DIY USB Emergency Power Unit

I needed a USB power bank for my Beatstep Pro and USB powered synthesizers that can run without needing to be charged.

Just plug in a new set of batteries when needed.

Not the kind that uses AA batteries and work for an hour but something that can charge mobile phones and run for up to 10 hours.

Not finding any on the market I set out to build one myself.

Energizer states that their D-sized Max battery holds 20000mAh when full so I based my power source on that.

And a battery holder for easy replacement of the batteries.

Since I needed something that could deliver 1000mA I used 2 cheap chinese 5V boost converters. They deliver 500mA each with a 3V supply.

Using 2 of them means I have 2 USB jacks suppling 500mA each.

Looking at the schematics you can actually parallel them for 1A since you have schottky diodes at the 5V outputs.

On with the build.

I glued the 2 boost converters to the battery holder and added a power switch.

Next solder the battery leads in parallel to the boost converters.

You dont really need the power switch if you desolder the LED’s on the converters. Without the LED the standby current draw is about 0.6mA for each converter.

I 3D-printed a nice case for it looking like a vintage car battery.
Not really necessary but adds a nice touch to it.

Testing it trying to charge a Huawei P20 mobile phone showed it pulling 570mA from 3V and indeed charging the phone.

Why didnt I just use one D-battery and one converter since the datasheet for it states it can run from 0.9-5V?

At 0.9V it doesnt even start and at 1.5v the output only delivers 80mA so at least 3V is needed for the output to deliver 500mA.

A very nice build and easy even for a beginner  as a first project.

And the best of all, even with a power outage or around the campfire, you can have 5V for charging or playing your synthesizer.

A tip, 100uF smoothing capacitor is quit small for pulling 500mA off the USB output. If you add a 4500uF cap on the output the performance will be much better.

STL-files for printing the battery box are available here:
Download STL files

PDF-files for printing the the vintage labels are available here:
Download labels PDF

(If you want to build this yourself a kit with the parts and the 3D-printed case are available at

(more DIY available at