Extend iPhone Battery's Lifetime

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.

*Oxidation at the negative electrode*

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.

*Cells age faster at high temperature*

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.

*Effect of charging voltage on discharge capacity for NMC cells*

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.

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