Thanks a lot for all the answers. sorry for asking this much basic question.

What i basically want to know that, I want 12v@2amps dc for my device.

*If i will use a 9v dc battery , Will it work for even for quite some time?

* If i will double the battery in parallel then what will happen ? if I will do the same in serial then ? which way will be batter. ?

If your load draws 2 amperes when it's connected to a 12 volt source and you only have a 9 volt source, then you'll need to connect a boost converter between your 9 volt source and your 12 volt load in order to convert that 9 volts to 12 volts.

Then there's power (P), which is the product of current (I) and voltage (V), so your load will dissipate:
P = IE = 2A X 12 V = 24 watts.

On top of that, nothing is perfect so if your boost converter puts out 24 watts and it's, say, 80% efficient, it'll need to see Pin = Pout / 0.8 = 30 watts on its input.

With a 9 volt source, the input current required to get 30 watts into the boost converter will be:
I = P / E = 20W / 9V = 2.2 amperes.

Now, if you know for how long you want to run your load, and you want to run it from a battery, you can select the battery you need by knowing its capacity (C) at the rate current is taken from it.

For example, if you want to run your load for one hour, the battery's capacity must be such that it can supply the current needed by the boost converter's input until the battery voltage falls below the voltage needed to run the boost converter, which will be something greater than 2.2 ampere hours at 1C.

So... Why can't you just use something like a 12 volt sealed lead - acid battery to run your widget and make life a little easier?

 

I am concern about size as well. So i want the battery as much small ..

Sometimes what you want is made from unobtainium, so you have to step back and figure out what you can do with what you can get.

Or Quit.

 

As wonderful as batteries are........that's the problem with them......not enough charge density(capacity).

If you could increase our current densities one thousand fold..........you would be richer than Bezos and Gates combined.

 

I am concern about size as well. So i want the battery as much small ..

Hi,
What type of device or circuit do you intent to use this battery for? The smallest 12V SLA Lead acid is 12V 1.2Ah with dimensions 97mm (3.82") x 43mm (1.69") x 52mm (2.05") and weighs around 500g. Lead acid cells are pretty cheap but weigh more compared to other battery chemistries. Also if you could post the max dimensions and weight of the battery pack you intent to use, it will be a lot easier.