Hi.
I have a small project where I have an Arduino that controls a servo. Simple stuff. But I would like to know if it's possible to run both on the same battery to save space.
I can only find examples where the servo is powered by 4xAA and the Arduino by a 9V or usb.
How long do you need to run it for?
Running an Arduino off a 9V battery is really wasteful.
You'd be better off with a cheap buck converter and maybe a 7.2V battery.
There should be no problem running the servo and a 5v Arduino (such as an Uno or Mega) from a pack of 3 x AA alkaline cells (4.5v) or 4 x AA NiMh cells (4.8v) with the battery pack connected to the Arduino 5v pin.
...R
Thanks a bunch! I'll try the 3xAA pack and see how long it can run on a fresh set of batteries. I'm building a automated windows opener that can open my window a couple of times during the day to ensure a better indoor environment.
Buck converters are also interesting. Just curious... why is a 9v wastefull on an ardiono? And why not use a 9v and a buck converter for the servo?
Just curious... why is a 9v wastefull on an ardiono
Because the linear regulator on the Arduino converts excess voltage (the difference between the input voltage and the output voltage) to heat.
9V batteries are relatively expensive, given their low capacity.
jeppeburchardt:
I'm building a automated windows opener
That sounds like a stationary project that could be powered from the mains - perhaps using a USB charger that can produce 2 amps
...R
jeppeburchardt:
I'm building a automated windows opener that can open my window a couple of times during the day to ensure a better indoor environment.
I like the project but how big is the servo?
What are its specs to lift a window?
Thanks.. Tom.. 
I a small power supply that came with a kit. 9V 500mA 4.5VA. Could that power the Arduino and the servo if I connected the servo to the arduino 5V and GND? Or should I never power a servo fom the board?
I got the below battery packs to test with a WeMOS D1 ESP8266 development board. The larger one powered the board (with very little transmit time) for 220 hours. If your servo has to do any real work, it probably needs 6v instead of 5v.
HD Frameless Blind Spot Mirror - Fan Shaped 2.5" Convex Glass Mirror - 2pcs/set 781509267278 | eBay?
Ultrasonic Cavitation Fat Remover Anti-Cellulite Body Slimming Machine Gel USA | eBay?
The clear blunder is not comprehending what the "Vin" or "RAW" terminal is. The regulator on the Arduino UNO/ Nano/ Pro Mini/ Mega2560/ Leonardo/ Pro Micro has very little heatsink, so will not pass very much current (depending on the input voltage and thus, how much voltage it has to drop) before it overheats and (hopefully reversibly) shuts down. It is essentially a novelty provided in the very beginning of the Arduino project when "9V" power packs were common and this was a practical way to power a lone Arduino board for initial demonstration purposes. And even then it was limited because an unloaded 9 V transformer-rectifier-capacitor supply would generally provide over 12 V which the regulator could barely handle.
Nowadays, 5 V regulated switchmode packs are arguably the most readily available in the form of "Phone chargers" and switchmode "buck" regulators are cheap on eBay so these can be fed into the USB connector or 5 V pin to provide adequate power for most applications. Unfortunately, many tutorials or "instructables" are seriously outdated or misleading and have not been updated to reflect the contemporary situation.
If powering from batteries, as long as the battery pack cannot exceed 5.5 V, this must be connected to the 5 V pin.
Err, yes! OK! 