Granddad_Mike wrote: ↑
Mon Feb 04, 2019 2:01 pm
The Apple PSU is outputting 5.4V with a maximum rating of 2.4A. The USB cable core is 0.5mm (17AWG),1M long and is made of aluminium. (aluminum in the former colonies). No Volt resistance is 0.8 ohm.
I have never seen a micro USB cable with 17 AWG power wires, so I'm dubious of that spec. Make and model of cable?
5.4V is too high for the Pi computers. Recommended range is 5V +/- 5% (4.75-5.25), with voltages slightly above 5V preferred.
Another problem is the micro USB connector on the cable, which won't be rated for 2+ amps either. In addition to the cable losses you will also have connector losses. Standard micro USB cables just aren't good for much more than 0.5A without significant voltage loss, and one with an aluminum core is probable a pretty crappy cable.
You need a good quality cable designed for higher current charging. The charging cables included with smart phones or tablets usually have fatter power wires and higher current rated connectors. Of course Apple products use proprietary connectors, so you may not have a good micro USB charging cable. Also, as you have already discovered, shorter is better.
And if all of that isn't discouraging enough, charging phone (or tablet) batteries does not require precise voltage control (most phones only need a little over 4.2V to charge). Because of that, chargers often have poor voltage regulation and will not maintain a stable 5V under load. I have a 5V/2.4A charger that drops well below 5V at only a 1A load. It charges my phone fine, but it's a lousy Pi PSU.
So to answer your question...
... given the numbers above, are the tolerances built into Pi 3 B+ too tight?
The simple answer is no, but you need to be using a proper power supply.
The more complex answer is, too tight for what?
The 3B+ is the most powerful Raspberry Pi computer available at this time, so it also consumes the most power. Which means you are more likely to run into power problems with that model. It's not a fault, it's just the reality that, all else being equal, a more powerful computer will use more power. The official Raspberry Pi Universal Power Supply
for the older Pi 3B still works fine on the newer 3B+ model.
The problem with using phone chargers, especially those with separate USB cables, is that you don't know if they'll work without a proper load test. Even if your 3B+ booted and seemed to run fine with your existing arrangement, you could run into problems later when you start doing something heavier on your Pi, or connect a power hungry HAT or USB HDD. That uncertainty makes the more experienced users here wary of them. The official PSU is a proven performer that is usually very affordable (depends on where you live and who you buy from).
Now here's the disclaimer part:
Your Pi may very well be faulty, but it's unlikely, as defective Pi computers are uncommon. It's more likely your charger and cable are the issue. The charger may be experiencing transient voltage drops with sudden load changes. These may be too brief to measure with a DVM, but the Pi's low voltage warning will still react to transients, and if the voltage dips enough, boot will fail.
Not all chargers have crap regulation. I have a couple of older RAVPower chargers that work great as Pi power supplies. Unfortunately that model is no longer in production. And even if they were, the official PSU is half the price I paid for those chargers (so I'd still recommend it).
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?