dcvoltage
If I connect 2 6V batteries or two individual sources serially the combined voltage should be the sum of the voltage of each battery (i.e. 12V). Is it possible to get 12 volts from a single source or battery of 6 volts? Actually I have to power my DSL router of (12v-700mA) from my laptop's USB port (5volts).
Schematic diagram will be highly appreciated.
You cannot connect two USB ports in series to obtain a higher supply voltage. You will short out the PC's power supply and although USB is specified with protection for a similar situation, many motherboards do not implement that. This means you'll short the power supply and if you are lucky you PC will restart fine after removing the short circuit.
As an alternative look for a step up / buck / boost converter. These are available from many webshops/ebay for couple Dollars/Euro's and do exactly what you want: input voltage eg. 5V and output voltage most of the time adjustable.
Here is a random example of what I mean.
Constant LED brightness means constant current, and constant current means a current limiter.
simulate this circuit – Schematic created using CircuitLab
Select R2 as (VD2 + VD3 - VbeQ1)/ID1. Don't be afraid to use a current slightly lower than the spec (i.e. 3 to 6mA).
Best Answer
Your laptop's USB port supplies only 500mA. So, that is impossible with the USB port. Because even in the ideal conditions, output power cannot be higher than input power. In your case:
Your output power: 12*0.7= 8.4 Watts
Your input power: 5*0.5(max) = 2.5 Watts
With a 95% efficiency of the converter, you need at least an ~8.9 Watt input power.
However, if you have a higher current source with 5V, you can do it. And the way to convert a voltage by increasing it to a higher voltage is done by a "boost converter"*.
A boost converter is a switch-mode power supply and it can be fairly easy to build one by using a Simple Switcher from National Semiconductor.
Here is a design I selected using WEBENCH. It uses LM2585 at 100KHz. So it will be fairly easy for you to make it work. Because higher the frequency, harder it gets to build it.
*Generally, I think. Correct me here if I'm wrong, guys.