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I want to use an 18650 battery pack to power a circuit with 5 volts at a low current. The 18650 battery pack has a USB output which provides 1 amp at 5 volts. I measure voltage drop across the left and right pins and it’s only .77 volts. When I plug in a device like a phone, it goes to 5 volts and a light turns on on the charger board.

How can I get power without talking to the charger board?

Ok actually I solved my own issue through dumb luck. I plugged it in and it turns out my clumsy self had shorted the usb shield to positive and it tripped protection until it was plugged into the charger. Now everything is working great @5 volts 300mA (I’m powering 4 hall sensors).

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    \$\begingroup\$ Either you've used the wrong pins of the USB connector, or perhaps your circuit draws such a tiny amount of power that an auto-off circuit in the powerbank doesn't realize it is there. Either way, enumeration is not something traditional power banks support, as they are not actually USB hosts. \$\endgroup\$ – Chris Stratton Aug 17 '18 at 1:22
  • \$\begingroup\$ Ok, I thought I needed to request the enumeration to get power going. I put a buzzer on and it’s still at .77 volts. I’ll try something more. \$\endgroup\$ – James Meas Aug 17 '18 at 1:24
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Power delivery from power packs doesn't need enumeration. As a matter of fact, none of standard power schemes do this, the entire power delivery or older battery charging protocols/handshakes don't rely on USB enumeration. Battery charging was intentionally specified outside the USB in-band signaling protocol.

However, many powerbanks have a design feature to shut itself down if the current drawn from it falls below 50-70mA.

Try to add an extra load (like 47 Ohms resistor), or find a powerbank that doesn't have the cutoff threshold. For more details see this discussion. There are references to models that don't have the shut-off.

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  • \$\begingroup\$ It seems to smart for me. I can’t get it to run even with a 10 ohm resistor. \$\endgroup\$ – James Meas Aug 17 '18 at 2:26
  • \$\begingroup\$ Also is it normal for the housing to have a negative voltage?? \$\endgroup\$ – James Meas Aug 17 '18 at 2:28
  • \$\begingroup\$ @JamesMeas, if no output with 10 Ohms, and housing has negative voltage, then you are connecting something REALLY wrong. What kind of "USB output" connector do you have? Is it Type-A receptacle, or something else? \$\endgroup\$ – Ale..chenski Aug 17 '18 at 2:35
  • \$\begingroup\$ It’s a type a female. I looked everywhere and I can’t find anything about it. Anyways, I found a dumb power supply (funnily enough also has negative voltage on the housing, go figure) and we are in business. \$\endgroup\$ – James Meas Aug 17 '18 at 2:47
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Its because the battery pack has its own internal circuit that transforms voltage from 18650s to 5V USB. To conserve power, it disables the transformer circuit when it detects no or very low current.

I had the same problem with powering my Arduino from a powerbank- the general idea was to "ping" the powerbank's rail by, for example, setting one of digital pins to HIGH.

Either way- add some small resistor for extra load, or somehow manage to short it every few seconds.

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