There is a difference between energy density and power density. Energy density is how much charge is stored in the battery. Power density is how much electricity can be delivered per period of time at a point of charge.

A battery may have 2,000 milliwatts of charge, but can only deliver 500 milliwatts per hour; thus, it could run at full blast for ~4 hours or half at ~8 hours if the power density is not dependent on the charge. Batteries normally have a power curve, the amount of power you can draw depending on the charge in the battery. When you over draw the power on a battery, the tendency is for the voltage to quickly drop.

Cycling the battery and age both play in lowering the energy and power densities, but they are not one-to-one correlated. This is why you'll see people who say that their battery will last to 40% and die. The battery's voltage is used to gauge how much charge is left in the battery; however, at that level, the power density has dropped such that any significant draw on the battery causes the voltage to plummet and the phone shuts off.

> 2,000 milliwatts of charge > 500 milliwatts per hour

It's bad enough journalists confusing watts and power and energy but better not in the middle of a post explaining the difference between energy and power!

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.

Resistance increases due to microcracking (fatigue) of the copper leads from thermal cycling.

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.

Is really the case? an you link something so I can find out more?

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.

https://www.sciencedaily.com/releases/2014/09/140903105638.h...

the difference is that a brand new battery would be able to deliver enough current to sustain the peak workloads, even when at 1% battery remaining. An aged battery would not be able to do this.