drgeoff
Posts: 8633
Joined: Wed Jan 25, 2012 6:39 pm

Re: Powering up Pi 3 using a power bank

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.

Jayce
Posts: 67
Joined: Sun Jan 29, 2017 5:53 am

Re: Powering up Pi 3 using a power bank

Mon Oct 09, 2017 9:19 am

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:
1.) Idle - 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.

W. H. Heydt
Posts: 9113
Joined: Fri Mar 09, 2012 7:36 pm
Location: Vallejo, CA (US)

Re: Powering up Pi 3 using a power bank

Mon Oct 09, 2017 2:26 pm

Jayce wrote:
Mon Oct 09, 2017 9:19 am
From a site I've found, a table regarding the Pi State and its power consumption:
1.) Idle - 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)
When quoting data like that, you need to specify which model of Pi it is. Without that datum, the power requirements don't mean anything. By comparison, I did tests on a Pi0 with a 3.5" LCD display. Just idling with a static image, the total power draw was in the 180mA to 200mA range, and half of that was the LCD.


User avatar
Imperf3kt
Posts: 1386
Joined: Tue Jun 20, 2017 12:16 am
Location: Australia

Re: Powering up Pi 3 using a power bank

Mon Oct 09, 2017 8:08 pm

You have the formula right but you put the wrong values in.

Your Pi will not use 2.1A under most conditions, so you are actually using far less.

Look on the battery bank and find it's watt hours. This is more accurate/truthful.
I have a 12,000 mAh battery which has 44.4Wh, meaning I can draw 1 watt of power from the device, for ~44 hours. I did a calculation a few months ago and assuming a total efficiency of 80%, I could power my Pi, a small monitor and a fan, for approximately 18 hours off a single charge.

I am yet to actually test this though as I don't have the time to keep an eye on a Pi for 18 hours.
Stop plugging your fan directly into the GPIO 5v
https://www.electronics-tutorials.ws/power/transient-suppression.html

Return to “Beginners”