My understanding is that the internal resistance of the battery increases as the battery ages.
You basically have a voltage divider - with some of the resistance being the load (phone electronics) and some being the internal resistance of the battery. The voltage from the battery is split across these resistances. Normally the load gets almost all of the voltage.
When the battery is old and the peak current demand comes through the battery “sees” this as a lower resistance. The larger internal resistance causes a bigger voltage drop internally in the battery and the load therefore gets a lower voltage. When this is lower than the phone needs you get a brownout/reset.
That can't possibly be true. There are only be on the order of 1000 day/night charge/use cycles in the life of a battery and that number is too small for high-cycle fatigue to appear. Low cycle fatigue can happen but only with extreme strains where it's obviously deformed.
Also, if you could make batteries last longer just by using more finely stranded wire, somebody would have done that.
Not only that, but a crack doesn't lower resistance by much because the length of the current path is so short that the narrower cross-section hardly affects resistance over the whole length of a conductor.
That shouldn’t be that case for a well designed battery pack.
An unavoidable issue with all lithium ion batteries is plating during charging. This process increases the output impedance of the battery reducing its ability to deliver power to a load, regardless of how much energy is inside it.
You basically have a voltage divider - with some of the resistance being the load (phone electronics) and some being the internal resistance of the battery. The voltage from the battery is split across these resistances. Normally the load gets almost all of the voltage.
When the battery is old and the peak current demand comes through the battery “sees” this as a lower resistance. The larger internal resistance causes a bigger voltage drop internally in the battery and the load therefore gets a lower voltage. When this is lower than the phone needs you get a brownout/reset.