaged under normal circumstances. However to my understanding, as part of that process, if it is left in the charger beyond the point when it is charged, it will start going through charge cycles - discharging a little bit and filling back up - as part of a safety measure, and those little charge cycles will add up over time. Since Lithium Ion batteries are rated to have 250-500 cycles during its overall lifespan before it is worn out, it's better to preserve those cycles when possible, especially given phone and therefore battery usage is oftentimes pretty heavy. A good example is eating cheeseburgers: one or two won't kill you, but if you do it daily for 40 years, you'll experience some pretty significant health issues. Same concept with leaving a phone plugged in.
When my customers purchase a replacement battery from me, I oftentimes recommend they alter their charging habits. The most common is to plug it in prior to bedtime and leave it in all night. While certainly not dangerous or damaging to the battery, overall it will wear it out faster than if you didn't do this. Instead, I suggest they change their habit to charging their phone when they get home from work/school/shopping and go about your daily routine: cook dinner, wash dishes, watch television, etcetera and periodically check in on it. Once it's fully charged, unplug it.
The charge controllers which charge the battery are really smart. They are designed by really great companies such as ST Microelectronics.
These charge controllers typically have many features. (I don't know if the ST ones I linked have all of them. I just pulled those parts out of the air). Every Lithium battery charging IC will have a smart charging algorithm which WILL PROTECT AGAINST OVERCHARGING. Lithium batteries are different from NiCd batteries in which you can pump a small current through continuously which will be converted to heat directly. A Lithium battery will get destroyed (violently) if subjected to minimal overcharging.
Some of the higher-end chips, like the ones used in Apple devices are really smart. They understand how a Lithium battery will last the longest and will adjust the charge and discharge to ensure the longevity of the life cycle of the battery. As mentioned in an answer above, Apple fast-charges the battery from 0-80%. The reason why they do this is because as you get closer to 100%, the charging efficiency becomes much lower. After this stage, the charging slows down. It typically goes into constant voltage mode where a specific voltage is set and the charge circuitry waits for the voltage to reach a certain threshold. However, after the battery is fully charged, there's no point in recharging it when it's voltage is 1/1000th a volt lower than what it should be. Instead, the charger has a set cutoff voltage when it will start charging again. This is often pretty low, like 50-70%. The battery is simply disconnected from the phone internally and the phone is powered off the power cord. The charging will only begin once the battery goes lower than the threshold which is set up by the manufacturer.
This prevents the wear and tear on the battery.
As a side lesson, the isolation between the battery charge circuit and the power adapter/cable you use makes very little difference to the charge quality as long as the design is not so piss poor that it causes too many voltage spikes and dips.
If you use your phone while plugin
it's not good if you do it on regular basis it can hurt your battery life. I don't recommend that you should charge your phone all night. I read some suggestions that you should turn off your phone and then plugin all night.
I recommend Just charge it to 40-50% before going to sleep.