drgeoff wrote: ↑
Mon Oct 09, 2017 8:17 am
Jayce wrote: ↑
Mon Oct 09, 2017 7:31 am
Hence, the shorter the cable the better as the power can consistently flow faster through short cables than long.
Shorter cable drops less voltage. "consistently flow faster" is mumbo-jumbo.
The formula you mention is theoretically correct but in practice:
1. Your RPi is not drawing 2.1 amps. Possibly half or even less of that.
2. The capacity claimed for a power bank is almost always that of the cell which has a lower voltage than 5 volts. To get x amps at 5 volts the converter in the powerbank needs to take significantly more than x amps from the cell.
3. The cell in a powerbank degrades over time (even if not used) and loses capacity. A year or two later the capacity will be much less.
4. Marketing lies. Purchasers evaluate powerbanks mainly on price and capacity. The price is known but how can you know the actual capacity, even after purchase.
You're right regarding the less voltage from shorter cable as the length is proportional to resistance of a wire. I do not think I am wrong too, maybe just a wrong choice of words.
From a site I've found, a table regarding the Pi State and its power consumption:
- 260 mA (1.4W)
2.) ab -n 100 -c 10 (uncached)
- 480 mA (2.4W)
3.) 400% CPU load (stress --cpu 4)
- 730 mA (3.7W)
From there, (I chose 2 for neutral, as I just use Pi for presentations and lectures), so 10 / .48 = 20.83 hrs (21 hrs approx). Doesn't sound right.
Let's say 1.5A, 10 / 1.5 = 6.67 (7 hours approx.), sounds right to me.
Nevertheless, I just ask that for curiosity sake. Thank you for the explanation.