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I've already investigated how to power the RPi with batteries (I did some tests with AA batteries and with 18650 batteries) using a 5V "boost" step-up DC-DC converter, in some other questions here.

But now I'd like to test directly with 

18650 3.7V battery ========> 3.3V of the Pi

Are there special thinks to know before trying? Should I put a 3.3V regulator? A DC-DC step down convertor? (I'd like to avoid regulators that dissipate the over-voltage into heat...)

Or can I plug it directly ? Will 3.7V harm the RPi's 3.3V input ? Does it need exactly 3.3V?

Why do I want this? I'd like to avoid 

3.7 battery === DC/DC converter ===> 5V ===> Raspberry Pi  (converted in 3.3V again !)
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    You can try feeding 3V7 into the Pi's regulated 3V3 line. I expect you will destroy the Pi. Please let us know the result of your experiment. – joan Jun 17 '15 at 10:35
  • Haha, ok, so a DC-DC converter is mandatory then ;) ? What would you use? I added a comment at the end of the question to explain why I want this – Basj Jun 17 '15 at 12:00
  • I would feed the battery input into the 5V line. I believe that will produce the core voltages needed (3V3, 2V5, and 1V8) from as little as 3V5 input. It certainly won't damage your Pi. Of course you won't be able to use anything which needs 5V such as a HDMI screen or USB devices. – joan Jun 17 '15 at 12:06
  • Do you think feeding 3.7V into 5V would result in a clean 3V3, 2V5, 1V8 ? – Basj Jun 17 '15 at 12:11
  • I'm a software rather than a hardware person. I'm going on what I've read rather than any personal understanding. If I had a single cell lipo I'd give it a try. – joan Jun 17 '15 at 12:20
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For a lack of other schematics I will use the schematics of the Pi 1 B, Rev 2.1 here. Unfortunately the description of the Broadcom chip itself lists no electrical spec (BCM2835).

Let's have a look where the +5V (coming from the powering USB port and passing through the poly fuse) are used on a Pi:

  • the main 3V3 switching voltage regulator (an NCP1117)
  • +5V0_HDMI pin at the hdmi connector (including the CEC lines)
  • the down-stream USB ports
  • the VDD_BATx pins of the BCM2835

So assuming USB and HDMI are not going to be used on this application we still have two points to consider.

  1. NCP1117 regulator:

    It's a low-dropout positive, fixed voltage, linear (non-switching) regulator. At 3V3 output and an current of 500 to 800 mA a dropout voltage of 1.0 to 1.1 V will occur. So it is not suited to provide clean 3V3 from a single cell lithium ion battery.

    Bonus "Will 3.7V harm the RPi's 3.3V input?" - I do not know yet, but I'd guess so...

    So the save way would be a switching regulator (probably a buck-boost DC/DC converter) to provide the 3.3V, e.g. here.

  2. The VDD_BATx pins

    These are tied to the BCM's internal SMPS (switching-mode power supply) to create the "core voltage". You certainly want your main processor's core voltage clean. The use of the BCM2835 in mobile applications at least strongly suggests that this converter will operate safely from a single cell lithium ion battery. This wiki entry documents the core voltage to be 1.2V and it is not unreasonable to expect the internal SMPS to generate 1.2V safely from 3.3V.

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