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I recently bought a RPi 3 but got a concern with its reliable operation in relation to the power source.

A store near me has something like this but it has only two ports - http://www.amazon.in/Charger-iPhones-Samsung-Android-Devices/dp/B00MUSNG6K

(5V 3.2A two USB ports)

Can I reliably use this kind of adapter? Utilizing only one port for full 3A source.

And for the cable, I got a spare cable lying around which came with a 5v 500ma adapter. Are they generally rated for high current sources?

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    The Pi requires 2.5A at 5V, so the power supply you suggested should work fine. The spare cable might work, but I strongly recommend getting a cable that is rated for 2.5A at 5V.
    – Darth Vader ♦
    Oct 22, 2016 at 10:02
  • @DarthVader that 2.5A value is only the maximum it might draw with USB perhiferals that use a lot of power. The Pi itself doesn't use nearly that much
    – Jodes
    Oct 22, 2016 at 10:22

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Without USB devices attached to the Pi it should use under 1 amp., certainly nowhere near 2A.

Mainly guessing, but your power supply is probably ok, since most now have 2.1A ports - and the fact that it only says it gives a total of 3A out implies to me that all ports share a single supply bus.

Your cable might be a problem, some are better than others, but a higher rated cable would help. It might help to also have a shorter cable, but not necessarily.

If you get unreliable operation from your Pi then I would first change the cable to one rated at 2A, and if still a problem, get a PSU that has a dedicated 2.1A output - both dirt cheap on eBay. But my guess is you won't need either unless you attach thirsty USB peripherals on the Pi.

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  • Does the Pi monitors its power requirements? Can it be accessed from the command line? Or wattmeter is the only way to measure this?
    – arjun
    Oct 22, 2016 at 13:26
  • @Arjun - that's an interesting question. Although I doubt very much the Pi has the capability of doing that, since it is designed to be very low cost. It should be quite easy to do with any multimeter though. They are also very cheap on eBay. You'll just need to split a USB cable to measure the current running through. Multimeters are very useful - even if you get a really cheap one (2-3$!), I'm sure you'll use it in the future.
    – Jodes
    Oct 22, 2016 at 14:46
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The Pi 3 will draw up to about 800 mA. If you attach USB peripherals, each of them may draw an additional 500 mA maximum, as per the USB 2.0 specs. Thus, in theory, it should work.

However, in practice USB chargers may present two issues:

  • Voltage may be lower than the 5V indicated on the label. The Pi 3 will show a voltage warning if voltage drops below 4.7–4.8V.
  • The charger may not actually be capable of supplying the current indicated on the label.

The cable may also matter, as their resistance may differ. For both voltage and current, this may make the difference between just above and just below the threshold. If you experience such issues with one cable, go for another which is shorter and/or has thicker wires.

None of these are major issues when charging a USB device—it’ll just take a bit longer. The Pi, on the other hand, is pickier. If voltage or current are below its requirements, you may get the power warning on screen, USB peripherals may fail, the Pi might reboot spontaneously or even fail to boot up in the first place.

I’ve just learned this the hard way with two Pi 3s, one of them with an external hard disk connected.

  • Charger #1, rated at 1200 mA: The Pi without the hard disk would occasionally show the power warning, while the Pi with the external hard disk would not even boot.
  • Charger #2, rated at 1000 mA: Both Pis initially worked fine. However, on the Pi with the external hard disk, the disk would frequently fail under high load.
  • Charger #3, a dual-port charger rated at 1000 mA: seems to work, the Pi with the external HD occasionally displayed a power warning but no failures so far, even after several hours of high load (same use case that would very soon cause disk failures with charger #2).

Takeaway: it depends on the charger. There’s an official power supply for the Pi 3, which has been designed specifically for this use case (but may not work as a generic USB charger). USB chargers may or may not work—unfortunately there’s no easy way to tell other than by trying it out.

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