If there's one thing that rechargeable batteries do not like, that's getting hot. According to Apple, iDevices are good operating in a range of 0 degrees and 35 degrees Celsius (32 degrees to 95 degrees Fahrenheit), and the company warns that using its devices "in very hot conditions can permanently shorten battery life."
I'm quite fortunate that I live in quite a mild to cold climate here in the wilds of Wales, but I have noticed that I put my iPhone through thermal stresses several times a day.
And that's when I'm charging it.
I've been keeping an eye on how devices recharge -- especially iPhones -- for some time, and that's allowed me to build up some patterns, even how the temperature fluctuates.
Here's what I've noticed.
The faster the charge, the more heat is generated, and wireless charging also warms up the iPhone more than charging the iPhone with the supplied charger.
The longest time my iPhone spends on charge is overnight. I plug it in before I close my eyes and unplug it as soon as my eyes open. That's between five and nine hours a day.
Apple introduced a feature called Optimized Battery Charging in iOS 13, the idea being that when the iPhone is hooked up to a charger for extended periods -- specifically overnight -- that controls the charging so that the device is fast-charged to 80%, with the final 20% added in time for when you wake up (for me, charging is always completed before 5:30 am).
So, how have I changed my charging?
Bottom line: I've switched to using the supplied charger for overnight charging. This is perfectly acceptable for overnight charging. Why fast charge something while it's on charge overnight? All that does is cause more heat and more unnecessary battery wear. Same with wireless charging (which is a bit flakey for overnight charging anyway, because a small shift in the phone's position on the pad can stop it charging -- unless I was using MagSafe), it just generates unnecessary heat.
I still use a fast charger during the day for quick recharges, because that's when the extra speed is useful, but for overnight charging, using the slower stock charger is perfectly acceptable. I've even tried the slower USB-A chargers and found them great for overnight charging, as they do the job and cause a minimal amount of heating.
Also, when charging devices during the day, I only partially charge the battery, up to about 80%, and avoid long-term idle charging where devices remain attached to chargers indefinitely.
This has been such a successful strategy that I've adopted it for all my smartphones (yes, I have multiple devices, including devices I'm carrying out long-term reviews on), and it works great.
I've also found that by leaving my devices alone while they are being recharged, the battery level readings and predicted discharge times are more accurate. I'm assuming that putting a load on a device -- especially a heavy load -- while it's charging skews this data.
My hope/prediction/wish is that this strategy gives me the best of all worlds -- slow, safe charging overnight that doesn't stress the battery too much and allows me to get the longest life possible out of it.
It's also a great way to reuse older chargers, so there's less e-waste being generated all around.