I want to see our devices to work with input power when available and use battery only when there is no input power. In other words, a device must be able to p

I want to see our devices to work with input power when available and use battery only when there is no input power. In other words, a device must be able to power on and function correctly without a functioning battery plugged in. I think all notebook computers pass this test. Why can't phones and tablets do the same?