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In this project, I power cycle a RaspberryPi Zero over a 5V USB powerbank, please have a look at my previous question: Blocking current from Backup battery (3x1.5V=4.5V) when powerbank (5V) is turned on my timer switch

I use the TPl5110 as a timer switch and 2xAA as backup battery. My circuit looks as follows:

schematic drawing

  • The power bank is 10.000mAh, 5V, noname. EDIT: I've checked with the Varta consumer service and according to the datasheets of most powerbanks, they require a minimum current of 50mA -> this question is more universal than it initially looked.
  • The P mosfet is a NDP6020 with a low Vth of max 2V
  • The Diode is standard silicon 1N4001

According to the manufacturer, the powerbank needs a minimum of 50mA to be turned on/stay on.

I've set the potentiometer of the timer switch to 5min and I've verified it is working.

The problem is: after the first cycle, the RPI0 won't power up anymore since (for the 2nd and subsequent cycles) 0mA current (measured with multimeter) is drawn from the powerbank when the switch kicks in again. Clarification note, as requested: In the first power cycle, 130mA current are drawn (if I turn on the powerbank manually by pressing the on switch).

Can you see a flaw in this simple circuit that prevents current to be drawn from the powerbank on the power cycling events?

Please see the overall setup here: enter image description here

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  • \$\begingroup\$ Well, I don't get it. You said that you want to draw 100mA from the powerbank to turn it on ? But you can't draw any current from it if it's turned off... maybe I am missing something here. \$\endgroup\$
    – Blup1980
    Mar 11 '21 at 16:29
  • \$\begingroup\$ @SamGibson I've edited the respective part in my thread, most powerbanks need a minimum of 50mA as I've checked with the customer support of VARTA and from the datasheet of other powerbanks. For clarification: Once the switch is one, I want current to be drawn from the powerbank, I'll try to clarify it in my post. \$\endgroup\$ Mar 11 '21 at 16:33
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    \$\begingroup\$ The power banks I've got don't deliver power until you press the button. Press the button, and as long as the current stays above the minimum the power bank stays on. If the drawn current drops below the minimum then the power bank shuts off and you have to press the button again. This is why one project I am working on bypasses the 5V output and taps the cell in the power bank directly. \$\endgroup\$
    – JRE
    Mar 11 '21 at 16:39
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    \$\begingroup\$ In other words, measure the voltage at the power bank output. I expect you'll find it to be zero when the Pi fails to power up. \$\endgroup\$
    – JRE
    Mar 11 '21 at 16:40
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    \$\begingroup\$ You can also go on your favorite online retailer and buy a battery bank that takes 5 protected 18650 cells. Cheap and because it's cheap doesn't have the fancy low current shutoff feature. You have to buy the batteries separately, but you end up with a massive power bank at low cost. \$\endgroup\$
    – K H
    Mar 12 '21 at 7:05
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The power banks I've got don't deliver power until you press the button.

Press the button, and as long as the current stays above the minimum the power bank stays on. If the drawn current drops below the minimum then the power bank shuts off and you have to press the button again.

In other words, measure the voltage at the power bank output. I expect you'll find it to be zero when the Pi fails to power up. If the power bank isn't putting out any voltage, then you won't be able to draw any current from it.


This is why one project I am working on bypasses the 5V output and taps the cell in the power bank directly. To turn the power bank on again, I'd have to open the case and add something to "push the button" to get power on again. That means an extra battery (like you have,) modifications to the power bank, and wires coming out of the power bank.

I found it simpler to just go ahead and bypass the 5 volt output and use the lithium cell directly with my own external regulator.

Make sure you connect to the correct side of the low voltage protection circuit for the cell (in the power bank) if you go this route.

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  • \$\begingroup\$ All of my power banks are controlled by the button... \$\endgroup\$
    – Solar Mike
    Mar 11 '21 at 19:36
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The problem according to my experience is not the power draw, it is because of the output capacitor inside the powerbank that is more than capable of driving your circuit after initial load, power bank senses no current draw and turns off,

One way to solve your issue If you don't care about power loss is to add 50 Ohm 0.5Watt resistor on the output of power bank, parallel to your load (circuit)

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After talking with an application engineer from VARTA consumer electronics, I can conclude the following:

  • Power banks autostart after around 50mA are drawn (100Ohm resistor in parallel)
  • Powerbanks only discharge until around -5°C and can be charged around 3°C, so powerbanks might nto be ideal for me in this outdoor project.
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