Large capacity batteries to provide extended charge on smart phones, smart watches, fitness trackers, earbuds, head phones, etc. have become ubiquitous. Built-in batteries seldom provide enough runtime to keep customers satisfied, especially when traveling. Laptops and tablets are every bit as portable these days and yet most have failed to accommodate these plentiful and low cost, yet high capacity, spare batteries to keep our large screen devices running longer as well (although iPads do so by using the same connectors and chargers as iPhones).
A simple solution to give laptops and tablets an extra charge would be to build this capability directly into the external power adapter by integrating a single-chip step up dc-to-dc converter into the step down ac-to-dc power supply that comes with virtually all such devices, which typically run on 14-20 Vdc. This could even be done in a standalone "universal power supply" intended to serve as a replacement or spare to the one provided by the manufacturer.
The connectors to such a an adapter would be the 120 Vac plug, a large USB-A connector to charge other devices from the account plug, a micro-USB to RECEIVE power from a spare battery, and of course the proper connector to supply power to the laptop or tablet at its required voltage. The supply source would always be the AC plug when voltage is available but if not it would switch over to check if 5 Vdc is available from the micro-USB port. Amperage limitations may prevent the spare battery from fully powering the device while it's running, but it would at least be able to provide a trickle charge to transfer the external battery's energy to the internal battery over time even if periods of "rest" were occasionally needed by shutting down the device for a little while.
This is a simple fix to give extra charge and extra life to existing or new laptops and tablets.