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A project log for LiFePO4wered/Pi+

Next Gen LiFePO4 battery / UPS / power manager for Raspberry Pi, ideal for headless and IoT use

patrick-van-oosterwijckPatrick Van Oosterwijck 03/25/2018 at 00:130 Comments

Time for another update!

First of all: the question of Raspberry Pi 3 B+ compatibility!  I have to admit I was nervous when I heard that the Raspberry Pi Foundation was going to announce a new product on 3/14.  What if they announced something new that wouldn't work with the LiFePO4wered/Pi+?  That would have been horrible: to make a product that would have been obsolete before it was even released!

Luckily that didn't happen.  A quick inspection of pictures made me pretty confident that there were no mechanical issues and after ordering and testing one I can now confirm that the LiFePO4wered/Pi+ is fully compatible with the Raspberry Pi 3 B+! :)

Aren't they cute together?  Also, I'm quite happy with how the naming turned out: the LiFePO4wered/Pi+ works great with the Pi3 B+. :)

I also received my test fixture boards and built three of them last week.  There were a couple of issues I ran into.  For one thing, I accidentally ordered some OPAMPs in MSOP8 packages that didn't fit on the SO8 footprints, so I'll have to order the correct ones and add those manually.  I also had some trouble initially with the on-board USB hub, after a day of debugging I found that I needed to add 15K pulldown resistors to the USB data lines and that made it all work.

Here's the one working board I have (the others are still missing the OPAMPs):

The big power transistor on the left will be working as a shunt regulator to simulate a battery at various levels of charge.  The two other power transistors will be electronic loads connected to the main 5V and switched battery outputs that can be configured to different load current levels while I monitor voltages.

And here's how the Launchpad board fits on top to do the programming of the MSP430G2332 microcontroller:

By integrating the USB hub, it has become a nice compact system that only has one USB connection to the controlling PC.

I still need to add the pogo pins to the exposed end that will connect to pads on the LiFePO4wered/Pi+ to do the testing.  But before I do that I'll be writing some test code first to make sure the circuitry in that area works as expected since access to this area will be much harder once the pogo pins and supporting PCB are in place.  Once the USB hub was working, it was quick work to program the Espruino firmware on to the micro, which should speed up the test development nicely.

I also have been doing more testing of production LiFePO4wered/Pi+ boards, including some statistical characterization.  The final numbers still require more samples, but it's looking good.  The 18650 battery version supports 2A load current with some margin both when plugged in and running only from battery.  The 14500 battery version supports 2A when powered by a quality high current power supply, but when running from only the battery it looks like only 0.7A is reliably supported.  The reason is the higher internal battery and circuit resistance when using the smaller battery, which makes the battery voltage drop under high load.  So if your system is using a Pi 3 or Pi 3+ under high load, the 18650 version is the way to go.

Meanwhile the thermal image under 2A load looks very good with the production boards:

I have the impression that with the 2oz copper production PCB the heat spreads even better across the PCB than it did with the 2oz prototype boards.  The heat also nicely stays away from the microcontroller and 32 kHz crystal, which is exactly what I want to see.

I've been updating the product briefs for the LiFePO4wered/Pi and /Pi3 to include more information on hardware connections.  I'll be using the document for the /Pi3 as the basis for the new document for the /Pi+, so it made sense to fill it out with as much up-to-date info as possible before doing that.  Hopefully next week I'll find some time to work on that.

Lots going on and getting closer to general availability, stay tuned!

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