As you know a laptop battery consists of a number of battery cells
inside. A pack of cells, produces the correct amount of voltage and
current to power the laptop. This is the reason laptop batteries are
often called laptop battery packs.
External laptop batteries, as the name suggests, do not fit inside the
battery bay of the laptop. They simply sit outside, plugged into the
laptop by the necessary cables. These batteries consists of a greater
number of cells. Allowing for a greater capacity to power the laptop
for a longer time.
Standard laptop batteries have an internal circuitry with chips to
control the flow of current etc, Universal laptop batteries contain
these circuitry, but the tips that plug into the laptop detect the
voltage and current. The various tips that are supplied with the
external laptop battery allows it to be used with almost any brand of
Universal external laptop batteries, give extended capacity/running
time and if you change your laptop it still can be used.