Resolving the Low Voltage warning (aka the "lightning bolt" ⚡) on a Raspberry Pi.
Please note that this research was done on Pi 3 and lower. I don't own a Pi 4 which uses another USB spesification and different power requirements.
This is a document that was produced from a mess of personal notes in order to try to create a resource that might be useful for someone that runs into the same issues as me when it comes to powering a Pi in different scenarios.
The Low Voltage detection warning that a Pi emits surely is annoying, but should not be dismissed or disabled. It's crucial that the Pi gets the juice it needs to operate normally. Not supplying enough power impacts the performance heavily (because the CPU will run at a lower speed), and in the worst case lead to instabilities (which in turn can corrupt your storage).
From my testing the 0.7" touch screen seems to be drawing ~0.5A in normal operation.
🚀 If possible power the LCD from a dedicated custom plug that goes to a 5V source (instead of via GPIO) to avoid drawing the power from your Pi directly.
Avoid connecting anything that sucks a lot of juice directly to the Pi, like a hard-drive.
🚀 Go for a powered USB hub instead.
This is the most important part of your chain. A high quality power supply is crucial.
🚀 Unless you're deploying this into a place where you don't have access to AC wall power or a high-current rated device/battery USB port, get the official power supply
1.5A-2A or even more on the label would be enough to drive your Pi. But that's not entirely true. Those things are not designed to handle power spikes and maintain a fully stable voltage of >=5V under full load. These are designed for charging, not powering a computer.
From my testing the following ("fully expanded") setup seems to draw around 2A on maximum load (not synthetic).
- Pi 3B+
- 7" Touch Screen
- Serial USB Dongle
- USB SSD Storage
- Connection to USB Hub
- Ethernet connection
On idle it sips ~1A, and when power save kicks in ~0.5A.
Supplying 5.0V always seemed to trip the low voltage detection. Bumping it up to 5.10V-5.20V seems to put everything into equilibrium (this depends on your cables).
I don't have an official power supply so I don't really know the operational specs here with that sort of equipment.
5.25V here as this will potentially damage the power supply on the Pi itself.
In my case I wanted to have a Pi deployable in an automotive setting without using a cheap cigarette adapter that could introduce all kinds of noise and other horrible stuff, so I bought the Mean Well SD-15A-05 and a USB type A female connector (soldered onto a thich gauge power wire) and Type A automotive connectors for making adapters (USB, Battery claws/clamps, cigarette plug, or even a permanent setup). [1]
Now this is a wild one.
Finding good quality cables with high enough gauge to handle these kinds of loads without dropping the voltage too much over medium distances (1-2m / 3-7ft) is not easy, so I just made my own to eliminate this variable [2].
Even some of my short (10-20cm / 4-8in) cables weren't able to handle this because I got them from some random online marketplace, while some of the stuff I had from more reliable sources (like mobile phone and battery bank manufacturers) worked just fine.
I highly recommend reading this article on this subject:
https://goughlui.com/2014/10/01/usb-cable-resistance-why-your-phonetablet-might-be-charging-slow/
Sadly there's no way to get the value from the Pi power supply as there's no circuitry available for the SoC to read any such metric.
Getting something like a "Keweisi KWS-V20" inline USB voltage/current/energy tester is a nice way to monitor the power over your USB connection [3]. This won't give you the measurements at the end of the chain, but give a good idea about what's going on.
50mV) increments with the Pi detached to read the supply voltage with no load. Don't use this as a part of a permanent setup. And always try to test without this device entirely after tuning to eliminate any introduced variables.
With a little bit of hacking, it's possible to bypass the USB to power your board while still going through the fuse.
Solder wires to PP1 (5V) and PP5 (GND) on the bottom side of the PCB and run to your supply.
