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I am trying to power my Pi (3B) from 6 AA batteries, via an UBEC (rated 5V/3A). By itself, the UBEC is outputting some 4.94V.

But plugging the UBEC output into "GPIO" pins 4 and 6, m yPi does not seem to start correctly: the power does not stay consistently on, and I start hearing worrying "hissing" noises from the Pi.

Am I doing something incorrect ? Or what can I do to get more informations about a potential problem ?

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    Just a thought but do you have the polarity correct? Are you sure you have the correct pins? Can you include some pics of the connections in your question? Did the Pi work when powered through the micro USB port? – Steve Robillard Oct 23 '16 at 16:55
  • The Pi worked through USB and thankfully still works. I matched the black wire with pin 6 and the red with pin 4. The Pi's LEDs went on at some point so I guess the polarity was OK. I could bother making/uploading pics though, if bad circuitry is a likely possibility. – Gnurfos Oct 23 '16 at 17:10
  • This probably has nothing to do with the Pi. A Pi3 requires ~1A - this would mean ~750mA from the AA cells. They are not rated for this continuous current (although some rechargable cells may just make it). – Milliways Oct 23 '16 at 22:59
  • Interesting, but I read somewhere that you can in theory draw a lot of amps from anything, it just would drain it fast. Would your theory be visible with a voltage drop if measured, and would it explain the hissing noises ? – Gnurfos Oct 24 '16 at 5:58
  • @Milliways I have managed to power a Pi from AA batteries (used 6 alakaline cells with a 5v regulator), it doesn't last very long... – Wilf Aug 31 '17 at 17:54
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By itself, the UBEC is outputting some 4.94V.

The Pi 3 needs at least 5.1 V to run correctly.

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    That is not true. A supply voltage in the limits of the USB spec (5 V +/- 0.25 V) is perfectly fine for the Pi. The Pi's (model 3) undervoltage detection has a threshold of 4.65V. See also: raspberrypi.stackexchange.com/q/48329/19949 – Ghanima Oct 23 '16 at 19:26
  • I read 5+-0.25 too, but is there an official spec ? When powered through USB, the pins have a 5.04V voltage. – Gnurfos Oct 23 '16 at 19:45
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    The microUSB socket specification requires 5V+/- 5%. You can run the Pi itself down to about 3.5V fed into the 5V pin. Just don't expect any USB peripherals or a screen requiring 5V to work. – joan Oct 24 '16 at 8:02

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