After getting the Pi 3B+ and 3.5" LCD display to work nicely, I ventured down the path of battery power, so the Pi could shake the shackles of the Plug Pack. The Pi and LCD running draws about 500mA.
- The Pi Foundation made a big mistake powering the Pi 3B+ via a Micro USB connector.
- Most Micro USB cables will cause too much voltage drop as the wires are too small.
- Buying a 5V supply with a higher current level will not solve the situation unless the supply is rated at some very high current level so the 5V does not drop too far.
- To adequately power the Pi via a battery source and at a reasonable current level (2.5A max) the voltage regulator needs to generate 5.3V so the voltage droop will not fall below the trigger level.
The decision to use a Micro USB connector leads people to think the Pi can be powered by any USB source using any Micro USB cable. Generally it will work; but the power supply problem Icon and associated pop up messages will appear causing people to think there is a problem with their USB source.
Of all the USB power sources I have; the only one that does not generate the power supply problem Icon is the “Official” Plug Pack. I decided to investigate why in more detail. The Plug Pack is stated as being 5.1V, as if 0.1V would make a great difference. Below are my findings.
Raspberry Pi 3+ Power Supply (Official)
Different Resistor values were connected to the Plug Pack power cable and the voltage measured. Current levels were calculated using the resistor value and the measured voltage. Placing a current meter in circuit altered the measurements too much to be accurate.
No load = 5.29V (this was a surprise, the spec says 5.1V)
0.519A = 5.19V (10ohm)
1.0A = 5.11V (4.7ohm)
2.2A = 4.90V (2.2ohm)
2.4A = 4.87V (1ohm + 1ohm)
Beyond 2.4A the voltage fell below the level that would trigger the Icon.
Cygnett 10,000mAH Power Bank
Same testing as above.
No load = 5.2V
0.5A = 4.98V (10ohm)
1.0A = 4.84V (4.7ohm)
2.1A = 4.69V (2.2ohm)
Micro USB Cable
Power Bank connected to Pi via Micro USB cable, voltage at Pi for 500mA = 4.43V, for 1A = 4.38V.
Various cables produced varying results, one cable was especially bad.
Generally the cables dropped 0.4V to 0.6V.
Power Bank connected via wires soldered to the Pi GPIO pins. The Icon appeared intermittently meaning the voltage must droop for a short period of time. Kind of backs up the thought that Power Banks are not really USB power supply’s.
A variable regulator board rated to 3A and set to 5.3V was used to power the Pi. The power cable was soldered to the GPIO pins of the Pi using wire similar in size to the Plug Pack cable.
The Icon no longer appears.
The battery pack for the regulator is 6 x NiMH AA cells. The Pi & LCD should last about 7 hours with this setup.
So the journey continues …
I post this here as I have read much on forums and such that does not give the full picture. Any portable supply for the Pi needs to have a higher voltage and enough current capacity. Most Power Banks are not suitable. Most Micro USB cables are not suitable as the wire size is too small. IMHO.