Powering the Raspberry Pi from standard RC batteries

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The Raspberry Pi is a fantastically versatile, cheap and tiny computer. It's very easily comparable to the ever popular Arduino, except that it provides significantly more processing power and programming options. One of the major drawbacks to this power, however, is the increased power consumption which makes powering the device problematic.

Power

For example, a standard Arduino Uno requires mere tens of milliamps (mA) to function at full power (nanoamps in sleep mode), while the RPi is rated at 700mA. This means it can't technically be powered from a standard USB port because USB 1.0, 1.1 and 2.0 permit a maximum power draw of 500mA.

You need a wall adapter to supply that 700mA, or more with peripherals such as wifi. The idea if powering it off a small lithium-ion battery or standard AAs is completely out the window.

One field which is particularly experienced at supplying huge amounts of electrical power is the field of remote control (cars,boats, airplanes, flying blimps with faces on them [LINK]). All of the batteries and technologies we need already exist, we just need a way to join the two in the middle.

Enter the lithium polymer (LiPo) battery, and the ironically named Battery Eliminating Circuit (BEC)!

  

LiPo batteries are relatively new to the RC party and are more or less the favourite due to their high energy densities and performance. The photo above is my 2,200mAh Turnigy battery which I got off eBay for less than £10 (about $15). It can power my RPi plus wifi for about 3.5 hours on a full charge.

STOP!

An import safety note about LiPo batteries!!!                

LiPo batteries are not for beginners, not for kids and not for adults who don't read safety precautions! Slightly over charging, over discharging or improper charging can cause serious explosions. Just looking at them funny can cause them to burst into flames!

Click here for a good primer to LiPo batteries

Anyway, safety concerns aside, these are great batteries to make your RPi more mobile. Unfortunately a 3 cell (series) [more info] LiPo provides 11 - 12v, enough to fry the lovely RPi which you had to wait for14 weeks for delivery. That's where the BEC comes in!

BEC

A battery eliminating circuit is used to power the electronics in a RC airplane, which as chance would have it works on a RPi-friendly 5v. It takes the higher and variable battery voltage and turns it into 5v for your electronics. BECs are sometimes built into a larger speed controller and/or RC module, however standalone ones are available.

A note on efficiency:

BECs come in two varieties: switching and linear/unswitching. Linear ones rely on inefficient voltage regulators which waste a lot of the input power. Switching ones are a lot more efficient (mine is about 92%). It's worth the slight cost increase for a switching BEC.

Note that the terms UBEC (unswitching/linear) and SBEC (switching) are often confused. Particularly, Ultimate BEC is/was a brand of BEC, which was switching. Be sure to read the description when purchasing. (Thanks to the anonymous commenter for pointing this one out)

So that's what you need, now how do we put it all together?                         

1. Connectors for batteries

When buying a BEC, if you don't like soldering, make sure you buy one with the same connector as your battery. There are a few standard connectors, such as XT60 (my fav), JST, EC3, banana plug (should be avoided!!), and Traxxas. Make sure you buy a matching pair.
Obviously if you're a fan of soldering you're not limited here and can choose which ever BEC and battery you want.
Incidentally, you don't have to use LiPo batteries, you can use any battery which supplies a voltage which your BEC can handle (car batteries are 12v and around 40,000mAh!)

 2. Connection to RPi

The only modification you need to make to your BEC is on the 5v side, at the connector to the RPi.
The pinout of a RC connector is one of the following:

http://www.coastalplanes.com/articles/pinout.htm

We need to access the +5V and 0V pins on the RPi. The pinout of the RPi's pin header is:

http://elinux.org/RPi_Low-level_peripherals#General_Purpose_Input.2FOutput_.28GPIO.29

If we were to plug our BEC in as it is we'd hear a pop and our RPi wouldn't work any more. Luckily its very easy to change the BEC without soldering or fancy tools
The connector of the BEC is made of a plastic case with metal pins inside, which clip in place with a springy bit of plastic on the case.

Simply take a knife, pin or screwdriver and GENTLY raise the spring. You may need to wiggle the wire up and down a bit, but it will come out. Now just make sure it's lined up right and pop it into the correct slot.

That's it! Now, this bit scares me because one slip and you potentially fry your RPi. It may help to disconnect your battery from the BEC first, connect the BEC to the RPi then reconnect the battery (you should never leave your battery connected to your BEC anyway when it's not in use).

To connect, set your RPi with the Ethernet and USB ports towards you (the SD slot facing away from you). Locate the golden pins at the top right of the board. Slide your connector on, with the RED wire to the TOP, RIGHT-MOST pin, and the BLACK wire two pins towards you.



Provided that the RPi lights up, you can begin to enjoy mobile and more flexible projects!

DONT STOP READING YET!!

Remember what I said about exploding batteries? Unless you want that to happen (hint: you don't) then pay attention!

When do you turn it off?

Unlike your laptop, the RPi has absolutely no way to monitor the voltage or remaining capacity of the battery. This is a problem for two reasons:
1 - for LiPo users, over discharging your battery causes copper deposits to form and damages your batteries. If this goes too far it can actually cause an internal short, and when you charge it again the whole thing goes on fire (seriously)
2 - for everyone, if your RPi is busy doing stuff, it'll just shut down all of a sudden when it runs out of juice. That may or may not be a problem, but will probably lead to data corruption if you're writing data at the time (like when you pull the plug on your PC)
The only solution I have at the minute is to time the how long it has been running and pull the plug manually. I do mean literally pull the plug, because the RPi uses a significant amount of power when "off" as it does while running (106mA according to my meter, vs 300-400mA when running. This is OK for NiMH batteries but a disaster for LiPos. I'd love to hear any suggestions anyone has for how to get around this.