# Battery Supply Question

This question may have been asked several times, regardless:

It's been quite sometime since I took my electrical courses in tech school and had a question about battery powering my RPi. I currently power it with a USB wall charger. Works great, no issues. Would it be possible to say create my own custom battery pack, perhaps about 10 AA batteries all daisy chained together, and then the negative and positives wired to the prongs on the wall charger? Would the wall charger handle that appropriately and provide the correct ammount of juice to my RPi? The idea being that I can create a battery pack, or perhaps use an old laptop battery or something to that effect, and have the +/- just fed to my wall charger and be able to cheaply and easily battery power my RPi for hours!

Any and all input/thoughts/constructive criticism is very much welcome. Thank you!

Your wall charger needs line voltage to work (120V in the US) batteries cannot be used to drive it.

You can make a battery pack but you won't be able to use your existing wall charger. You need batteries and a voltage regulator to ensure the batteries provide 5 Volts.

AA batteries are 1.5V each so 4 in series would give you 6 volts which is too much. You need a voltage regulator to take this voltage and make it a solid 5 Volts.

This article has instructions for creating a battery pack from 6 AA batteries and some off the shelf parts.

• Thanks for the information! Would it be possible to just wire a resistor inline from the four batteries? I read there is some variance in what the true output of AA batteries is. 1.2-1.5 V it sounds like. So I would assume we would want to assume the top end of that spectrum to protect from any damage. I could make my own battery pack with a resistor to shave off a volt? Or is that not reccomended? Apr 16, 2013 at 19:00
• That won't work. You can use a simple resistor based voltage divider to get the voltage down to 5V but the output voltage will quickly drop below the needed 5 volts as the batteries drain. A voltage regulator is designed to maintain a constant output as long as the input is above a certain level, thats why the article uses 9 volts instead of 6V (6AA instead of 4AA) as input into the voltage regulator. Apr 16, 2013 at 19:26
• In the comments, following that article, they said that using the gpio pins(2 and 6) to connect the power would bypass the pi’s internal voltage regulator. I don't think that is correct. On pi's schematic, I see a zener diode connected between the 5v rail and ground. I don't see any other protection. Will someone more knowledgeable, please confirm this? Apr 16, 2013 at 21:45
• I agree about the voltage regulator, but you would be bypassing F3 which I assume is a fuse of some sort. So you would be bypassing a safety device by using the GPIO. Apr 16, 2013 at 22:01
• Good point, Craig. Thank you for pointing that out. I guess adding an inline fuse would be a good idea. To be clear, there is an onboard voltage regulator that supplies the 3v3. Apr 17, 2013 at 1:17

The joy of electronics questions like this is one of the reasons I love the Pi. Unfortunately wall chargers pretty much everywhere in the world work on AC current, which they turn into DC current that powers the Pi. One letter difference how had can it be? Well, alternating vs. direct as it turns out matters somewhat.

In alternating current, the electrons in the wire bump back and forth, back and forth in the same position. Each electron knocking its neighbour and vice-versa is what carries the power. The fact the electronics essentially never move is what makes it great for transmission over long distances, like from the power plant to your house. However, because the electrons go back and forth, trying to power a car with an AC engine would be like watching it go forwards and backwards repeatedly. (Not to say that you can't power a car with an AC engine, but I'm using it as a metaphor here.)

In direct current, the electrons travel the full distance. This makes the car with that same motor move in one direction only. If we move the car metaphor, where powering of an AC motor is actually trivial, to most consumer electronics, powering them with AC is non-trivial. Hence DC. Why not use DC for everything? The fact that all the electrons are moving means the experience resistance (think friction) which causes them to heat up (hot wires) which further increases resistance (you slow down on a hot day) which means you have to put more power in. In room temperatures it's a loosing proposition. (In near absolute zero conditions it works amazingly well---that's where the super conductors of fabled sci-fi enter in.)

Anyway, the difference between AC and DC is why using the wall wort from DC power won't work. They use clever bits of electronics (previously transformers, now switched mode regulators) to make the transition. But there are such things DC to DC convertors. Switched mode regulators do that too. The MoPi board has one, will take any input voltage from around 8 to 25 V (so basically as many AAs as you can find, or any whacky old laptop battery) and make it so that the Pi can use it. It will then also tell you when the battery is about dead. Nifty!