almost all apple idevices will not run plugged in if the battery is dead. thought I could do this on an old click wheel iPod but nope, had to replace the battery before i could use it. it did not use to be common, i had no issues keeping other devices like my xperia play powered on when removing the battery, but that is also a device where the battery is meant to be user replaceable.
Lots of battery-powered gadgets cannot run off the charger alone, because they rely on the battery as a massive capacitor to handle transient current spikes that are beyond the capacity of any cheap, compact charger.
But that's entirely different from replacing the battery itself with an expensive bench power supply. That can definitely work, especially if it's a high-end source/measure unit capable of being programmed to emulate the discharge curve of a real battery.
Every laptop I've ever owned will run with the battery disconnected.
Except the Macbook, of course.
The simple fact of the matter is that Apple routinely makes design decisions that are not only questionable, but compromise the usable life of the device in the process. And keep repeating the same mistakes over and over.
It's not just Apple. I can confirm that a Samsung Galaxy Note S4 will not run from its original charger on its own. A Battery is required during boot (whether to act as supply for current peaks, compensate for losses on the thin cable during such current peaks or whether the system expects to talk to the controller built into the battery, I cannot say).
Your argument is that iDevices like the iPod and iPhone require more stringent 'massive capacitor' power delivery with an inline battery over... more power demanding/complex devices like laptops? And other phones that had removable batteries?
Sounds like an attempt to handwave away bad design principles.
The extent to which a device relies on the battery as a capacitor even when plugged in depends on both the peak current draw of the device, and the size of the charger.
Smaller devices like phones tend to be paired with downright tiny chargers, but modern smartphone SoCs can draw a lot of power in short bursts (cf. all the controversy about Apple trying to prevent brown-outs when operating the phone with a worn-out battery). Apple's laptops are usually paired with power bricks that roughly match the maximum sustained power draw of the laptop. Windows gaming laptops tend to ship with power bricks that are 2-3x larger than any Apple power brick.
