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I'm working on a mobile computer unit with a Raspberry PI 3 motherboard. The computer unit needs to run on battery so i can bring it with me anywhere. I'm going to solder both the battery pack and power supply (buck or buck boost) myself, since it need to fit as perfect as possible in a custom made 3D printed case.

I have decided to use LiPePO4 battery cells, either 2 (6.4VDC) or 3 cells (9.6VDC), I guess 3S is best since I only need a buck converter to get 5V contra 2S that may need buck boost. I'm thinking of soldering a) power supply and b) cut of (high voltage/low voltage) on the same PCB.

But I'm not so very experienced with electronics other than I have some basic electronic education, so I'm wondered if someone has some advice or inputs to this project. I have mainly three questions.

  • Is it better to use 2S or 3S battery to this project, I'm then thinking of efficiency of power supply, as far as I understand 6.4VDC is closer to Vout (5VDC) so it will be a more efficient power supply than Vin as 9.6VDC. Runtime is importent, so more efficiency power supply is longer runtime.
  • I'm looking for power supply circuits that can deliver 2-3A, is it enough to run just the RPI 3 board?
  • I have found several DC/DC converters that delivers 2A@5VDC, but these use Linear Technology IC that are pretty expensive, I have been told multiple times that LT's IC is overrated, what other IC brand can I search for?

And if you guys have been doing similar projects with RPI 3, or have other advice, please inform me.

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  • \$\begingroup\$ For the first two things you need to do some reading of datasheets, for the converter how efficient it is at the desired operating points and for the RPi on how much power it needs the way you use it. For the third, recommendations are offtopic here. \$\endgroup\$ – PlasmaHH Sep 18 '17 at 13:55
  • \$\begingroup\$ I know that Raspberry PI 3 B recommended power supply should be 2.5A (ref. raspberrypi.org), but I'm not going to plug in any external USB/HDMI/Ethernet, etc. units. It's only going to use bluetooth and WiFi. Where should I go to get some recommendations? Because everyone is telling me that LT is overrated but no one tells me what I rather should look for, and it's hard for a newbie to search when I don't know what to search for. I just need a little jumpstart. \$\endgroup\$ – BufferOverflow Sep 18 '17 at 14:04
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    \$\begingroup\$ I have never had anyone tell me that they are overrated. Just go to digikey or wherever you buy your parts, do a parametric search for whatever you need and start browsing around \$\endgroup\$ – PlasmaHH Sep 18 '17 at 14:06
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    \$\begingroup\$ Rpi 3 peaks at 1.75 amps during stress testing with peripherals according to the rpi faq. raspberrypi.org/help/faqs/#powerReqs \$\endgroup\$ – Passerby Sep 18 '17 at 14:10
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...6.4VDC is closer to Vout (5VDC) so it will be a more efficient power supply than Vin as 9.6VDC.

That is true if you would be using a linear regulator since then the current remains constant. But then the 3 cell solution would give a longer battery life as the individual cells can drop to a lower voltage until the regulator cannot provide 5 V anymore.

But you should use a switched regulator since that is going to be much more efficient for the currents needed by an RPi. And for a switched regulator efficiency will be better when the input voltage is higher.

The most efficient solution is 3 cells and a switched regulator.

I suggest not to make your own switched regulator as that might end with all kinds of issues. There are cheap ready-made switched converter modules for sale which will just work saving you a lot of trouble. My suggestion: find an LM2596 based module, these are cheap as chips and will just work. These can also supply up to 3 A or so, more than enough for an RPi.

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  • \$\begingroup\$ First I was thinking of 2S solution with 2x2 cells, or 3S with 3x1 cells. But if higher voltage is better in SMPS design, then 4S with 4x1 cells is best? I not thinking of designing my own SMPS, I'm thinking of finding a pre designed SMPS circuit and then design the PCB from it? What can go wrong? I have a lot of experience from soldering. \$\endgroup\$ – BufferOverflow Sep 18 '17 at 14:16
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    \$\begingroup\$ With 4 in series you'd get the best battery life. I would not go for 2x2 due to lower efficiency. If space or price is a concern 3x1 is a good choice as well. I would also recommend against making your own SMPS PCB unless you have a design including the PCB layout. PCB layout can be critical in SMPSs. But anyway, you often cannot beat a cheap LM2596 module on price. The cost of the separate components is more than a complete module including shipping from China. Yeah, crazy but true. Only if you're going to mass produce would I go for my own PCB. \$\endgroup\$ – Bimpelrekkie Sep 18 '17 at 14:22

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