If you need to work hours with your laptop where there is no place to plug it, for example at the airport while waiting for the endless time between one fly and the other. :( ..
You may be needing something like this, the good thing about this external battery is that it is universal, the bad thing is that is another package to worry about, and to carry also, but almost everybody is carrying two Laptops batteries.
Its specifications are:
weight = 0.75 kg dimensions = 298 x 216 x 15 mm power = 70 VA/hour
And it is a Li-polymer battery so you will not have to worry about discharging, to charge it again it has no “memory” effect, and it is designed to last for 2 to 4 years.
It should add some three hours of work, with WiFi on, and working hard, but of course depending on your Laptop, and the voltage it may needs the more voltage the less time it will last.
The output voltage range between 15,16,18,19,20 volts, so as you can see it is universal.