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Roast My Circuit:  LiPo Battery Protection for Raspberry Pi

Sean_Miller

I dreamed up a circuit that would prevent my Raspberry Pi project from damaging its LiPo battery.  A 3 cell LiPo should not go below around 10V or it will balloon and become unusable - if not a fire hazard.

 

The Pi and circuit work together to shutdown cleanly and disconnect the LiPo entirely.

 

I bread boarded the basics and it appears to work fine.  Being that I scribbled it out of my noggin versus copying from a trusted source, it probably has all kinds of theoretical holes.

 

Here is the gist:

  • 14.8V LiPo is switched with a momentary switch.
  • When switched, power gets to a voltage regulator which powers the Raspberry Pi.  (The user knows to keep holding the switch until they hear the click of the relay.)
  • The Raspberry Pi outputs a pin high when it starts to boot which switches a NPN transistor.
  • The transistor takes the relay to ground latching in power to the regulator/Pi.  It's latched in as long as the Pi keeps that pin high.
  • The user can let go of the momentary switch once they hear the click as the relay is now latched in.
  • There is circuit block that monitors the main battery voltage and takes a pin low once it decreases to ~10V.
  • The Pi detects this and takes action to alert the user and shutdown safely.
  • A 1000uF capacitor is on the signal line so that the circuit will remain latched for 20 Seconds more to give the Pi time to finish the shutdown process after it takes the signal pin low.

 

So, please roast my circuit - what could be better (not the crappy schematic - but the design/components)?

 

image

 

image

 

Thanks,

Sean

Parents

  • Relays are horribly power inefficient - the draw of the coil could easily add several tens to hundreds of mA to your power consumption. Perhaps consider buying a "latching" relay if you can - those only need a "pulse" of power on a coil to change the state, thus not taking any power while otherwise running. Electromechanical relays may be a bit vulnerable to vibration and bumps ...

     

    Not sure the 1000uF would keep the circuit latched that long ... relays are pretty power hungry.

     

    To be honest, you're probably better off with a proper 3s battery pack protector circuit that will disconnect the battery regardless - no intervention from a Pi or anything, as that could crash or hang. Such a circuit would unconditionally protect the battery while only using very small amounts of current (tens of uA). Usually they also incorporate cell balancing as well, but need connections to the intermediate tap points. They often also provide short circuit and over-voltage protection too.

     

    Then you're basically reduced to having the RPi detect the voltage of the battery and shut down below a given voltage. Something like a cheap uC with an ADC input could provide that data to the Pi, or if you're daring, an ADC hooked into an I2C/SPI port in much the same way.

     

    In essence, that would achieve a similar result, but minus the mechanical relay and power consumption associated with it, and the risk of software crashes causing the protection to fail.

     

    - Gough

  • in reply to Gough Lui

    I like the sound of the latching relay.  My breadboarded circuit drew about 43mA, which is acceptable for the application, but 0mA - now that's a bargain.

     

    The capacitor in my circuit is actually on the base of the transistor that is completing grounding the relay.  So, it is very slow to drain.  The breadboard proved it to be about 20 seconds to drain through the base - plenty of time for the Pi to shutdown cleanly after the pin voltage is dropped.

     

    To address the software failure concern, I'll add a second voltage supervisor.  One supervisor will use a voltage divider as a LOW alarm that triggers a user warning and a shutdown timer for a clean shutdown by the PI.  The other will be monitoring for the LOW-LOW interlock which simply punches the latching relay regardless of the PI state.  Ideally, that one would never be reached.  This will be the best of both worlds and will prevent SD card corruption from hard power downs.

     

    I'll update the schematic once I find one of those latching relays.

     

    Thanks for the assessment.

    Sean

  • in reply to Sean_Miller

    This latching relay looks like a winner - plenty of amps, lapping, and works good with my battery:

     

         G5RLK1EDC12 - Omron - General Purpose Relay, G5RL-K Series, Power (newark.com)

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