Using Mobile Power Banks to Power Electronic Circuits
Mobile Power Banks can be used as an excellent power source for simple electronic circuits and DIY projects. However, as they are designed primarily for charging mobile phones they possess certain inbuilt features which need to be suppressed.
Basically the power bank shuts down if the output current drops below a set threshold. To do this the internal circuitry continuously polls the output current over a period of 12-15 seconds and shuts down if the value is below a threshold.
A simple way to overcome this would be to have a bleeder resistor continuously connected across the power bank output. This is very inefficient and would cause unnecessary power drain.
Based on experiments carried out on two power banks and the references cited below I present a simple circuit which needs to be inserted between the power bank and our experimental circuits. This circuit generates a pulsed current load of 150 mA for approximately one second every 10 seconds. This satisfies the sensing requirements of the circuitry within the power bank while effectively reducing the power drain to one tenth as compared to that of an equivalent bleeder resistor.
1 TI application note: Port Detection for Power Banks
"To extend battery life and minimize power when a power bank is powered off during system idle, shipping, or storage, the power bank can turn off the discharge path of the battery so that the output voltage is zero to minimize the battery leakage current. Once the power bank is signaled to be charged or discharged, it wakes up the system from idle mode and re-enables the discharge path; several methods are implemented to detect the input and output attachment events."
2. Forum discussions: USB Battery Bank Prevent Shut Down
Based on NE555 Timer IC
The circuit is based on the popular NE555 timer IC which has been setup as an 'Astable Multivibrator'. Capacitor C2 is charges through R1+R2 and discharged through R2. The waveform at the capacitor is shown in blue in the oscilloscope trace. The square wave output of the NE555 at Pin 3 is fed to a PNP transistor to drive the 22 Ohm load (R4). The PNP transistor collector waveform is shown in red. A LED in parallel blinks whenever the PNP transistor is on and a pulsed current flows through R4.
I rigged up the circuit and checked it out on a bread board and tried out this scheme with two power banks.
The EVEREADY power bank needs to be switched on for it to start and has a dwell time of approximately 15 seconds before it switches off in case the output current is below the threshold value.
The ivoler power bank comes on automatically when the circuit is connected based on even a small current being drawn. The dwell time is approximately 12 seconds.
I found that the power banks remain ON at an approximate threshold current of 100 mA and have selected a safe pulse current of >150 mA. With a 1:10 duty cycle this is equivalent to a 15 mA continuous load.
My test circuit
I have used this scheme to power a PIC microcontroller which with associated components draws approximately 50 mA and the power banks were shutting down with this load after 12-15 seconds.
Other power banks
Other power banks may need minor changes in the circuit in terms of the current pulse magnitude, duration and repetition rate. The Ne555 based circuit has been chosen as it can easily be modified to cater to a large spread in values.