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I have 10+ various older model RPis running already. I bought 2 new RPi 3 B+ starter packs (with original power supply!). When I insert an existing SD card it shows a low voltage warning in the top right corner and does not boot.

The SD card works fine in the older models. The out of the box SD card also works fine. I have made many customizations, so I always clone the SD cards when adding new RPi's, up to today it worked fine. Any idea why it might show a low voltage warning and how to solve it?

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From The Boot Problems Sticky :

If you have one of the latest PI 3B+ which won't boot and it keeps stuck on the rainbow screen, while also displaying the under voltage (lightening bolt) icon, and/or the red LED flashing then you are simply are using boot code that is too old. Please download the very latest version of Raspbian, or do an upgrade to it.

sudo apt update
sudo apt upgrade

Other OS's users might have to wait for their OS to get an upgrade. Also note that a PI3B+ won't accept just any old charger, like older models, you really need a good reliable 2.5A power supply with thick wires, like the official one, or you might experience crashes, and memory failures. Still the under voltage icon in the rainbow screen -only- indicates you are using the wrong boot code, it probably doesn't at that point indicate a power fail. Due to the RPI3B+ new power chip, (which includes the under voltage detector) old software no longer will find under voltage info in the same I/O registers, as its now reads that info through I2C. This also means that the red power LED is now also under CPU control, so it can flash (even without a real power fail).

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#Special Case Here (Probably)

Use Etcher instead of Win32DiskImager to install OS.

Apparently, when you use win32DiskImager for writing the image file to the card, it shows the under-voltage warning and doesn't boot up. I got my B+ model yesterday and faced a similar issue. changed 3 cables. Re-plugged HDMI hit keys and tried everything but the warning didn't go anywhere. Re-installed raspbian stretch(old one saved on my hdd) using etcher and it worked perfectly.

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The low voltage warning is shown when the voltage drops below 4.65V. It drops below 4.65V when the power supply is inadequate.

The solution is to buy a 5V power supply capable of delivering more amps at 5V. I.e. the power supplies you are using are not good enough for the Pi3B+.

  • I have read this answer mutiple times. However I am using the original raspberry power supply! If you read my question you can also read that using the supplied SD card, the RPi 3 B+ is also working fine, so there is no issue with the power supply. It seems to be a software setting or bug in the existing SD cards, maybe the low voltage check can be disabled?! – L.Gibbs Sep 26 '18 at 7:58
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    You can disable the voltage check if you want. That will remove the symptom. – joan Sep 26 '18 at 9:19
  • How can I do this? I found some config.txt lines (avoid_warnings), but this did not help. – L.Gibbs Sep 26 '18 at 11:17
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Like the hundreds of similar questions with similar issues your power supply is inadequate and/or your cable is unsuitable. See Raspberry Pi Power Limitations

Despite the other answer the problem is not a supply with inadequate current rating, but rather one with inadequate voltage regulation (or high resistance leads).

The official supplies are better than many, but still inadequate - particularly with the Pi3B+ which is more demanding.

The only way to be sure is to measure the voltage at the power pins on the header. NOTE the Pi will still work even if the voltage is low, and the lightning bolt may show if the voltage briefly drops.

If it does not boot you may have other problems - in particular if the rainbow indicator shows.

  • Yes it is the rainbow box with the lightning bolt icon top right corner. RPi3 B+ with original PS and SD, works great! Older exisiting RPi's with exisisting SD works great. So existing SD's are good, and original PS is good. RP3 B+ with existing SD, rainbow box and lightning bolt, doesnt boot. – L.Gibbs Sep 26 '18 at 11:25
  • In which case you probably haven't updated the OS (specifically the firmware) which MUST be done on an older Pi which boots. You haven't said which OS but the latest Raspbian works on all model Pi. – Milliways Sep 26 '18 at 11:29
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    Thanks this works. New OS works with the 3b+ and also older models. Issue is fixed. As I suspected a software issue, not the power supply. -Edit: os now installed is 9-Stretch 2.3 (march) – L.Gibbs Sep 26 '18 at 12:47

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