iPhone Batteries Are Two-Stage Lithium-ion Models
The vast majority of batteries in mobile devices are lithium-ion models, so this part isn’t so surprising. Li-on batteries are simply the most efficient type of chemical batteries available. Apple uses an advanced type with two different charging stages. The first is a fast charge stage that quickly charges the battery during the 0 – 80% charge phase. The second is a slower, trickling charge from 80% to 100%. This helps keep the battery safe and efficient for as long as possible.