BBB - Rechargeable on-board battery system

Note April 2017: There are different revisions of the BeagleBone Black since this post was written in 2013, and without testing them all, it is the user's decision whether to try this solution or not. The revision changes are listed here and a possible option for at least some revision boards is shown in the photo here but I've not investigated this. The newer BeagleBone Blue already has a battery connection, so that is another option.

This posting is about implementing a rechargeable battery system for the BeagleBone Black. It is simple, safe and very low cost (less than 6 Euro).

First, some brief information about the power circuitry on the BBB.

The BBB has a built-in power management IC (PMIC) based on the TI TPS65217C device. This device contains multiple switch-mode regulators and LDO regulators to provide all voltage levels needed for the entire board. It handles wake-up using a push-button fitted on the BBB. Automatic power-down via the button requires some software to be implemented (to do the equivalent of 'shutdown now' from the command line). When the button is pressed, an interrupt is generated and the microprocessor is supposed to query the PMIC (via I2C) to learn that the button was pressed, and kick off the shutdown sequence. In the event of a failure here, the power can be switched off by holding the button down for 8 seconds.

The IC also contains built-in battery charging capability.

Apart from the USB requirement of 5V, there is no need to run the BBB from 5V; it can happily run from a lower supply. This means that a single 3.7V cell could be used to power the entire board. No need to step-up to 5V or to run dual cells and step-down to 5V, both of which could have been inefficient.

Why is this useful?

It makes it an excellent platform for outdoor/portable use.

For indoor use a battery is useful because it serves as a backup supply in case the main power (from a mains powered supply or from USB) gets disconnected. It could prevent filesystem corruption. If the main supply gets disconnected, the battery immediately takes over. In fact, I permanently leave the battery connected even when I'm running from the mains supply, in case I forget to safely shutdown the board.

What battery can be used?

Any small Lithium Ion (Li-Ion) or Lithium Polymer (Li-Po) single cell can be used, preferably as long as it has a built-in protection circuit. If it doesn't have an in-built circuit, it is highly advisable that one with a built-in thermistor is used. A cell in the range 700mAH to around 2AH is advisable. The one that I used in the photo above is from Olimex part code BATTERY-LIPO1400mAh. It should last around 3.5 hours (EDIT: Now been measured, it lasts 2 hours 50 minutes - see notes below, and comments below) on a full charge (not measured) and should fully charge in around 2 hours. This particular battery is just the right size to fit in between the two rows of headers and is flush so that a cape can still be plugged on top. So, the entire thing can fit inside any enclosure.

You will also need a connector (see next section) and one resistor.

Construction detail

The BBB has four holes that are suitable for connecting up the battery. They are detailed in the BBB system reference manual (SRM):

This is what they look like:

The Olimex Li-Po has a built-in protection circuit, so I soldered a 10k resistor to TS and GND to simulate the thermistor. (EDIT: You may or may not wish to do this, please study Li-Po and use your own judgement - see comments below) It is desirable to use a connector for the Li-Po.

The LiPo connector was convenient to solder to pins TP6 and TP8, and then  a zero-ohm link between TP5 and TP6 on the underside.

Here is how it was done step-by-step (there are just two steps):

1. Solder the 10k resistor, and a zero-ohm link (both are achieved on the underside of the board) as shown here in the yellow box. These are simple 0603 resistors; I used a 1% tolerance resistor, but 5% should be fine.

2. Solder in the connector.

This is straightforward, but complicated slightly by the fact that the connector has 2mm pin spacing, but the board has 0.1" spacing. It means that the pins on the connector need to be splayed or bent into the correct spacing. The connectors are available in vertical mount or right-angle (RA) mount. If you use a vertical mount connector, there may not be enough space to splay the pins. Instead, I used a right-angle connector and bent the pins into a vertical orientation, and then I had space to bend the pins and still have the connector flush with the board as shown here. You can see another view of the connector from the first photo.

I'm fairly sure that the desired connector is JST 'PH' series.

That's it; plugging in the battery, the board worked instantly. Note that the Li-Po charge method is usually to have a constant current supplied to the battery until it reaches a certain, very precise voltage. After that the charger switches to a constant-voltage mode. For the Li-Po battery that I used, the charger needs to be set to 4.2V, but the BBB by default sets it to 4.1V (It can be set to 4.2V via I2C control but today it doesn't). I'm not sure what the impact of this is (beyond storing less charge), but I believe it is safe. I have been using it daily for three months and the battery is always cool to touch.

Controlling the PMIC

The TPS65217C PMIC is very programmable; it has dozens of configuration settings specifically for charging and it has safety timer capability. The PMIC is configured upon startup via I2C. There are three I2C busses on the BBB, and one is dedicated to on-board peripherals. Control of the PMIC is not normally possibly by the user; it requires driver code or possibly there is access by the device tree infrastructure. Checking the .dts file in /boot did not reveal how to control the battery charger functionality. There are two current Google Summer of Code (GSoC) projects that touch on PMIC:

1. IIO, ADC, PMIC, LCD debug/patchwork (summary page, blog page) - Zubair Lutfullah

2. MINIX I2C drivers (summary page, blog page) - Thomas Cort

Hopefully the guys working on the projects (Zubair and Thomas) can offer some advice on how to set the level to 4.2V. Zubair's project also includes how to use the in-built ADC inside the AM3359 to monitor voltages.

Notes

There really should be some more detail including measurements to show how long the battery lasts and to observe the charging status (via I2C reads). Unfortunately I didn't have time to collect this information. But I've been using it for three months regularly and it functions well.

EDIT: The following simple test was conducted using the Olimex battery referred to above.

1. Power up the BBB using the DC power supply and let it charge the battery while powering the BBB

2. After about 4-5 hours, the DC supply was disconnected, and a script was run on the BBB to log the current date/time to a file, every minute. The script would sleep in-between. The Ethernet connection was left up, and the BBB was left alone until the battery died, and then the log file was examined.

#!/bin/bash
while true; do
date >> log.txt
sleep 60
done

The result was that the log file showed that the BBB ran for 2 hours 30 minutes before it died. After this, the test was repeated. The second time, it ran for 2 hours 50 minutes. The discrepancy may be because this battery has never been fully charged followed by such a long discharge, and so perhaps it is related to that. No test has been run with the Ethernet disconnected, but the BBB should run for longer in that case of course.

In a third test, the battery again ran for  2 hours 50 minutes (to within 1 minute). Again, this was with the Ethernet interface up.