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Calculating how long a device will run (power consumed)
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Calculating how long a device will run (power consumed)

by stewkingjr on Fri Jan 10, 2020 10:34 am

Hi,
Looking at the 2.2 PiTFT data sheet and i find these relevant details:

(Panel+LSI) Power Consumption (VCC 2.8 typ)
Black Mode - 18.10 - m W
8 color Mode - 8.16 - m W
Sleeping Mode - 0.035 - m W

Backlight Ta=25℃ (typical values)
Forward Current IF 15 mA For each LED
Forward Voltage VF 3.2 V (4 LEDs in parallel)
Power Consumption WBL 192 mW (4 LEDs in parallel)

And this Adafruit power bank:
USB Li-Ion Power Bank with 2 x 5V Outputs @ 2.1A - 5000mAh

My question: How do i calculate approximately how long the power bank will power the display (yes, ignoring any other power drain, like a Pi, for example, ignoring inefficiency in the process, and not worrying about running to empty)?

Can i simply do this to get the approximate answer in hours?

5000mAh / ((18.10 mW / 2.8V) + (192mW / 3.2V)) = hours? (Doing this i get about 66 hours.)

Thanks in advance.

stewkingjr
 
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Joined: Wed Oct 02, 2019 9:12 am

Re: Calculating how long a device will run (power consumed)

by adafruit_support_bill on Sat Jan 11, 2020 9:55 am

Assuming the input numbers are right, that should give you an approximate run-time.

Keep in mind that power-bank rated capacity is often stated as just the battery capacity. Bu the battery is just the input to the boost converter, so the actual usable 5v output capacity is typically less.

The laws of physics say that any increase in voltage requires a proportional decrease in current. And the boost conversion process is never 100% efficient. Boosting a 3.7v LiPo to 5v typically incurs about a 1/3 drop in mAh. So if your Power Bank has a 5000 mAh 3.7v LiPo, you can expect about 3333mAh at 5v.

https://learn.adafruit.com/energy-budgets

adafruit_support_bill
 
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Re: Calculating how long a device will run (power consumed)

by stewkingjr on Sat Jan 11, 2020 10:58 am

Hi,
Thank you for the answer and thanks for the link too. Yea, i was only looking approximately to see whether adding the display to a headless system would require an inconvenient recharge time.

stewkingjr
 
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Joined: Wed Oct 02, 2019 9:12 am

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