I want to run the Arduino Pro Mini on batteries, and I was wondering what are the available options.
Sparkfun list 3.9V LiPo batteries as an option. But those need an external charger.
I wonder if I can use 2 x 1.5V AAA bateries or do I need 3x1.5 AAA?
The same goes for coin cell batteries, could I use 1x3V CR2032 or it's the voltage too low?
Interesting option. If i use a 5V step-up converter and connect to the RAW pin in arduino pro mini, how efficient is it? I mean the pro mini runs on 3.3V. I heard that the voltage regulator wastes a lot of energy. Is that true?
I would go with 2 AAA cells into the VCC input.
The regulator on the sparkfun device is pretty efficient at getting up to 5V, but I'm not familiar with the Arduino Pro Mini, so it might well burn off 40% of the power to get you back down to 3V.
If you need a minimum of 3V though, I'm not sure how well 2AAA would work as the terminal voltage drops over its life.
Hi Oracle,
I haven't use the pro mini but I believe its clock is 8mhz. The ATmega168 datasheet says its specified for 10mhz operation at 2.7 volts so I would guess that a couple of alkaline cells would be fine.
I had a look at the schematic for the pro mini, it has some sort of voltage regulator and the spec page for the pro mini says a minimum voltage is 3.35 V http://www.arduino.cc/en/Main/ArduinoBoardProMini
I'm not sure if that minimum is related to the regulator, since the schematic labels the regulator ###V. I suppose you can connect the batteries after the regulator though.
Yeah, the specs are a little confusing. I would doubt the regulator could handle 3.35 volts on the raw input.
I think what the specs mean is a minimum of 3.35 volts connected directly to the VCC input and up to 12v on the raw input (the input to the regulator). Not sure why the arduino specs say 3.35volts and the Atmel specs say 2.7 volts. Perhaps someone more familiar with the Mini Pro can clarify.
I found this interesting link in the sparkfun forum about batteries and voltage regulators
http://forum.sparkfun.com/viewtopic.php?t=7023&postdays=0&postorder=asc&highlight=pro+battery&start=0&sid=002224d75ec4bd05c53edd2775180204
In my case since I want to power also an XBee I cannot use the LiPo battery directly connected to the Vcc because that will destroy the XBee (2.1 -3.6V). right?
I'm just wondering if the 3.35V spec on the Arduino Pro Mini is because they want to maintain compatibility with 3.3V peripherals. Even if the MCU will work at 2.7V, it doesn't mean that 3.3V I2C eeprom will as well.
Hi Oracle, FWIW, The nominal 3.3volt stuff I have come across like I2C EEPROMS and Xbee are are specified to work at 2.7 or 2.8 volts, so I would think would be ok on a couple of alkaline cells. YMMV
Just for the record, the Arduino Pro Mini voltage regulator appears to be a Micrel 5205 MIC5205 http://www.micrel.com/_PDF/mic5205.pdf labeled as KB33 on the chip.
/me would recommend NiMH, Li-Ion or LiPo rechargeable batteries
You will save a lot of money over time and help the environment too 
NiMH batteries usually last for 800-1000 charges
Li-Ion and LiPo batteries should last even longer
I just found these two options
Lithium Backpack Stealth
http://www.liquidware.com/shop/show/BP/Lithium+BackPack
Arduino AA Undershield
http://www.instructables.com/id/The_Arduino_AA_Undershield
Lithium Backpack Stealth
FWIW, I've got a pair of these and they work pretty nice.
Note that you'll need to manually flip a switch on the shield to charge the battery - not a big deal, but could be a problem if you want to mount the shield in a remote location.
Brad.
The AA backpack is pretty attractive too
Just use some 1.5V NiMH AA batteries, you will save a lot of money on batteries that way
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXKFP2&P=FR
With a 15min charger you'll up and running in no time
Just FYI, nimh is 1.2 volts per cell, unless I'm missing something here.
These NiMH batteries are 1.5VDC and are available in 2500mAh
And can be charged in 15min
You can usually find them on sale online and at walmart in the US for less than Amazon
Some people claim other brands of high-capacity NiMH batteries are even better so do your research and get whatever is best for you
"These NiMH batteries are 1.5VDC and are available in 2500mAh "
Same batteries, different data sheet showing 1.2volts.
http://www.batteries.com/productprofile.asp?appid=330000
All single cell NiMH batteries have the same terminal voltage, it's a chemistry thing 
Here is an extract from Wikipedeia:
The charging voltage is in the range of 1.4–1.6 V/cell. A fully charged cell measures 1.35–1.4 V (unloaded), and supplies a nominal average 1.2 V/cell during discharge, down to about 1.0–1.1 V/cell (further discharge may cause permanent damage). In general, a constant-voltage charging method cannot be used for automatic charging. When fast-charging, it is advisable to charge the NiMH batteries with a smart battery charger to avoid overcharging, which can damage batteries and cause dangerous conditions. A Ni-Cad charger should not be used as an automatic substitute for a NiMH charger. [4]
Lefty
Aye, aye but mine routinely measure 1.5 - 1.55VDC or a bit higher when fully charged and are new enough
Virtually all my other NiMH batteries measure 1-2VDC less than the energizers so draw your own conclusions
I will post pictures later