External Batteries: How Do They Work?
Extended batteries are unlike the battery that ships with your notebook. The most obvious difference is that they come with a variety of tips, since they claim universality with each manufacturer's specific power connector.
Even within a single brand, it is possible to find notebooks that use different voltages or plug sizes. For example, Dell has a standard plug size, but the Inspiron Minis are exceptions to this rule.
The stock power adapter that ships with your notebook supplies voltage at a fixed setting. This is not so for the external batteries. In order to deliver the multiple voltages required by different devices, they must also be capable of outputting different voltages.
There are a couple of ways to do this. The easiest way is to use a voltage converter controlled with a manual switch. Some companies choose to go a step further by using a sense resistor in the adapter tip. The sense resistor drops the voltage as current crosses its path. The small voltage drops are fed to a simple comparator circuit, which is then used to control the VDC output from the power supply.
If the resistor is in the tip, there is an additional layer of protection because you cannot force an incorrect voltage over a plug. Physical switches give you more control, but there is a chance that you'll choose an incorrect voltage and damage your notebook.
The way voltage gets to your notebook is important to understanding how an external battery can affect run time. Say your internal laptop battery yields two hours and your external battery offers an additional two hours. If your notebook is at 50% and your external battery is at 100%, this doesn't mean you get three hours total. There is an efficiency loss involved whenever you make an energy transfer, and you see this happen whenever the external battery charges the internal battery. This ineffiiency is less than what you see with an AC to DC conversion if you were to use a UPS, but it is there all the same. Remember that this only applies during a transfer of energy from battery to battery. If your notebook was at 100% and external battery was at 50%, you would see three hours total.