Part of the Project Logs series.

I’m fully invested in the Ego line of power tools for everything from my lawnmower to hedge trimmer. I also have a need for a portable, long-lasting, rechargeable, power-outage-resistant, 12 volt power supply.¹ Ego sells several inverters/converters, but they all output 110 VAC and/or 5 VDC² USB; none of them have a 12 volt accessory power outlet.

So I set out to build an Ego-battery-powered 12 volt power supply. I was inspired by this Instructable project, but that design has some issues I wanted to rectify:

• There’s no fuse anywhere to protect against short-circuiting the battery!
• It’ll happily draw from the battery forever instead of shutting off when the battery voltage drops too far. As I understand it, it’s up to each Ego tool to monitor the battery’s voltage and shut off when the battery gets too low.
• In addition to the 12v accessory outlet and USB outputs, I planned to add a 5.5x2.1 output jack.

This is not a complete guide, and I don’t have progress photos to share, but I wanted to blog something about this project.

I started by putting together the electronics. The most complex part by far was putting together this battery monitoring circuit on a little protoboard. This circuit monitors the Ego battery’s voltage and uses a relay to cut off power to the 12v converter when the battery’s voltage drops below 43v:

The battery monitoring circuit itself is powered by this step-down converter, which allows efficiently stepping the Ego battery’s 56v down to 5v.

After verifying the electronics worked, I moved on to cutting and drilling the required holes in an Ego battery charger enclosure. Working with plastic with a Dremel tool is annoying, slow, smelly, imprecise work, but in this case it was good enough. Reusing an Ego charger enclosure made much more sense than 3D-printing something custom.

With the enclosure prepared, I started putting everything in place. The various parts are secured in place with a combination of nuts & screws, hot glue, and PETG from a 3D printing pen. I added still more hot glue and Kapton tape where I felt it might be helpful to prevent electrical faults in case something (like the heavy 12v converter) comes loose.

Here’s what the inside looked like before I added a lot more glue and put the rear cover on:

It’s definitely not perfect or elegant, but it works and I’m pretty confident it’ll last a long time.

The finished product: