We touched upon iPhone batteries in two previous posts. When the capacity of Li-ion batteries drops to 80% of brand new cells, they're considered EOL. What reduces the cells' ability to hold charges?
Apparently there're chemical processes at work between charging/discharging. The processes develop oxidation at the negative electrode. The oxidation forms a thin film covering graphite which prevents more charges to stay. The slide below magnifies the negative electrode of a Li-ion cell when it's fresh and aged.
Factors accelerates ageing
Temperature and charging voltage are two killers of Li-ion batteries. The chart below shows the effect of working temperature on remaining capacity after a number of cycles. The cells are Lithium iron phosphate (aka LFP). We can see more capacity is lost at higher temperate at the same level of use.
Impact of charging voltage is shown in the following slide of Lithium Nickel Manganese Cobalt Oxide cells (aka NMC). At a charging voltage of 4.25V, the cell retains 100% of its original capacity after 200 cycles. At 4.35V, no drop after 120 cycles. At 4.45V, the effect is catastrophic. It only takes 40 cycles to reduce its capacity to 60% of a fresh cell. Another interesting tidbit is the trade-off between charging voltage and the maximum capability to hold charges before break-downs.
Take-away for iPhone batteries
iPhone batteries are usually known as Li-ion polymer. Polymer refers to a gelled electrolyte but does not indicate any type of Li-ion technologies. Li-ion polymer batteries can be built using Li-cobalt, Li-posphate, Li-manganese, NMC or LFP.
I usually start charging my phone when the battery indicator is around 40% and unplug the phone when the indicator reaches around 90%. These two points refer roughly to a working voltage from 3.8V to 4.2V. It's considered optimal for longevity. I'll find out through the exercise mentioned in my previous post. I also ensure the battery does not go too hot during charging and use.
Above tips do not limit to iPhones. They also apply to other manufacturers' Li-ion batteries. So start taking good care of the batteries inside your gadgets!
Slides for this post are taken from this interesting talk.