Electronics > Beginners

Need cheap USB power bank with "always on" instead of "auto off"

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Peabody:
I've looked at the little single-18650 $1-3 power banks (battery not included), but all seem to have a "feature" that I don't want.  If the load drops below some value, say 50mA, they shut down.  I assume that's because the boost converter would run down the battery if allowed to keep running.  But I would like to use one of these for very low current projects, such as Arduino or MSP430 stuff.  So I don't want it to turn off.  Instead, I want an On/Off switch on the battery's positive terminal.

I found an always-on power bank on Amazon, but it's $25.  Does anyone know of a product that costs more like $5?
Or is there another solution to this problem that I should look at?

If it matters, what I want to do is have a device that can be connected via USB to a computer for power and for firmware updates, just like an Arduino, but which can also be detached from the computer and powered through the same USB connector from a USB power bank.  But I need that power to stay on, even during sleep.

sokoloff:
If you're willing to burn a 1/4 Watt anytime the switch is on, you could just wire a 100 ohm resistor across the USB +5V and GND pins.

kripton2035:
what about this kind of device ?
https://www.aliexpress.com/item/32870411748.html

darrellg:
Here's one with a 20 mA cutoff: https://www.sparkfun.com/products/15204.

Peabody:
I've found one that may work.  Julian Ilett did a video on it five years ago:



and that Ebay listing is no longer up.  But this looks like the same package:

https://www.ebay.com/itm/Aluminum-DIY-USB-Power-Bank-Case-Only-18650-Battery-Charger-2600mAh-Cover-Kit/361693405448

Of course in five years the innards may have been completely changed out, but if not, the video indicates that there's no auto-off, and it has an On/Off button.


--- Quote from: kripton2035 on November 15, 2019, 06:12:29 pm ---what about this kind of device ?
https://www.aliexpress.com/item/32870411748.html


--- End quote ---

I think that one is good in not having an auto-off, but the switch doesn't switch off the battery.  It just turns off power to the output USB connector.  And it has no proper "load sharing" circuit to allow charging while powering the load.  There is some chance the first device does have that, but there's no way to tell without testing it.

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