One of my Interlock projects has been to explore the capabilities of the Beaglebone Black's built-in Power Management Integrated Circuit (PMIC) that gives the BBB a pretty useful feature; charging and operating off of li-po batteries. If you look at the BBB board, you will see 4 through-holes behind the 5v plug. These are break outs for the PMIC and can be used to hook up to a battery.
Having a battery connected to your board gives you the ability to do things like making a UPS for you beaglebone so in the case of a power loss, it can politely shut-down, run a custom command, or just continue running for as long as the battery can charge it.
In most of the use cases, you're going to find a lithium polymer that can produce around 3.7V which is under the BBB's 5v requirement. 3.7V will work fine to power the board but of course your 5V USB port will not work while on the battery. Sparkfunhas a few nice ones that have the voltage protection circuits built in to help limit the risk you brick your li-po.
There are 4 pins we're talking about:|bbb-batt-srm|
You can see in the diagram what each pin-out is. Li-po's are riskier than some batteries because they're known to explode in some cases of over powering. A temperature sensor that is designed to check how hot the battery is getting and decide how to handle it, is built into the PMIC in case your battery doesn't have this capability already(many do). The directions below are going to show you how to jump this temperatur check with a 10k resistor, which is not recommended if you value your home or hackerspace. If you don't trust the battery you're charging, I'd suggest looking into accurately reading the temperature from your battery. My Sparkfun batteries do no break out the temperature sensor so this wasn't plausible.
- jumper pins TP5 to TP6 (or use an SMT zero ohm resistor)
- connect a 10K resistor between TP7 and TP8
- Install a JST connector on TP6 and TP8
- Connect your battery into the JST connector
With a little luck and the correct battery, you should be in business. You'll need to let your battery charge before you try and yank the power cable from it. In the mean time, you can query the status of the battery via the i2cget command built into the OS.
The PMIC is accessible using I2C and the builti-n OS for BBB has a simple command line interface to query its state. The following command will tell you whether or not battery is plugged in:
`` i2cget -y -f 0 0x24 0xA``
This will return information that contains this:
0 device 0x24 On battery power only? 0 STATUS: r[0xa]=0x88 Push Button = 0 USB Power = 0 AC Power = 1 CHARGER: r[0x3]=0x1 Active (charging) = 1
"Active" refers to whether it can recognize the battery you have plugged in. You can also read this state to detect a power failure and automatically failover. If you're using the default OS for the Beaglebone Black (the one that comes pre-installed), the OS will automatically shut itself down in the case of a power loss. You'll want to either install another OS, or disable that service if you'd like to change how long the battery should stay online.
A decent amount of research went into this simple project. There are a ton of warnings and caveats that I'm not going to cram into this blog post (i2c address is read only, pin-outs are not a standard size, beware of jumping the resistor next to the pins). You can find out more information here:
- Detailed information on how to interface the PMIC with a 3 pin li-po http://www.beaglebone-asterisk.org/uninterruptible-power-supply-ups-for-beaglebone-black-a-diy-project/
- Python wrapper to query the PMIC via I2C https://github.com/pehrtree/beaglebone_snippets/tree/master/power
- Discussion of powering the BBB with 3.7V https://groups.google.com/forum/#!topic/beagleboard/Ahzk6Ut7xYE
Thanks to Alex for finding a fatal flaw in the 10k resistor I was using.