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Raspberry Pi Zero LiPoly board

An add-on to the Pi Zero that charges a LiPoly battery when the Pi is using external power, and powers the Pi when not.

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I've designed a LiPoly charger board. When the Pi is plugged into an external power supply, the LiPoly battery is charged. When external power is disconnected, the Pi can be run from this board. The board is sized to match the Pi Zero, but it works on others. I've used it on a Pi B, A+, Pi 2 and Pi 3.

The Pi works fine on the LiPoly 3.7 volts, as do some peripherals. I've also designed a three-port USB hub and that and a WiFi dongle work. I've gotten about 20 minutes from a 150 mAh battery with the Pi Zero, USB board and WiFi.

A number of folks talk about the Zero being missing too many important accessories to really be useful. I don't look at it like that. First, if the original Pi was designed to help teach software design, the Zero can also be looked at as a platform to help teach hardware design. In addition to that, it should make a great embedded controller and project base. My future designs will test that theory out with motor controls, sensors,

Update: I've got the final version 1.X design done. It removes a few minor mods and adds a direct battery port for my upcoming Pi Zero dual motor driver boards. V2 will have a battery "fuel gauge" for safer battery operation.

In my earlier logs, I mentioned a "version 2." I changed the numbering so that everything to date is a v 1.X.

Update: I do have a Pi Zero in hand now. I've also considerably reworked the design. I put a P channel MOSFET on the power line going into the MCP73831 LiPoly charger chip, which is the Pi 5 volt line. The power line out, has a Schottky diode, and then goes to the Pi 5 volt line. I use a comparitor across that diode to switch the input MOSFET on and off.

I put a P channel MOSFET on the power line going into the MCP73831 LiPoly charger chip, which is the Pi 5 volt line. The power line out, has a Schottky diode,and then goes to the Pi 5 volt line. I use a comparitor across that diode to switch the input MOSFET on and off.

If I'm running from the Pi, and charging the battery, the cathode of the diode has 5 volts, while the anode has the about 4 volts coming out of the charger chip, keeping the comparitor off. When I disconnect the Pi power supply, The anode has the 4 volts from the battery, and the Pi 5 volt line has that, less the voltage drop from the diode. This sets the comparitor, which turns off the MOSFET. That way, I won't be feeding a low voltage back into the charging chip when running from battery.

After the Schottky diode drop, I get about 3.7 volts on the Pi 5 volt line. That's enough for the 3.3 volt LDO regulator on the Pi. Most 5 volt peripherals won't be able to run, but the Pi and 3 volt peripherals will be fine.

[Pre update]

So... What were those assumptions I spoke of?

I assumed that, since the Pi is basically a 3.3 Volt device, I should put a regulator on my board and power the Pi via the 3.3 Volt line. With this being the case, I would run the 3.3 Volts on my board through a P-Channel MOSFET. The gate of the FET would be controlled by the 5 Volt power line. If the Pi has a 5 Volt supply, my board wouldn't supply power to the Pi. If not, it would.

What got me is that when you run power into the Pi 3.3 Volt line, there is still high enough voltage on the 5 Volt line to switch the MOSFET off.

With a bit of research and experimenting, I determined that the LiPoly batteries supply at a voltage level high enough to get through the Pi's low drop out 3.3 Volt regulator. Version 2 of my board removes the voltage regulator and puts in a comparitor to switch the MOSFET.

  • Back up to date

    Duane Benson05/12/2016 at 07:39 0 comments

    I've skipped a few logs. Since ,my last, I built up a V1.2 (I called them V2 in my earlier log) board. It worked, with a few mods, but I wanted a bit more flexibility with the I/O that I used, and some additional test points. That resulted in V1.3, which worked well, but needed a mod wire on the battery on/off indicator LED.

    A V1.4 is now on the way that has that mod and an added set of direct battery pins to drive my dual motor drivers that I've since designed.

  • They're here

    Duane Benson02/10/2016 at 00:57 0 comments

    And... I pulled the V2.0 boards out of their pancaking today. Hopefully, I'll have time to build one this evening and see where it stands.

  • V2 Approaches

    Duane Benson02/05/2016 at 20:47 0 comments

    I just got word that my V2.0 pcbs have been shipped and will be on my doorstep soon.

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Arsenijs wrote 01/05/2019 at 14:59 point

No schematics? Would be great to take a peek

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Lachlan Marschke wrote 04/10/2016 at 21:29 point

Seems cool and really complicated!! Keep going though!

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