Power Raspberry Pi directly with Lithium Ion rechargeable batteries

From reading Adafruit’s excellent dissection of the (then new) RPi Model B+ power supply I had learnt (from the schematics) that 5V GPIO pin was directly connected to RT8020AGQW step down DC/DC converter (5v to 3.3V/1.8V) which automatically switches to a low-dropout mode.  So it should be possible to power a RPi B+ with much less than 5V at the GPIO 5v pin. Relevant excerpts from the data sheet:

Two operational modes are available : PWM/Low-Dropout auto-switch and shutdown modes. Internal synchronous rectifier with low RDS(ON) dramatically reduces conduction loss at PWM mode. No external Schottky diode is required in practical application. The RT8020 enters Low-Dropout mode when normal PWM cannot provide regulated output voltage by continuously turning on the upper PMOS.

Rds_RPi

Since I did not have the right equipment to test for this at that time I tried calculating it from the datasheet. Assuming a worst case Rds of 0.5 ohms for the PMOS at a current of 700mA (model B+, camera module, USB wireless) I get a voltage drop of 0.35V across the PMOS. So if we ideally want 3.3v at the output, then VIN should be 3.65V. At this point I should add that I am not completely sure if that is the correct way to estimate the dropout.

Now that I own an adjustable power supply I wanted to test that out. The only RPi I had on hand was a model A+ so thats what I used.

DISCLAIMER: All numbers below are from a very rough test and come with no guarantees. This means that if your house burns down or something terrible happens because of using something I said then I am not  to blame for that.

Test A:

I tried booted the RPi at 5V, with a keyboard and HDMI monitor connected. Then I launched Minecraft (wish the RPi foundation included a stability testing benchmark) and started lowering the voltage. For the short duration of time that I tested I was able to run without any brownouts at 3.5V. Going lower to 3.4v or even 3.45V would cause random reboots, which brings us to the next test.

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Test B:

While the RPi was rebooting I increased voltage back to 3.5V but noticed that was not sufficient and the RPi stayed stuck in a continuous reboot cycle. I was able to see the RPi booting up messages but at some point during that it was get reset. In hindsight I should have tried to make a video to figure out when exactly the reset happened. I next tried 3.6V which also did not work. I was only able to stop the reboot cycle at 3.7V.

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Test C:

Next I unplugged the keyboard and plugged in wireless dongle and again tried lowering the voltage. I was able to sustain a download over wifi with X server loaded at 3.5V.

Conclusion

The calculation was fairly accurate and as expected the model A+ was able to operate at a lower voltage than calculated because of using less current.

Coming back to the title of this post, I went down this path because I wanted to minimize components and was wondering if I could directly power the RPi for my remotely controlled home rover from the Li Ion battery. Searching for lithium ion battery discharge curves I see that at 1C rate reaching 3.5V would be somewhere in the 60% to 80% discharge capacity of the battery. So if I want to use the full battery capacity I should use a step-up converter, but if I don’t care about that I should be able to power up a Rpi with a lithium battery connected directory to Vin. Monitoring the current and voltage to the RPi might also be a good option to ensure stability to raise early warning if the voltage goes too low.

Relevant products:

Adafruit INA219 breakout to monitor current and voltage to the RaspberryPi.

CPS-3205 power supply bought from ebay

RaspberryPi with SM5100b GSM GPRS cellular shield

RPi

I am going to try porting my PC python script for interfacing to the SM5100b directly to the RPi instead of converting it to an Arduino program.

UPDATE: Wow… it was easier than I thought it would be. I was able to quickly get to the same level of functionality as that previous post in less than an hour. Here is the push message posted to Prowl app using the cellular shield by a python script running on the Raspberry Pi.

Updates project files should show up on my github soon.

IMG_3379[1]

My RaspberryPi Config

Setup for first boot

I bought an 8GB micro SDHC card which came with a USB card reader and an SD adapter. I used the windows disk image writer for setting it up with the latest raspbian wheezy image.

Then I powered the card using a usb cable and used a ethernet cable to connect it to my router. I then setup my router to assign The RPi a reserved easy to remember IP address.

$ ssh pi@192.168.1.2

On first login its best to setup the following config using your preferences.

pi@raspberrypi ~ $ sudo raspi-config

At this point I was using ~1.5GB of the card.

Please note that the following tidbits have been scraped from various parts of the internet and I did not really have to figure out any of this myself.

Here is a good index page from the embedded linux RPi wiki for quickly looking up information.

rpi

Installing Packages

Next I upgraded all the installed packages, this takes a while to finish.

$ sudo apt-get update ; sudo apt-get upgrade

The upgrade pushes the card upto ~1.6GB.

General packages

$ sudo apt-get install vim ctags tcsh screen tightvncserver python-setuptools git

Automatically Adjust Clock speeds – Save Power

$ sudo vi /boot/config.txt

arm_freq_min=100
force_turbo=0

Install the cpufrequtils package, which allows control over the CPU speed based on demand and reboot:

$ sudo apt-get install cpufrequtils
$ sudo cpufreq-set -g ondemand
$ cpufreq-info
...
...
  current CPU frequency is 100.0 MHz.

GPIO packages

$ sudo apt-get install python-dev
$ sudo apt-get install python-rpi.gpio

Packages for OS X screen and file sharing

$ sudo apt-get install netatalk avahi-daemon
$ sudo update-rc.d avahi-daemon defaults

Miscallaneous setup

OS X File and Screen sharing

$ sudo vim /etc/avahi/services/afpd.service
<?xml version="1.0" standalone='no'?><!--*-nxml-*-->
 <!DOCTYPE service-group SYSTEM "avahi-service.dtd">
 <service-group>
 <name replace-wildcards="yes">%h</name>
 <service>
 <type>_afpovertcp._tcp</type>
 <port>548</port>
 </service>
 </service-group>

$ sudo vim /etc/avahi/services/rfb.service
<?xml version="1.0" standalone='no'?>
 <!DOCTYPE service-group SYSTEM "avahi-service.dtd">
 <service-group>
 <name replace-wildcards="yes">%h</name>
 <service>
 <type>_rfb._tcp</type>
 <port>5901</port>
 </service>
 </service-group>

Note that rfb port 5901 implies :1 display.

$ sudo /etc/init.d/avahi-daemon restart

I2C

sudo vim /etc/modules

and add these two lines to the end of the file:

i2c-bcm2708
i2c-dev

Next for python support:

sudo apt-get install python-smbus
sudo apt-get install i2c-tools

And finally try the following to test:

sudo i2cdetect -y 1
sudo i2cdetect -y 0

Setup config files

~/.vimrc

 set expandtab "et"
 set sw=4 "shiftwidth"
 set smarttab "spaces for tabs ?"
 set ts=4 "tabstop
 set shiftround
 "Indentations and line numbers
 "set number "nu"
 set autoindent
 set cindent "ci"
 set incsearch "incremental search
 set hlsearch "highlight search
 "line break - causes word granularity wrapping
 set lbr
 set wrap
 "Folding
 syntax on
 syntax sync fromstart
 set foldmethod=indent
 filetype plugin indent on
 syntax enable
 "set invlist
 set showmatch "sm"
 set smartcase "scs no ignore case when pattern has uppercase
 set backspace=2 "bs how backspace works at start of line
 filetype on "filetype detection on
 "dont indent when pasting with middle clicks
 set paste
 " Make shift-insert work like in Xterm
 map  
 map!  
 set nocompatible "dont emulate bugs
 " configure command line completion
 set wildmode=list:longest,full
 " setup status line
 set statusline=%F%m%r%h%w\ [%{&ff}]\ [%Y]\ [%07l,%07v][%p%%]\ [%L]
 set laststatus=2 " always show the status line

~/.cshrc

 set prompt="%~ %# "
 bindkey -v
 set history = 5000
 set savehist = (5000 merge)
 set histfile="$HOME/.tcsh_history"
 history -L
 # Make command history (arrow keys) use history search operations
 bindkey -k down history-search- forward
 bindkey -k up history-search-backward
 setenv MANPAGER less
 alias l ls -lh
 alias cls clear
 alias vi vim -p
 alias vim vim -p

Disable root ssh

$ sudo vi /etc/ssh/sshd_config

Change to following:

PermitRootLogin no

Measure Core Temperature

$ /opt/vc/bin/vcgencmd measure_temp
temp=45.5'C

For other options possible with this command run:

$ /opt/vc/bin/vcgencmd commands

Serial port setup

Remove the following text from the /boot/cmdline.txt file:

console=ttyAMA0,115200 kgdboc=ttyAMA0,115200

And comment out the following from /etc/inittab file:
T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100

You also might want to install minicom for monitoring:

$ sudo apt-get install minicom

To setup the serial port packages required for use with python:

$ sudo easy_install pyserial

Reboot time

As per my rough measurement it takes around 90-120 seconds for the raspberry pi to be able to accept ssh connections after initiating a reboot. I suspect that this is partly because of my slowish class 4 memory card. I was getting much quicker boot times with the 45MBps UHS-I SD card from my camera.

Change hostname

Update it in the following files and reboot:

$ sudo vim /etc/hostname /etc/hosts

And lastly

Once you are happy with your config remember to make a backup img of your card which you can restore if anything goes wrong.
A useful suggestion I found here was to zero out the free space to get better compression.

# cat /dev/zero > /zeroes ; rm /zeroes