@atexplosion
I'm an electrical engineer, and I recently spent a day debugging all the boards and wires in my Nespi case because of the undervoltage alarm. I also tried multiple cables, power supplies, and other brands of cases, with and without fans. My conclusion ended up being that all of my USB cables were inferior, and I will explain why.
For starters, Ohm's Law states that DC electric potential is equal to the product of current and resistance in a circuit (V = I*R).
All conductors (wires) have an associated resistance (R, measured in ohms [Ω]). At low current (I, measured in amperes [A]), the effect of wire resistance is negligible. At higher current values, that effect becomes significant, causing a "voltage drop" (V, measured in volts [V]). A Raspberry Pi running Retropie has a pretty high current load compared to generic USB devices (like an optical mouse, or a game controller).
For a given cross-sectional area, a copper wire has an intrinsic resistivity (that is, resistance per unit length, or Ω/m [ohms per meter]). Depending on the given USB cable, the power conductors typically range from 28 AWG to 24 AWG (0.08 to 0.25 square millimeters cross-sectional area), with the largest available (that I know of) being 19-AWG (0.653 square millimeters).
The resistances of these three wire gauges at 10 ft are the following:
28 AWG @ 10 ft -- 0.649 Ω
24 AWG @ 10 ft -- 0.257 Ω
19 AWG @ 10 ft -- 0.081 Ω
Assuming an average current of 0.800 A (800 mA) for a medium-usage Raspberry Pi 3B+, using Ohm's Law, that would yield the following voltage drops:
28 AWG @ 0.8 A, 10 ft -- 0.52 V
24 AWG @ 0.8 A, 10 ft -- 0.21 V
19 AWG @ 0.8 A, 10 ft -- 0.07 V
For a 5-volt power supply, that would mean the voltage at your Raspberry Pi's micro USB connector would be the following:
28 AWG @ 0.8 A, 10 ft -- 4.48 V
24 AWG @ 0.8 A, 10 ft -- 4.79 V
19 AWG @ 0.8 A, 10 ft -- 4.93 V
The shorter your cable, the effects become lessened:
28 AWG @ 0.8 A, 5 ft -- 4.74 V
24 AWG @ 0.8 A, 5 ft -- 4.90 V
19 AWG @ 0.8 A, 5 ft -- 4.97 V
28 AWG @ 0.8 A, 2.5 ft -- 4.87 V
24 AWG @ 0.8 A, 2.5 ft -- 4.95 V
19 AWG @ 0.8 A, 2.5 ft -- 4.98 V
Increasing the size of your power supply (higher current output) really won't get around these effects. Maybe going to a higher voltage would compensate for the drop, but your current changes constantly, causing the voltage drop to change constantly. You'd be risking damaging your Pi. The best solution is using the shortest standard cable available, or buying a cable with larger conductors if you need a longer length (the company Volutz offers the largest that I could find).
The PCB boards in the Nespi case do slightly exacerbate the issue, but the biggest culprit is the main power cable. Once it's of sufficient length and/or gauge, the effects of the Nespi boards are negligible. I even had undervoltage faults using no case at all when I had a long power cable with thin wires.
So, use a really short cable if you can, and you should have no issues, even using a Samsung phone charger.
Cheers,
Than MacKenzie
