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I need to power an arduino uno with a battery but for a very long period of time. And I read a lot of thing about this. Some say do not use a 9V battery with a regulator some other say that it is the best way. SO I am really confused and I need to know what's the best solution for this (taking into consideration the fact that I need my board to last a week at least, if it is possible?)
Thanks!
The Arduino Uno is not really designed for that. Simply keeping the power indicator LED, regulators and extra circuits on for a week will require in the order of 1500-2000 mAh (10*24*7=1680mAh). That is without any application at all on the MCU.
Now it is possible to modify the board and reduce this extra power requirements to less than 1 mA and as low as 0.1 mA. Or even better buy a Pro Mini clone and cut the wiring to the LED and regulator, and power with a 3.7 V/1500mAh battery directly.
Here is a great reference on the subject.
Cheers!
You will definitely not get a week out of a 9V cell - they have very small capacities (and are expensive relative to other types of cell).
Your Uno already has a regulator in it and this regulator wastes a certain amount of energy converting between different voltages. I'm certainly no expert, but from what I've gleaned over the last month or so of owning an Uno, powering it from batteries can be tricky if you want to do it for long periods of time.
It's probably worth trying a 12v sealed lead acid battery - these have capacities measured in Amp Hours, rather than milli-amp hours like most other types of cell. The built-in voltage regulator in the Uno is good up to 20V, just be sure to connect to the barrel jack on the board.
Of course, without knowing more about what you're actually trying to power along with the Uno, it's difficult to make proper recommendations.
I would also suggest looking at building your system with a naked MCU and a voltage regulator tailored to your application. That way you can throw away a lot of the stuff that uses power on the board that you don't need. (Start with a Breadboarduino and take stuff out from there!)
Three main things to do to decrease the Uno's power consumption:
Remove the power LED (you can Google for instructions if you're not sure how to do it)
Bypass the voltage regulator by providing ~5V of power to the 5V pin. You can arrange three sets of AA batteries in series to get about 4.5V which should be fine except that the crystal may run at a different speed (and you may get incorrect results from the millis() function for example). Obviously if that is unacceptable then you will need to provide a constant 5V voltage either by using the Arduino's regulator (which is quite inefficient) or buying a more efficient third-party regulator. The voltage provided by the batteries will vary somewhat, and over time the batteries will start putting out less voltage and eventually your Arduino may start to behave erratically or not at all. If reliability is of high importance then this may not be acceptable to you.
Use sleep modes to the greatest extent possible (see http://playground.arduino.cc/Learning/ArduinoSleepCode). If your Arduino only needs to wake up for a few seconds every hour, you can make a few batteries go a very long way. Note that if you're using a regulator, it will still draw significant power even while the Arduino is sleeping.
Bonus tip: buy a micro instead, or build your own board. (Avamander suggests a Pro Mini 3.3V in the comments below, he's probably right, I don't know much about that board)
