why most electronic devices perform worse and worse as the battery life shortens? shoudln’t they always perform at peek as long as there is battery?
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Battery voltage drops during discharge. For most “analog” devices like electric motors or incandescent light bulbs their output (rotational speed, brightness) depends directly on voltage. So if a Li-Ion battery drops from 4.2V to 3.3V you’ll see roughly a 25% change in output performance.
Battery voltage also drops the greater the load (current) is. This is called internal resistance. Older batteries in bad shape have a higher internal resistance which makes their voltages drop further. Internal resistance gives an upper limit for the power you can get out of a battery. If you put a thick wire between the two poles of a car battery (low internal resistance, a few mΩ) it will simply melt the wire. If you do the same with a measly AA alkaline battery or coin cell (~1Ω and ~20Ω internal resistance respectively) the wire will merely get slightly hot.¹
Devices with more complex electronics usually have a voltage converter in them which converts the battery voltage to a fixed voltage. For example from 3.3V – 4.2V battery voltage to a constant 5V. These devices shouldn’t see any change in performance when you discharge them. However, If the battery is very old and its internal resistance high the voltage can drop below what’s healthy (or even sufficient to power the device). That’s why Apple had to limit CPU performance on some older iPhones.
¹: Related fact: With LEDs you usually need a dedicated series resistor to limit current. However, when you open one of those tiny coin cell LED flash lights you’ll often only find a LED directly connected to the coin cell battery. Looks crazy at first when you’ve been taught that you *always* need a series resistor for LEDs. Until you realize that the high internal resistance of the battery acts like one.
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