Decreasing tension resistor

Hallo everybody,
i've a 12V barrery to feed Arduino by the plug.
As you now, the real tension of a battery is not properly 12v but a little bit higer.
i would insert a resistor to lower tension under 12V... but i've to know the current circulating to dimensionate the resistor... could you give me some reference values?

If this is a lead acid battery feeding an arduino you do not need it.

If there are shields consuming too much power however you could burn out or trip the regulator chip on the arduino.

i would insert a resistor to lower tension under 12V..

No that is not the way to reduce the voltage. It would only work if the current is fixed. The current into an Arduino is fluctuating on all sorts of time scales. Inserting a resistor is not the way to reduce the voltage, you need a regulator. I suggest connecting a 9V regulator to your 12V source and do not forget to include the capacitors.

If he's powering an Uno or other standard board, no matter what, his board is going to be pulling enough current (even with micro in sleep, due to serial adapter, power LED, and other crap on the board) that, with a 12v source, he'd probably get better battery life using one of the small cheap buck converters from ebay....

Ok guys,
thank you for the tip..
i will use a buck converter...

Go for the MP1584 if your project constantly takes a relatively high amount of power (as DrAzzy said, that is basically always the case when you use an Arduino Uno or Nano, or anything with a constantly glowing LED), else browse Polulu for a low quiescent current option.