This one has come up a lot recently on the Arduino subreddit and Electronics stackexchange – can I power my Arduino with a 9V battery?
The quick answer is yes, you can.
The long answer is yes, but only if your goal is to chose the most expensive and short lived batteries possible.
I think the boards and documentation should have an explicit warning about 9V PP3s for this reason!
9V is not a good input voltage to produce 5V through a linear regulator
The first problem is how to reduce that 9V to 5V that the Arduino board can use. Most Arduino boards have an external voltage input, and a range of 7-12V is recommended. So 9V seems perfect.
The problem is that most Arduino boards use a linear regulator to drop that 9V to 5V. If you are drawing a mere 50mA, 0.2W is being burnt in this linear regulator with 0.25W being used by the Arduno itself. This is very inefficient!
To solve this, either another battery should be used, or a switch-mode regulator used – these are much, much more efficient.
Even worse is if you try to draw large currents – say 250mA – from the 9V source. The regulator needs to now burn 1W of power. It will fry in no time!
9V batteries are very low capacity
An alkaline 9V PP3 has a capacity of between 500 and 600mAh. This really isn’t very high – a typical alkaline AA battery will be at least 2000mAh. 6 series AA cells will provide the same voltage, but a capacity at least 4 times the size!
This gets even worse if you look at rechargeable 9V batteries. A typical 9V PP3 NiMH battery has a capacity of less than 200mAh. This is so low it is hardly worth bothering with.
No matter how efficient the regulation is, these batteries are not going to last very long.
First thing first, you need to ditch that linear regulator. It is meant to regulate external voltages when power loss is not an issue.
Use one of the modern switching regulators like the LTC3525 – this can take an input voltage of between 1.0V and 5.5V and convert it to 3.3V or 5V as required. Bypass the linear regulator on the board.
Couple this new regulator with high capacity AA batteries. The LTC3525 is so versatile it can take one, two or three batteries and drain the last drop out of them.
AA batteries are common, cheap, and high capacity. There are excellent rechargeable batteries available as well. Please stop using the 9V PP3s!