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Charge/Drain iPhone to 40% Before Storing
There is an erroneous recommendation floating around on the Internet. It claims that iPhone’s battery should be kept between a 40% and 80% charge. It seems to be a misunderstanding of two reputable recommendations.
If you are storing your iPhone for an extended period, charge (or drain) it to 40% before turning it off. According to research, this will prolong the battery life while in storage. With too much charge capacity, the battery is under stress and discharges slowly, which prolongs the strain. If the battery is undercharged, it will completely discharge and the ions will cease being active. This lack of activity causes the battery to seize and damages its overall capacity. According to Battery University, a lithium-ion battery stored for 1 year at 32º F will still have 98% of its capacity when charged to 40%. At 77º F, the same battery will have 96% of its capacity when charged to 40% prior to storage.
Undercharging Your iPhone is a Tradeoff
Another myth suggests that you should never fully charge your iPhone. There is some truth to this, but there is a tradeoff. It is true that a fully charged battery suffers from more stress than one at the optimal charge — 58%. A battery that is only charged to 58% will have the most charge cycles. The problem is that this battery will also lose the capacity to hold a full charge.
If you decide to charge your iPhone to 58%, make sure to do a full charge occasionally. This will help retain the battery’s ability to hold a full charge. If you are periodically calibrating your iPhone battery, this should ensure that the full range of battery capacity is available.
I have never consistently or intentionally undercharged my iPhone. My iPhone 4 is going on 5 years of active service, with excellent battery life and performance. I personally feel that having 8-10 hours of active battery life is more important than prolonging its lifespan. Undercharging batteries is routinely done in applications where the device may be inaccessible. Satellites, for example, typically don’t fully charge their batteries in order to prolong battery lifespan.
According to Battery University, a lithium-ion battery charged to 58% can expect 2400 – 4000 charge cycles, compared to 300-500 with full charging. This isn’t specific to the iPhone, which does a trickle charge after 80%. Also, keep in mind, Battery University demonstrates that a 25% depth charge cycle will provide 2000-2500 charge cycles, regardless of overall charge capacity. If you go for the more optimal 58% charge, you are sacrificing capacity. Your iPhone won’t last all day. It will definitely provide the most charge cycles, and from 0-58%, these cycles have more depth than going from 80% to 100%.
I personally don’t over think optimal charging capacities. I charge my iPhone as needed, but try to top it off. A fully charged battery is under stress, however, a trickle charge produces less heat. Apple seems to have engineered the charging process to ameliorate the stress of fully charging a battery. I usually charge my iPhones beyond 100% and have never experienced diminished capacity or lifespan.
The science seems to indicate that under charging benefits lithium-ion batteries. It is something to consider, but I wouldn’t lose sleep over it. Just make sure the battery doesn’t get warm while charging, and you should have a battery that outlasts your iPhone’s usefulness. (continue…)