Developing a “memory” was a problem with older nickel-cadmium (NiCad) batteries. That’s where the whole “I gotta discharge the battery entirely” thing came from. Like we said, it’s not necessary on Lithium-ion batteries.
So why do Lithium-ion batteries not seem to last as long as they age? It’s not about “memory,” it’s about capacity. Your smartphone battery over its lifetime degrades enough that in the same amount of time charging, a new phone could hit a full charge, while an older phone might only get to around 82 percent in that same charge time. BatteryUniversity calls it “old man syndrome.”
Another way to look at it is that newer batteries are just hungrier to suck up all that power.
Apple claims that “Apple Lithium-ion batteries are designed to hold at least 80 percent of their original capacity for a high number of charge cycles” but also admits that the amount differs from product to product.
Apple iPhone batteries also support “fast charging” so they’ll get to 80 percent pretty quickly. After 80, you’ll see the capacity increase slowly, some of which is to prevent heat build up, and that extends battery life. But guess what? Fast charging isn’t great for Lithium-ion battery either—it makes the corrosion go even faster.
The current iPhones come with a 5 watt, 1 amp charger block which works, but of course, you can charge faster with a 5W, 2.1A charger. If you stick to the Qi-based wireless charging, also keep in mind, most support 7.5W, with wireless fast charging now available after recent iOS updates.
Best thing to do: If you are going to charge overnight, don’t fast charge. Use a slow charge. That means your charger should be lower voltage.