This is a migrated version of my Wordpress post, written on : 8 Mars 2015
I used the Pi as a brain of an autonomous car toy project, for such project, autonomy is alway a key factor that must be considered. So i did some researches to find an efficient way to power the Pi with battery. In my project, i used 4xAA batteries as power source ('cause those ones are very popular and easy to find).
To save the power, my suggest is to use the PI A+, this version of Pi is the less energy consuming in its family. In headless mode (without HDMI,camera, Idling in the command line), it requires around 100mA - 120mA (with or without an USB wireless attached for network communication).
The first thing you need to do is turn off the HDMI output, this can help you save about 20mA. Notes, without the HDMI, you can only access to the Pi via network using ssh (that is, you need to configure the network to work with ssh before turning the HDMI off).
The 4xAA batteries make totally 4,8V (1,2 each for the NiHM), which can be sufficient to power the Pi (which require 5v input) using the traditional way (via the micro USB), but it is not safe and efficient in term of power using. So we won't go that way, to find a more optimal usage of battery, we must to understand where the 5v power go :
Apart of these peripherals, the rest of the Pi works with 3.3v, so yeah, we can power the Pi with 3,3v. The problem is that some USB devices (which need) may not work properly in this case, i've tested with two USB wireless devices (we need this for network communication), and both of them run happily with 3.3v.
So how can we feed the Pi with 3.3v power? Well, we need a separate 3,3v LDO to regulate the 4,8v battery to 3,3v and power the Pi with it. The point is that we must choose the 3,3v regulator so that the Pi is stopped working only if the battery is flat. This depends on the dropout voltage of your regulator. For example, say you have a regulator with the dropout voltage of 1.5v, this means that the regulator works well only if the voltage supplied to it stays above 4.8V (3.3v + 1.5v). Below this value the output will fail to regulate to 3.3 volts, and thus will cause the Pi to stop working. This regulator, indeed, is not a good choice since the Pi will stop working before the batteries become plat.
To figure out which regulator is appropriated, we must know at which point the battery will be flat. For the 4xAA NiHM battery, the threshold is 4.4v (1.1 each). So the maximum dropout voltage needed for the regulator in this case is 1.1v. All regulators with the dropout voltage below this value are good.
In my project, i use the Microchip MCP1826S LDO with output of 1A (enough for my purpose) and dropout voltage of 250mV. That is, the Pi will work until the supply voltage fall below 3,55V. At this point the batteries are very flat indeed.
So here is how the wiring is done :
Note that here, we will bypass the built-in regulator by connecting the 3.3V and the 5V lines on the Pi (GPIO pin 1 and 2) to the same 3.3V line. The batteries are used as input of the MCP1826S, the Pi is powered by the output of the regulator. Here, the battery (before and after regulated) is stabilised by 2 capacitors.
That is how i power my Pi with 4xAA batteries supply, note that, following these instructions above will void your warranty, so do it at your own risk. I have no responsibility for any damage you cause to your Pi :D