I searched the battery types just now and found out that a 9V batter has only 500mA :/
Now all the calculations goes wrong..
How can I reach around 30 watt then with such a bettery? I don't mind adding another battery if that will solve it
Hello again Sara,
Well as you probably gathered by reading the other posts before this one, using a regular little 9v battery isnt going to work very well. That's because the energy stored inside the battery depletes fast because there just is not much in there to begin with. You need something bigger like Clyde posted, a lead acid battery. But even that may not be enough depending on how long you want to run your coffee warmer.
For example, a 12v, 7 AH battery (that's 12 volts, 7 Ampere Hours) can supply less than 1 amp for 7 hours. At 3 amps it goes down three times faster, so divide 7 by 3 and we get 2.33 hours, which isnt that long really. And because we are draining it with such a high current of 3 amps it wont even last that long. So maybe we'd get 1.5 hours run time and then the voltage would go down fast so the heater/warmer would not work anymore.
What you really need here is a good plug in power supply or wall wart that can handle the 3 amp load without a problem. An old PC computer power supply could easily handle this, but it would be kind of big and bulky. So you may want to look around for a power supply or wall wart.
If you absolutely must run this as a portable device, then you should consider the size of the battery and the run time you get. Here are a few examples:
12v, 9AH, about 3 hours run time (battery not too heavy, about 6 pounds),
12v, 18AH, about 6 hours run time (battery might be a little heavy),
12v, 36AH, about 12 hours run time (quite a heavy battery though).
These are based on around 5 ohms load which would cause a 2.4 amp current draw on the battery, not 3 amps.
Note that when we go to 12v we increase the total resistance to around 5 ohms, but if we go down to 6v we'd have to decrease the resistance to 1.2 ohm which would draw 5 amps not 3 so it would run down even faster. So 12v is the recommended battery voltage because batteries 6v and 12v of the same ampere hour are often priced about the same.
So the question now is do you still want to use a battery or use a power supply of some type.