Powering arduino UNO+microservo off power bank

Question

Currently, I have a USB powerbank powering my arduino Uno which powers my micro servomotor. I would like to have the motor running for long periods of time (~24 hrs). Sometimes, I am able to achieve this run-time but in most cases the power bank shuts off after about 3-4 hrs of running. I am not quite sure about the variability. It seems that my powerbank has a current threshold (don't know exactly but probably 50-60 mA) and my arduino/servo motor are probably right at that cut-off.

What is the easiest way to handle this problem? Could I program my arduino to draw more current from the power bank (have it calculate pi to many decimal points?)? Or, would just buying a proper grade resistor do the job.

Thanks

Answer 1

A direct resistor connection will work, but will be very wasteful. Trying to calculate PI won't actually increase the load much. Instead, a simple intermittent pulsed load will work. A resistor and suitable N-channel transistor or mosfet, pulsed every n seconds would trick the power supply into thinking there is a large load, but also reduce the average load, hence longer battery life.

^{simulate this circuit – Schematic created using CircuitLab}

A 1 second pulse of 75mA every 10 seconds is only a 7.5mA draw over 10 seconds, compared to a constant 75mA load.

Adjust the period and load as needed.

Answer 2

Most "powerbanks" are designed to cut internal operations off when the current falls below certain threshold (50 mA - 100 mA). Having additional load to keep it running will waste the battery runtime.

The best way is to modify the powerbank circuit to remove this protection. Some cheap powerbanks have a single control circuit that apparently has the current shunt circuit built-in, and it is not possible to disable. Some powerbanks will have low-Ohm sensing resistors on-board, so increasing their values will lower the cut-off threshold, but it may have adverse effect on the overall current delivery. It might be possible to somehow fool the cut-off circuit applying some internal bias, or else, it depends on powerbank control circuitry.

FOR ILLUSTRATION: My Powerbank (Model CJ-6000) has a pretty sophisticated circuitry, I might start playing with it:

See also https://electronics.stackexchange.com/a/270342/117785

ADDENDUM: after some reflection (based on powerbank circuit shown above) it becomes clear that a simple change in current sensing resistor will not work well - while the cut-off threshold gets lower, the overcurrent will also go lower, which might be unacceptable. The workaroud would be to use a non-linear element like a Schottky diode, with something like 10 Ohms in parallel (to define new cut-off threshold). This circuit has something like 3.5A overcurrent threshold (which amounts to 350 mV threshold over 100 mOhm shunt), so the diode must have a pretty low forward voltage like this one to avoid the overcurrent flip.