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LiPo battery questions

Former Member

I'm currently building a raspberry pi based phone similar to that of Dave Hunt (PiPhone – A Raspberry Pi based Smartphone) and also like Ben Heck's DIY Cell Phone. Using the Adafruit FONA ben used and the Pi with the Pi TFT I was wondering if the Pi could be powered by the same battery as the FONA.

 

Currently the FONA uses a 3.7v battery and contains its own charging circuit which is perfect. However I can't tell if it can provide power out to power the Pi. Therefore I was wondering if and how it would be possible to split a LiPo output to go to the Pi and also the FONA. I know I would need to convert the voltage to 5V for the Pi but I know how to do that. So I just need some help on the splitting battery output and also if anyone knows if the current charge circuit would still charge the battery?

 

Thanks in advance for any information.

  • What amperage does the LiPo battery provide and how much does the FONA require?

  • in reply to toxxn

    This is taken from a fact sheet on the FONA

     

    "How long can FONA last on my battery?

    We haven't done extensive testing with FONA but from preliminary reports, the FONA

    draws about 20-25mA while running, up to 200mA+ while actually making a

    call/sending/receiving data, and has very small spikes of up to 2A. The spikes are short

    and are absorbed by the onboard capacitors.

    A 1200mAh battery can run the FONA on 'standby' for a day or two."

     

    I have a 500mAh, 1200mAh and would be willing to get when they are back in stock a 2500mAh.

  • in reply to Former Member

    The battery-use of a 'pi is MUCH more than that of your FONA. A factor ten or twenty.

     

    The CPU on the rasbperry pi is designed to draw its power from a LIPO battery! Perfect!

     

    So... If you hook up the "what-is-supposed-to-be-5V" on the 'pi directly to the LIPO then the CPU will work just fine.

     

    What doesn't work is:
    * The 5V output on the USB side. Your USB peripherals won't work.

    * The 5V output on the HDMI. It is meant to power for example the in-cable HDMI to VGA converters.

    * The SD card and ethernet/usb chip on the model B(+) will work as long as its 3.3V is "reasonable".

     

    That "reasonable" depends on a few things. I've measured the ethernet chip as "working" with the 3.3V rails

    as low as 3.0V. I recommend not going below 3.3, and keep some margin. I have not tested SD cards, besides there are many of those.

     

    On the model B the "recommend not going below 3.3V" means that your 5V line has to be above about 4.3V: Not possible by the "direct connection to a LIPO". On the B+, I expect (but have not measured) that the voltage converter will keep the 3.3V operational down to a source voltage of about 3.4V.

     

    At that voltage you have less than 5% of your battery capacity remaining. You need to SHUT DOWN NOW to prevent damage to your battery. Just leaving your 'pi connected to the battery at that point in time will drain the battery into unrecoverable damage within a few hours, even if you halt the 'pi. So a discharge protection circuit is a must. If your FONA has this already I don't know.

  • in reply to Former Member

    Thank you Martin, that information is extremely helpful.

     

    As Roger mentioned above, the Pi can run at 3.3v minimum.

     

    To answer your question however.. a switching regulator. There are two different kinds, "buck" and "boost". A Buck Regulator (or Buck Converter) will step down the voltage similar to that of something like a 7805. A Boost Regulator will step up the voltage, which is what you will need.

     

    The problem is the current. The raspberry pi needs a minimum of 700mA and will obviously need more with anything connected to it. Regulators have an efficiency rating, meaning that some of the current is lost when stepping up or stepping down the voltage. For example, if the regulator you used were 90% efficient, than you would lose 10% of your current. This is something to take into account when looking for a regulator.

     

    As long as you don't alter the battery, the charge circuit should always charge the battery regardless of what is connected to it, unless your pulling way to much current from the battery at which point the charger just simply could not keep up. In which case could damage your battery.

     

     

     

    An alternative to your solution would also be to use an alternative battery of a higher voltage and using a buck regulator to step down the voltage for the FONA. This should also increase the mAh of the battery without to much of a substantial loss of current from the regulator. Just an option, options are good.