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LiFePO4wered/Pi+

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

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Many IoT and other projects are based on the Raspberry Pi, but usually little thought is given to the power supply. Most project use generic cell phone adapters or USB power banks, which is fine for one-off projects where the duct taped parts and cabling don't matter and it's expected that SD cards will die because power was removed with the Pi running.

But when you need reliable non-stop operation for your prototypes, or you're ready to turn your project into a good looking product, or you want to use different power sources such as solar, it's time to look for a serious power manager for your Pi.

Built on the solid foundation of the #LiFePO4wered/Pi, this project provides Pi bootup and shutdown management based on button or touch, input voltage, battery voltage and time, all while making sure the Pi always performs a clean shutdown before power is removed.
In addition it provides significantly higher power, RTC functionality and Pi current measurement.

It was a hard decision to start from scratch with a new project for this next gen effort since the #LiFePO4wered/Pi project has such good traction and a gazillion likes, but the more I thought about it, the more I realized that starting with a clean slate would allow me the freedom to make this what people want it to be instead of feeling shackled by legacy, and at the same time prevent confusion about the different generations. A new project also allows me to enter it for the 2017 Hackaday Prize. :)

There are some issues with the current #LiFePO4wered/Pi design that I hope to address in this Next Gen effort:

1. Make it easier to assemble. The two-PCB arrangement with the charger on the bottom and power manager on top makes for a nice and compact unit, but it takes too much time to assemble. The original design sprang from a desire to provide an example application for the #LiFePO4wered/USB, but the #LiFePO4wered/Pi has turned out to be a much more popular product in its own right. So this will be a single-board setup with the charger and power manager both on one PCB, significantly reducing assembly time and cost. It is the best way to reduce cost and support higher volume.

2. Alleviate power limitations due to physical size. Power supplies unfortunately generate quadratically more waste heat as the current goes up (P = I^2 * R), and the tiny PCBs in the current design have a hard time sinking this heat away. Customers always want more power, but recent experiments with higher power on the current physical design have convinced me that the only way to allow more power is a bigger PCB with more copper.

3. Improve hackability. For a product targeted at hardware hackers, the current design is not the most hacker friendly, mostly because there is very little room due to the small size. Customers want to connect different power sources, have the on/off button and LED remotely located, etc. The current design has some tiny surface mount pads to allow such things, but decent sized through-hole pads and more configuration and customization options would definitely be welcome.

4. Make load current independent of charge current. Some people don't need a big battery for long untethered run time, they just want a little bit of juice, enough to shut the system down safely when external power fails. But in the current design, maximum UPS load current is linked with charge current, which in turn dictates battery size. I hope to separate these two concerns and provide high load current with a small battery as well.

5. Improve physical stability. This isn't usually a problem, but the current mounting method with one screw and the weight of the battery supported on the 4-pin connector between the boards is not ideal. I plan to use a 40-pin header and connect to at least 2 screw holes.

So here is my current plan to make this happen:

  • Start with a Pi Zero-sized PCB, extended past the GPIO header so the battery can still sit next to the Pi, but upside-down. Have one PCB design that can accommodate both the 14500 and 18650 cells. The 18650 will be located more forward than in the current design, making sure it doesn't stick out farther than the USB ports on a Model B+ any more.
  • The circuitry will be over the Raspberry Pi as is the case for most HATs. The components will be mounted on the bottom of the PCB, with only the on/off button (mechanical switch) and LEDs mounted on top. Most of the top of the PCB will be copper plane for heat sinking, on which an additional heat sink can be mounted if deemed necessary for high power applications.
  • There will be through holes for remote on/off button and LEDs, to easily connect input power other than USB, give access to 3.2V battery power and switched 5V output power, connect external LiFePO4 cells and loads, and adjust the charger's MPP for use with solar panels.

Anything I've missed? Comments and requests are not only welcome but highly encouraged!

LiFePO4wered-Business-Pitch.pdf

Buniness Plan/Pitch for Hackaday Prize Best Product

Adobe Portable Document Format - 308.92 kB - 07/21/2017 at 19:20

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sheet - 10.73 kB - 07/18/2017 at 21:06

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LiFePO4wered-PiPlus-CC-BY-SA.pdf

LiFePO4wered/Pi+ schematic as of 6/12/2017, CC-BY-SA licensed

Adobe Portable Document Format - 45.87 kB - 06/22/2017 at 19:38

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  • 1 × CN3801 Power Management ICs / Switching Regulators and Controllers
  • 1 × TPS61236P Boost converter with bidirectional load switch
  • 1 × MSP430G2232 Microprocessors, Microcontrollers, DSPs / ARM, RISC-Based Microcontrollers
  • 1 × SSM3J332R Discrete Semiconductors / Diode-Transistor Modules
  • 1 × DMG2305UX Discrete Semiconductors / Power Transistors and MOSFETs

View all 10 components

  • PCB and firmware update

    Patrick Van Oosterwijck10 hours ago 0 comments

    I got my new proto PCBs from OSH Park, 0.8mm 2oz copper this time:

    It has the improvements described in the previous project log, you can see the new DFN power MOSFET footprint for instance.  I was getting ready to populate some prototypes by updating the BOM, until I noticed I had the wrong power MOSFET!

    That AON7408 I talked about last time?  Well turns out it's an N-channel MOSFET, and I need P-channel.  How I ever missed that while tearing apart the datasheet, I don't know.  So, I decided to use an Infineon BSZ120P03NS3 instead.  It's more expensive, but the safe operating area looks even better:

    Since I'm ordering production quantities to save time, I think this MOSFET should definitely have enough margin to work at high input voltages.

    On the firmware front, a LOT of work has been done.  I mean a TON.  Unfortunately most time has been spent chasing an elusive bug.  I think I finally got to the bottom of it.  Although it's more a patch than a root cause solution, it's as good as it will get, since the bug seems to be related to a hardware limitation.

    I really don't like the I2C peripheral in the small MSP430G micros I'm using.  Calling it a hardware I2C peripheral is an overstatement, it's more like a glorified shift register.  At every I2C related event, it requires software assistance.  8 bits are received: interrupt, set up for sending ACK.  1 bit sent: interrupt, set up for receiving 8.  Start condition, stop condition: all handled in software.  It stinks.

    So when I needed to add shadow buffering to ensure the 4-byte RTC would be written atomically, I seem to have upset a delicate timing balance.  Something in the timing was screwed up so sporadically the end of one packet and beginning of another would not be detected, causing the 8-bit I2C address with r/w bit and some other bytes (usually 0xFF because of reading) to be written over registers.  Registers such as the LED state (not good) or the I2C address (very bad) would be overwritten, with symptoms ranging from a flashing LED to killing communication.

    I tried for weeks, even months to track this down and fix it, but I think I just can't make it 100% reliable with this hardware.  With the shadow buffering added, there's just too much code running and based on when different other interrupts (timers, ADC) end up running, I can't guarantee the I2C baby sitting code will always run on time.  This is of course compounded by the fact that the Raspberry Pi has a retarded I2C peripheral as well that pays no attention whatsoever to I2C clock stretching, but just barges on even if the peripheral can't keep up.

    So I did the next best thing: I implemented a patch to prevent stray writes to I2C registers.  After the register to be written is received, the LiFePO4wered/Pi+ now expects an "unlock code" that is a combination of the I2C address, a magic value and the register to be written.  Once this is received correctly, the other bytes are written.  It just requires a small update to the host driver code.

    Other firmware updates: boot and shutdown timeouts, 32kHz crystal timers, RTC code, higher resolution oversampled ADC values, touch timing fixes, 10-second button hold to force off.  Many of these improvements will also carry over to the #LiFePO4wered/Pi as well, except those requiring a 32kHz crystal.

  • New prototype PCB layout

    Patrick Van Oosterwijck11/23/2017 at 18:59 0 comments

    Since blowing out parts on my existing prototypes has gotten them to the limit of their useful life, it was time to get some new prototypes. :)

    Time to make some changes related to things I've learned:

    • The locations of the mounting holes for the 14500 battery was wrong on the first prototype layout so I fixed that.  I also added extra holes to be able to mount the battery with a little gap so it's possible to mount the Pi in a case and have the battery outside.  This should work with the official case.
    •  Breaking the 18650 mounting PCB part off worked fine, breaking the complete battery holder part off was another story.  I did manage to break it off, but it put way too much strain on the PCB for my liking.

          So I now have added a gap that should make this process simpler.  Based on demand, I may also decide to have separate panels without the battery holder for those that want to use their own LiFePO4 battery.

    • I adjusted the layout around the 32kHz crystal so it now has a ground island and guard ring around the sensitive crystal signals.
    • I moved the programming pads a bit farther from the inductor for easier access.
    • I added some extra capacitors since ceramic capacitors have the habit of not having their full capacitance under bias voltage, and I want to make sure the supplies are clean.  Victim is the 4.25V output voltage option, which nobody seemed to be asking for anyway.  Yes, it would allow higher output current, but the output wattage would still be the same.  Since downstream there are usually switching power supplies, the lower voltage won't change the available power.
    • I changed the micro's supply ferrite beat and capacitor to a small resistor and bigger capacitor.  I had observed that under light load the battery voltage had some low frequency noise on it because the boost converter goes to PFM mode, and the periodic on/off puts little dips in the supply at around 30kHz.  The resistor / bigger capacitor combination will filter this better.
    • The fused battery voltage output was replaced by a battery output switched by load switch.  It provides protection and also turns the output on and off with the 5V output.  I just was not comfortable with the concept of having people connect stuff to the battery and be responsible for not over discharging the battery.  Now the LiFePO4wered/Pi+ circuitry can ensure the battery is not over discharged.
    • The charger switching MOSFET was upgraded to a higher power and current device since the current one blew out at high voltage and load current.  This required a new footprint.
    • I still have the HAT EEPROM circuitry on the board but don't intend to populate it.  The HAT standard is completely retarded in that it only allows ONE HAT, and only reads one HAT EEPROM address, even though it uses a bus.  This means that if people would use a LiFePO4wered/Pi+ they could not combine it with another HAT.  Since most of my customers order stackable headers, many must be using my current products with other HATs and it would be a disservice to populate the EEPROM on mine.  But since the LiFePO4wered/Pi+ is intended to be a base for other products that use the full HAT size and add custom circuitry, I'm reserving the space for the EEPROM so it is available for those spin-off products.

    I think that's about it for significant hardware changes.  Here's the new layout as sent to @oshpark :

  • Progress in learning and code

    Patrick Van Oosterwijck11/06/2017 at 18:29 1 comment

    I figured out some stuff since I burned up my power MOSFET last time.  Stuff like: "Read the datasheet more carefully dummy"! :)

    The SSM3J332R datasheet has the following graph in it:

    This makes it obvious that current capability of the MOSFET is seriously reduced at higher VDS.  The graph shows lines for pulse lengths but I'm not sure how repeated pulses play into this.  To be safe, I'm just scaling the DC line value by the duty cycle.  At 20V, that means the SSM3J332R is only rated for... about 260mA with 15% duty cycle.  No wonder it burned up!

    So, need to find a better MOSFET.  Unfortunately that means more expensive as well.  I stumbled across the AON7408, which is a significant step up and still low cost.  Unfortunately it's in a DFN package, and I prefer leaded packages.  But since my quest to replace the TPS61236P boost converter didn't turn up any good alternatives, I already am stuck with one DFN anyway so I might as well add another one.  On the upside, there seem to be many MOSFETs available in this package so if the AON7408 doesn't work out it should be easy to find an alternative in the same footprint.

    Here is the same safe operating area graph for the AON7408:

    In the same conditions at 20V with 15% duty cycle, the AON7408 should be able to deal with 4A.  A significant improvement, and the MOSFET fits in the same area as the original one.

    This also makes me wonder if some of the heat at 20V, 2A I saw in the previous project log near the Schottky diode was actually the MOSFET starting to burn up.  The resolving power of the thermal camera is just not good enough to tell where exactly the heat comes from if two components are close together.  Hopefully, the overall power dissipation and heat in the system will go down with this change, since the MOSFET has significantly lower RdsON as well.

    On the software side, I have finally implemented code for the 32 kHz crystal and RTC functions.  It's pretty cool to see the Pi wake up at an exact time. :)  I need to do some more testing, but I've started work on the next PCB revision and have started some volume component orders.  Exciting! :)

  • 2A load current! *

    Patrick Van Oosterwijck10/21/2017 at 04:16 2 comments

    Success!  I finally managed to get the LiFePO4wered/Pi+ to 2A continuous load current in UPS mode! :)  Without active cooling, and with the charger keeping up and keeping the battery charged.  So what changed?

    As I have previously explained, it all comes down to limiting the heat production on the board.  One commenter suggested I switch the 60V 5A SS56 Schottky diode for a lower voltage model, such as a 40V 5V SS54.  The reason is that the diode drop in the forward direction is lower for a diode with lower reverse breakdown voltage.  With a lower forward voltage, it will dissipate less heat.  I tried that, but unfortunately it didn't seem to make much of a difference.

    So I decided to look over some of my old thermal camera shots again to see what was producing the heat.

    In this one I was measuring the diode temperature, but as you can tell, there's another hot spot, between the 2 and 0 of the "> 120°C" text.  It even seems a little brighter than the diode spot, actually.  This is transistor Q4 in the schematic, it's present (in combination with Q2 and Q3) to disconnect the charging circuit from the battery when the battery isn't charged, to minimize current leakage.  Of course, while charging, it has the full charge current flowing through it.  Assuming conversion from ~3V to 5V at about 90% efficiency, this transistor has a continuous current of about 3.7A flowing through it when the load draws 2A.  Unfortunately the Vgs voltage is only 3V or so, so the transistor isn't turned on as hard as would be ideal, and likely has a RdsON of around 100mΩ.  This results in burning 1.37W of power in this little SOT23 transistor and it gets HOT.  Too hot probably, the DMG2305UX spec mentions 1.4W max mounted on 1 inch square 2 oz copper, which I'm not meeting in that location, especially since the diode is also heating the board.

    So I decided to try a different transistor, a Toshiba SSM3J338R (with the original SS56 diode).  It is specified to have 20mΩ typical RdsON at 2.5V Vgs, so it should be a good improvement and it has a SOT23 package as well.  It should only dissipate 0.27W at the same current, resulting in 5 times less heat.

    And it works!  Here's a thermal shot:

    And having it loaded with 2A, coming up from discharged battery, the system manages to supply the load with 2A and charge the battery, as it required for UPS functionality.  Success!

    But you must have noticed the little star, asterisk "*" in the heading.  You know what that means. :)  Fine print, exceptions, disclaimers, etc.  Well there are some here as well.

    In this case, it's about the input voltage to the system.  These tests were all done with 5V USB input, but I've designed the board to accept input voltages up to 24V.  So I decided to test what would happen at 2A with 20V input voltage.  The result isn't good:

    Yikes, frying the diode again!  This makes sense unfortunately.  At high input voltages, the PWM duty cycle is very low, which means that current is most of the time flowing through the freewheel diode.  At 5V input, the diode only conducts around 40%  of the time, but at 20V input it's about 85% of the time.

    Every power system is subject to trade-offs like this.  I'm all right with lower current capability when powered by higher input voltages--it's a trade-off the customer will have to make in their system design.  As long as it can do 2A with 5V input I'm happy.

    So how low do we need to go to have it work as a UPS and not overheat?  i tried 1.5A load:

    Better, "only" 110°C, but still too hot, since the battery discharges while plugged in.

    Ok, try 1A then:

    That's better, only 82°C and the system stays cool enough to allow the battery to charge!  It's not 2A, but it still can act as a UPS for a Pi3 at 100% CPU.

    Then I wondered: since at the higher input voltage the diode becomes the...

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  • ETA1096 evaluation

    Patrick Van Oosterwijck10/18/2017 at 17:31 0 comments

    In my quest to reduce the cost and manufacturability of the LiFePO4wered/Pi+, my attention was drawn to the ETA1096, a Chinese boost converter.  It is significantly lower cost than the TPS61236P, and it comes in a nice leaded package, so I thought I'd evaluate it to see if I could use it on the LiFePO4wered/Pi+.

    I found out about this part while investigating some of my competitors.  I found the Geekworm UPS HAT and although the creator tries very hard to hide which parts are used by scraping the surface markings off, some smart people on the internet figured it out anyway.  Since the UPS lists 2A output current and the chip's specification lists up to 3A, it looked promising.

    What was harder was to get a hold of samples to evaluate.  The Chinese distributor refused to ship samples in the mail, and I wasn't going to spend $100+ to have a couple cheap chips shipped by Fedex.  In the end I decided that the cheapest way to get a sample part was to buy the Geekworm UPS HAT. :)

    Now it might be considered a stupid idea for my business to talk about a competitor here, but honestly the Geekworm UPS HAT is a pretty lousy device compared to the LiFePO4wered/Pi and Pi+.  Of course I'm biased, but check out this attempt by an enterprising individual to make it usable and you'll know what I mean.  Calling it a UPS is a bit of a stretch in my opinion, the out-of-the-box functionality is really limited.  It's something you can make usable if you want a project, but if you just want something that works so you can get on with the project you're already doing, you might want to get a LiFePO4wered/Pi instead. ;)

    I decided to hack in my own battery and charger (a #LiFePO4wered/18650 base) so I could focus on the ETA1096 performance without worrying about the rest of the system.

    [Just one quick note on the "rest of the system".  The charger chip (likely an ETA9741) is an odd part that actually puts 5V back on the input USB port even if the ETA1096 is off, and seems to draw 40-200uA from the battery.]

    I used my electronic load and started with a load current of 1A.  The ETA1096 seemed to be doing fine, the voltage didn't sag and the chip didn't seem to get warm.  I bumped the current to 2A and the whole thing shut off.  OK... start over and take smaller steps I guess?

    So I went to 1.1A, 1.2A, 1.3A, 1.4A, 1.5A, 1.6A and it shut off again.  Start over and let's keep it at 1.5A.  After a minute I decided to touch the chip to check how hot it was and I yelped--blazing hot!  I don't have access to the thermal camera at the moment so I used my IR thermometer and it reported 107°C!  And it's always been conservative compared to the FLIR One since it averages a larger area.

    Well, that's... unexpected.  I knew it would get warmer than the TPS61236P since it has 40/55 mΩ MOSFETs instead of the TPS61236P's 14/14 mΩ.  But that still only translates into 0.3W instead of 0.1W (based on 2.8A switch current estimate due to 3V->5V @ ~90% efficiency guess), not the "blazing hot" I was getting.

    I guess I'll forget about the ETA1096 and stick to the "expensive" TPS61236P for now. ;)  It bugs me though that I don't quite understand what's going on and cannot explain the amount of heat produced.  I seem to be missing something and would love some fresh ideas on what might be happening.

  • Current sense bypass check

    Patrick Van Oosterwijck08/16/2017 at 22:42 0 comments

    One concern I had was that when using the small 14500 size battery, the current sense resistor that limits the charge current also is in series with the load when running from the battery.  My concern was that it would drop so much voltage that the software would decide that the battery was depleted and shut down.  This is, of course, not a problem since the battery voltage is measured directly at the battery, and not after the sense resistor.  But I had temporarily forgotten that. :)

    So I did a test where I used a MOSFET to bypass the 0.18 ohm resistor when the input (charge) voltage was not present.  I wanted to see if it affected run time from the small battery.  This is still a valid test, because the boost converter does see lower input voltage due to the voltage drop across the sense resistor, so its efficiency might go down.

    The test was done with a Pi3 with 4 cores @ 100% and an active SSH connection over WiFi.  Here are graphs of the battery voltage in mV:

    Oddly, the results look better without the bypass.  The worst result was after a recharge a day ago, the voltage would droop and threaten to fall below the shutdown threshold, but then recovered.  The longest run time is without any bypass, and was about 23 minutes.

    I don't know for sure what caused the initial droop, it could be a chemical effect in the battery when charging wasn't recent and ion mobility may be lower.  I will have to do a little more testing on that since it comes dangerously close to the shutdown threshold.

    Overall this is good, the little battery did hold up so it's promising as a solution for those who are looking for UPS functionality and just want enough power to get through a clean shutdown.  The Pi was pulling ~950mA at 5V, which translates to about 1.6A at the battery voltage.  Testing with my electronic load seems to indicate 1.2A output current is the limit for the small cell, at which point the boost converter output droops to ~4.85V, still within the USB spec.

  • Getting to 2A... or not

    Patrick Van Oosterwijck08/10/2017 at 23:43 2 comments

      In trying to get to my goal of 2A output power, I've run into my old nemesis again: heat.  After getting stable operation at 1.8A continuous output current, I thought I could get to 2A with minimal effort just by changing some current sense resistors.  But this doesn't seem to be the case.

      The main culprit is the asynchronous charger's Schottky diode.  For all other power components, it's pretty much up to me how much heat they generate.  If it gets too hot, just get a MOSFET with lower RdsON, or an inductor with lower resistance etc.  But for the Schottky diode, it's another story.  Sure, you can get a diode in a larger package, allowing it to dissipate more power safely, but it's still going to generate the same amount of heat, because due to physics the diode will always drop voltage.  So it will always dissipate power (P=V*I).  Worse, the amount of voltage it drops goes up with higher current as well.

      Now 2A doesn't seem to be that much current, but keep in mind that I'm aiming for 2A output current at 5V, which means that with a battery voltage of minimum 3V, and counting on converter efficiency around 85%, the diode sees average currents of (2A*5V)/(3V*0.85)=3.9A.  Now I understand why, even though the power components are external, the CN3801 datasheet indicates 4A maximum output current.  How much power is dissipated?  At 4A, the SS56 Schottky diode I use drops around 0.6V, burning off 2.4W.  As it gets hot, it starts to exhibit an appreciable reverse current as well, leading to even more losses.

      The mechanism preventing me from getting to 2A due to this heat seems to work like this: as the circuit (mostly the diode) heats up, some of this heat warms up the CN3801, eventually at high temperatures making the current sense voltage reference drop.  Nominally 120mV, I've seen it go as low as 95mV when drawing high currents.  This will in turn limit the amount of current drawn (for the same sense resistor, lower current will generate this lower sense voltage).  So the whole thing is self-limiting: as it gets hot, it will limit the current.  This could be considered a feature I guess, but the bottom line is that even when you lower the sense resistor, as things get hot the sense voltage drops enough to negate the lower sense resistor.

      I've been considering possible solutions and what this means for the project:

      1. Is there a way to turn this into a synchronous converter (replace Schottky diode by MOSFET) with some clever circuitry and if so, how much circuitry and will it fit?  The chip has no complementary output for this so it will have to be a hack.
      2. My simple topology of running all current through the charger is showing its limits.  Another topology would be to only run the charge current through the charger and provide a parallel path for the load current directly from the power input.  Downside is that now the 5V converter needs to become step-up/step-down, since the input voltage can be as high as 24V and the battery voltage as low as 3V and both need to convert to 5V.  A step-up/step-down immediately pushes you into chips that are in the several-dollar range.
      3. A spec fix: instead of 2A max, advertise the system as 1.8A max.  The heat still bugs me, and will limit the current even lower at higher temperature.  2A is also a nicer number, and what the Raspberry Pi foundation recommends for a Pi3 power supply.  On the other hand, 1.8A is double what the current #LiFePO4wered/Pi3 can do without active cooling, so it's still a pretty nice upgrade.

      I would very much like user feedback on this.  How much current do you need?  The current design works well at 1.8A, and can probably go into production this fall.  Completely changing the topology will get me past 2A, but is a big redesign that will likely not reach production this year.  Would it be worth it to delay and get the spec I wanted?  It would...

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  • Load test results

    Patrick Van Oosterwijck06/22/2017 at 23:40 1 comment

    Finally got around to doing some serious testing, and overall the results are looking good!

    I used a prototype with the battery holder area broken off and an external battery connected. This configuration has the smallest PCB copper area, and as such will have the worst thermal performance. I tested with a Pi3 @ 100% CPU first to get a baseline. This first thermal shot is running from the battery without charging:

    Very good, doesn't get hot at all even though the Pi is cooking!

    Now a shot while also charging the battery:

    The hot spot is hotter and has shifted to the charging area, which makes sense. The charger is an asynchronous step-down design, which is less efficient than the synchronous boost converter.

    To be able to see the components (which are on the bottom, facing the Pi), at this point I decided to switch to my electronic load instead. First I ran the system at a load of 1A, similar to a Pi3 at 100% CPU:

    As expected, the culprit is the asynchronous charger's Schottky diode. Unfortunately, the diode will always drop a voltage, so it will always dissipate heat. Because I anticipated this, I put a ton of vias around the diode to try and pull as much of this heat as possible to the top side ground plane. As the images show there's only a few degrees C difference between the diode temperature and the temperature measured on the top side, so this seems to be doing the trick. My prototype PCBs only use 1 oz copper as well, for production I intend to use 2 oz copper instead which should increase the heat sinking capability of the ground plane.

    Next I wanted to see what would happen if I really loaded this puppy. I cranked up the electronic load to 2.5A. I only designed the system for 2A loads, but I like to be mean during testing to make sure things aren't marginal.

    Ouch, hot! Unfortunately the FLIR camera can't seem to measure hotter than 120C, so I don't know how hot the diode actually is. On the plus side, the diode is rated for 150C, and I had it running for several days like this without anything going up in smoke. :)

    The boost converter got pretty hot too at this load:

    As this chip is rated up to 125C, I don't think this is something to worry about at this point. If the system is going to be used at high load and elevated ambient temperature, more thermal management will have to be added. I intend to do some future tests with a heat sink on top of the PCB as well.

    On the low power side, the system is pulling 3-4 uA from the battery with the Pi off, so it seems my scheme to disconnect the charger from the battery when not charging is working as well.

    I tried to run at high load continuously but I ran into an issue with that. I would try to run overnight but would find the system off in the morning. I finally figured out the charging chip was regulating to a lowish 104 mV across the current sense instead of 120 mV. So the system was discharging the battery instead of charging it. I had to lower the load to 1.8A to keep the battery charging. This means that I need to make some adjustments in the current sense resistors to account for tolerances due to temperature etc and still reliably charge the battery at 2A load.

  • It's alive!

    Patrick Van Oosterwijck05/19/2017 at 23:35 0 comments

    After hacking on firmware for the past two weeks, I'm happy to report that the prototypes are working!

    Here are my two protos: running a Pi 3 and a Pi Zero W:

    The prototype in the back uses a 18650 LiFePO4 battery and stackable header, the one in the front uses a 14500 (AA) LiFePO4 battery and normal header. To remove the excess PCB used for the bigger battery from the unit with the smaller battery, I had added perforations to the PCB, but I was worried that the copper foil might peel when breaking it off. I decided to see what would happen with and without scoring with a knife, so I scored only part of the perforated area. Here's the result:

    The section of PCB broke off very clean, and I honestly cannot see any difference between the part that I had scored and the part that I hadn't. Very good news! I'll have to see what happens when breaking off the bigger section when I'm ready to build a prototype with external battery.

    On the firmware side, I worked on the following:

    • Different pin assignments since I had to switch to a 20-pin package to accommodate an extra signal and had run out of pins.
    • The extra signal allows measuring the Raspberry Pi current consumption.
    • Separate single point calibration values for VIN, VBAT, VOUT and IOUT instead of a single calibration value based on VBAT. Since the range of VIN is now huge (24V) compared to the range of VBAT (5V), the single calibration value wasn't working well.
    • Added shadow buffering to the I2C driver. This allows atomic read and write of multi-byte values to prevent race conditions, and will be very important when implementing RTC functionality.
    • Timeouts for boot and shutdown phases. At the moment they are hard coded to 5 minutes for boot and 2 minutes for shutdown, but the idea is that they will be settable in 10 s increments. This improves behavior when the Pi is not present or unresponsive.
    • Support for mechanical switch (default for LiFePO4wered/Pi+) in addition to capacitive touch. As you can see in the pictures, the prototypes have a side-facing switch as well as a top-facing one. The side-facing will likely be standard and the top-facing an option. There are also pads to connect an external button or cap touch pad.
    • Improvements to reduce power consumption when off (still monitoring voltages and button), around 3uA now. Initially I had a problem where the current was ~80uA if the Pi wasn't connected.

    Once functional, I could do some preliminary testing, and things look promising:

    • With a Pi 3 running at 100% on 4 cores (~950 mA), the board was warm but not hot. This is promising for extra power capability!
    • The split sense resistor scheme to allow much higher load current than charge current seems to work! The prototype with the small battery was able to power a Pi 3 @ 100% while also charging the battery quickly, but not above its rating when under low load.
    • The 40-pin header with 2 screws seems to be mechanically solid.

    Next I'll need to do more detailed high-power testing to see where the limits are. I still need to add registers to set the boot and shutdown timeouts and implement RTC functionality.

  • First prototype build

    Patrick Van Oosterwijck05/08/2017 at 23:53 0 comments

    I decided to build up my first prototypes of the LiFePO4wered/Pi+ today. Since I wanted to do all 3 boards I received from OSH Park, and there are quite a lot of components, I figured using solder paste and reflow was the way to go instead of manual soldering.

    I had not ordered a stencil so I used the laser cutter at the Tinkermill to cut my own stencil from plastic film. It never turns out as well as the professional ones you can buy, but it's free and usable. :) Below is a picture of the stencil over a bare PCB:

    Time to apply solder paste:

    The result with the stencil removed is fairly decent, even for the tiny and weird TPS61236P boost converter pads:

    I decided to paste all 3 boards and then build them up in batch:

    The disadvantage of doing this is that the paste tends to dry out during the long build. Placing the components took around 2 hours and the paste was definitely getting dry, but it seems to have had no ill effect. Here are the boards with all components placed except the inductors:

    And here's a closer shot, ready to go into the reflow oven:

    Here's the first board after reflow:

    Looking good! :) Even the TPS61236P connections seem to have soldered fine. I was a little worried because the stencil wasn't exactly accurate for those tiny pads.

    Here are all three boards done:

    I still need to add the LEDs on the other side of the boards, and maybe a top button. As you can see, there is a side-facing button already on the boards but I want to evaluate different options such as side-facing and up-facing. I also need to add the 40-pin GPIO headers and battery holders.

    Then I need to take the existing #LiFePO4wered/Pi firmware package and port it to this new platform. This new design needed more pins than were available on the micro I was using before, so I'm using a different part from the same family and some pin assignments have changed. Plus, I need the firmware to sample a physical button instead of a touch button and use the 32kHz crystal instead of the internal RC oscillator.

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alanmcdonley wrote 10/01/2017 at 00:08 point

Hi Patrick,

You wrote: "I was intending to provide an option to lower Vout to 4.75V, because it will allow more output current. Can you expound on why you'd want higher Vout instead?"

With the existing LiFePO4wered/Pi3 unit running at Vout: 4958, cooled by the passive 15x15x15 heatsink, in the Tindie case (on 1/2 inch feet), a load factor greater than 2 will eventually report under-voltage throttling and/or high temperature throttling.  Perhaps a processing spike is trying to draw more current than the unit can supply causing the voltage to sag momentarily. Starting nearer the upper limit of the board spec 5.25 might offer me greater protection from these momentary voltage dips.

I read reports from some Raspberry Pi3 owners of under-voltage throttling when they ran at 4.75v (at the board).  Perhaps the voltage sensor on the Pi3, which supposedly triggers at 4.65v, is overly sensitive. 

Offering 4.75v as an option, sounds like it might extend the run time on battery as well.  Options are good.

The new unit with double the power sounds great. 

Regards,

Alan

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Jay wrote 08/05/2017 at 13:37 point

Any new updates since the initial load test results you posted 06-22? I'm interested in using a Raspberry Pi as a hub for an environmental monitoring device and your design makes my plan for it much more practical. Interested to know how the 2 oz copper works as a heatsink vs the 1 oz from before!

Thanks Patrick!

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x893 wrote 06/16/2017 at 09:19 point

Hi,

possible use TPS61089 ?

I have some problem with download schematic - can you publish info on github ?

Thanks in advance

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Patrick Van Oosterwijck wrote 06/22/2017 at 19:41 point

That chip is more expensive than the TPS61236P I'm using, any particular reason for the suggestion?

Looks like the Hackaday.io CDN choked on the "+" in the file name of the schematic, I changed the name and now it seems to work.  Hackaday, fix this so it either works or tells me there's a problem when I upload the file!

Wonder if the "lack of schematic" made any different in the Hackaday Prize judging... :(

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Antoine wrote 06/14/2017 at 08:35 point

Hello Patrick,

I have already purchased your LiFePO4wered-Pi3 ™ from Tindie.
Our prototype is based on Pi Zero W for AQI.
Now I have to select Solar Panels and there is a big gap between the average output panels (Vmpp 12V to 17V) and the LiFePO4wered-Pi3 ™ V.in Max (9V).
As you said, the MPPT is also less relevant for panels.


That's why this new version is very clever.

Our vote goes to You.

We keep in touch from South of France

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Stevo wrote 05/29/2017 at 21:36 point

I would like to buy two when your have finished the development. How many amps can it supply to the Pi? I have a bunch of peripherals attached to one of mine. I love the battery backup idea. I have lots of 18650s and one of them would be great to power it in the event of power loss until it can be safely shut down. I have a lot of little nieces that tend to trip over chords, unplug things for the fun of it, and create a general state of complete havoc.

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Patrick Van Oosterwijck wrote 06/22/2017 at 19:43 point

Hello, I'm designing this version to be able to supply 2A to the Pi and whatever you may have connected to it. :)

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Magnus Eriksson wrote 05/27/2017 at 19:22 point

Hi. Patrick Van Oosterwijck
It looks very interesting.
I wonder if I can buy 2 pieces of LiFePO4wered / Pi
Then I'm building a bit of a Rasberry pi 3 type that I'll have in my vehicles
I am also interested in being able to drive them with solar cells.
I'm most interested in the model with 18650 cells.
Sincerely
Magnus Eriksson

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Patrick Van Oosterwijck wrote 05/29/2017 at 18:58 point

Hello Magnus,

You can buy the existing LiFePO4wered/Pi3 on Tindie: https://www.tindie.com/products/xorbit/lifepo4weredpi3/

It will be a while longer before this next gen version will be available since I need to do a lot more testing on it. :)

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doubleodoug wrote 05/26/2017 at 22:21 point

Could you post your schematics? Ideally as pdf. I would really appreciate it.

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Patrick Van Oosterwijck wrote 05/29/2017 at 18:55 point

Planning to post the schematics soon... I just need to overcome my reluctance to give my hard work away. :)

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x893 wrote 05/26/2017 at 19:06 point

Super project !!!

Can you publish schematics for LiFe.../Pi+ ?

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denplis wrote 05/11/2017 at 19:53 point

Could it be possible to use a few 18650 cells ?

  Are you sure? yes | no

Patrick Van Oosterwijck wrote 05/29/2017 at 18:54 point

Yes, several 18650 LiFePO4 cells in parallel is supported, you will be able to connect them to the BAT through hole terminals and break off the existing battery holder if required.

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oshpark wrote 05/09/2017 at 18:45 point

Awesome project!

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SlumberMachine wrote 04/06/2017 at 23:40 point

Looking forward to your progress.  I have 2 of your lifepo4weredpi and they have been working great for me.  They even helped me detect and deal with a bad charger I had received by safely shutting down the pi because the usb charger was giving intermittent power (also started smelling like burnt plastic).  Probably would have cost me some corrupted SD cards if it wasn't for your lifepo4weredpi.  BTW - I'm using it for a time lapse setup which I leave connected to usb battery bank. 

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Patrick Van Oosterwijck wrote 04/30/2017 at 20:56 point

Thanks for the feedback and glad to hear the LiFePO4wered/Pi are working well for you, I always enjoy hearing how people are using them. :)

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