USB Electronic Load to prevent power-bank low-current shut-down

USB battery packs are a convenient way of powering low power USB devices such as WSPRlite. Unfortunately many USB battery packs switch off (go into a sleep mode) if the current being drawn is too low. This means that your device works for a few minutes and then stops. Often just a few extra milliamps current draw will keep the battery pack alive. There are various circuits on the web that give a pulsed load which works with some battery packs - but unfortunately our tests have shown that such circuits don't work reliably with all battery packs.

Our solution is a variable electronic load. We have designed it to be very simple to use. Just plug it into your battery pack and increase the current until battery pack stays on reliably (6 - 150 mA). Depending on the current that your powered device takes that may be a little as a few milliamps. An on-board LED indicates if the battery pack is live. The load supports a standard USB data connection. USB plug and socket for easy in-line connection. Clear heat-shrink sleeving is provided to protect the unit in the field.

Available as an easy-to-make kit suitable for a beginner or fully built and tested. Note that the built unit is supplied encapsulated in clear heat-shrink sleeving.

Easy to use

To use, just plug the Keep-alive load between your battery and item to be powered and adjust the blue potentiometer on the unit with a fine screwdriver. Set it fully clockwise at first (maximum current) and gradually decrease the current until the battery pack just cuts out. Then increase the current until the battery pack stays on reliably. That way you only draw just enough current for reliable operation - increasing the life of your battery. 

Instructions are available HERE!

Also available from our international agent(s):

Frequently asked questions

Why not use a pulsed design?

We initially tried a pulsed design, but found that it did not work reliably with all battery packs. We believe this is because some battery packs measure their output current at infrequent intervals. This is fine if the measurements are guaranteed to occur during the pulses often enough to keep the battery pack turned on. But depending on the frequencies of the measurements and load pulses, the measurement times may eventually line up with the low current times for long enough that the battery pack turns off, at an unpredictable time after powering up that depends on the difference between the frequencies, the duty cycle of the load, the turn-off delay of the battery pack, and the initial offset between measurement times and load pulse times. This means that even if a battery pack initially appears to work fine with a pulsed load, it may shut down at some random time later, and it is difficult to determine in advance whether that will happen until it suddenly does.

Since the inner workings of battery packs can vary widely between different models and are usually not well documented, the only way to be sure that a USB keep-alive load will work correctly with all battery packs is to use a constant current load. This wastes more power than a pulsed load, but is more reliable.