Well I am able to see the voltage of the battery(LiIon) of my phone, and my laptop, so it"s not flat, but they are designed to report their voltage by the OEM. I don"t know much about hardware, so for me it"s a mystery how they do it exactly, but they do it. A 3.7V LiIon cell is 4.2 volts when full, and around 3.4 when the integrated circuit kicks in, and shuts it off(to prevent cell damage from full discharge)
Someone who knows more about electronics can say exactly how, and is it relatively easy to implement.
Maybe, and this is my imagination speaking, we could achieve at least a low battery warning by using a logic gate, which turns on at certain voltage, and sends a signal to a GPIO pin.
Most probably there will be some soldering required, but I"ve been thinking about learning how to solder, and I think this is a good occasion