Power consumption
The Arduino boards use a fair bit of power compared to other embedded systems with similar functionality.
There are three main factors:
The NCP1117 (datasheet) 5V linear regulator in the Arduino UNO
R3 (schematic) has a quiescent current of around 6mA.
The ATMega328P (datasheet) draws around 5mA @ 8MHz and 5V, and probably more than double that at 16MHz.
user2973: The ATMega16U2 used for USB communications also draws approximately 13mA.
LEDs and other peripherals also draw some current. In your circuit, the LCD backlight is probably drawing 4mA as well.
When dropping 9V to 5V via a linear regulator, almost half of the power is lost by the regulator due to its 4V drop. Duncan comments that this nearly doubles the quiescent power draw from 9V as well as the power needed for every mA of 5V, since 4/9ths of the power gets wasted as heat by the voltage regulator. An efficient switching regulator would put out 5V with little wasted energy, effectively reducing the current draw seen by the battery by 4/9ths.
A duracell 9V battery (datasheet) drops from 9V to 7V in around 7.5 hours with a 50mA current draw. Therefore, a rough guess is that your circuit draws around 25mA, which sounds about right based on the description of your circuit.
Note, alkaline battery life is non-linear with respect to current. For very small currents (<1mA) the life of an Alkaline approaches that of a lithium battery.
Getting current down
Here are some tips to get the current consumption down:
Regulator: Replace the regulator with one with a low quiescent current, or better yet, a switching regulator (also with low quiescent current). A switching regulator uses 'pulses' of current and some external inductor and capacitors to give a reasonable steady voltage output. It doesn't waste energy like with the voltage drop of linear regulator and efficiency in the high 90% are possible.
- There are buck (step down) converters that take the battery as input, then connect directly to 5V and GND, bypassing VIN and the regulator. This one from Pololu both steps up and down, and has 0.1mA quiescent current.
- Alternatively, you could use some 1.5V alkaline batteries, and a boost (step up) converter to get the voltage up to 5V (e.g. this product from Sparkfun). It seems boost converters are more commonly stocked.
- Finally, you could buy a rechargeable lithium battery with charging shield. This advantage of this is not having to buy new batteries, and for a tiny bit bigger than a 9V battery the lithium one has much greater capacity. A really cool product is the seeeduino stalker waterproof kit that includes a charging circuit, battery, solar panel and other goodies.
ATMega328P: Rather than using delay for timing and spinning in loop endlessly waiting for something to happen, re-write your code so that it goes to sleep in between sensor reads, etc. There are a few low power libraries out there that use the watchdog timer for periodic wakeup from sleep which are handy. You can get the current consumption of the ATMega328P below 0.1mA during sleep.
LCD: Turn off the backlight, or even the entire LCD. Add a button to the design that the user can push to activate the LCD and have it turn off after a set amount of inactivity.
Peripherals: Most peripheral chips also have a sleep mode that drastically reduces their power consumption. Remove power LEDs and other indicators that are not necessary.
ATMegu16U2: user2973 comments It seems this chip is quite power hungry (user2973). It could be removed to save power and just use the UART instead, but that seems like overkill. There are Arduino Pro boards that are just the bare bones Arduino without the USB interface that could be used instead of the UNO.
Batteries: Other alkaline cells have much greater capacity. For example, a 1.5V AA has over 2000mAh for low currents. Using AA cells plus a boost converter can increase the time before battery replacement. Use D cells (16000mAh) and it will run for quite a while. :D
Summary
With appropriate power supply and coding you can get a reasonable amount of life out of a battery. Using the above principles, I have made an Arduino derivative board that measures a few sensors and stores the readings to an SD card every half a second. It can last for about 4 months on 2 AA batteries, so it is definetly possible to have low power and remain in the Arduino ecosystem.
The chip I used for the low power in my board is the LTC3525-3.3V. It takes an input voltage as low as 0.8V and boosts up to 3.3V and a 5V version is also available. I designed a PCB for this chip as there was no ready-made breakout, and in the datasheet there are reference designs. The main critera for choosing this chip was it still had high efficiency at very low currents. Some other converters need a small minimum current draw.
The biggest consumer of power on the board ended up being the micro SD card. It can vary between 0.1mA and 1.5mA of idle current depending on the manufacturer. I have found Verbatim and Lexar cards to consume the least amount of power. I will this EE.SE question up to date with the results of my micro SD card power consumption tests.
