Cordless Tool Battery to USB Charger




There are plenty of high capacity battery chargers available to buy and they are certainly a good idea. You don't want to be caught with a dead phone during a power outage. However rather than buy one of these devices why not take advantage of the high capacity batteries that most of us have in our garages already? The battery packs for my Ryobi brand cordless tools are 18V devices. With a few inexpensive parts we can easily turn that into the 5V needed by your cell phone.


1/4 inch plywood

wood glue

1 UBEC (universal battery eliminator circuit) DC/DC step-down converter - 5V@3A outut (available from AdaFruit Industries

1 USB shell connector, type A socket

1 22k Ohm resistor

1 20k Ohm resistor

1 6.2k Ohm resistor

2 picture hanger hooks


perf board

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Step 1: Build the Enclosure

Carefully measure your battery and cut the plywood to the correct dimensions to fit over the battery connector clip. Using a razor blade, cut out slots for the battery's spring clips to fit into. Assemble your enclosure using some wood glue.

Step 2: Make Battery Contacts

I had some brass hooks for hanging pictures in my toolbox already. They can be bent and trimmed down to make good battery contacts for our enclosure. We want to make the contacts have some spring so they make good contact and allow for a snug fit onto the battery.

Solder the UBEC onto your contacts paying careful attention to the polarity. Make sure the negative side of the UBEC (black wire) goes to the negative terminal of the battery pack and that the positive side (red wire) goes to the positive terminal.

Cut and glue some pieces of plywood to secure the clips in the proper location.

Step 3: Build Voltage Divider

While the UBEC gives us the 5V needed to charge our USB devices, many devices want to see around 3V on the USB data lines in order to charge. To go from 5V down to 3V we need to make a simple voltage divider circuit.

Note that I made two separate voltage dividers (one for each data line) but this shouldn't be necessary.

Cut a hole in your enclosure for the USB connector and then solder the voltage divider to the connector and the UBEC.

Step 4: Finishing Touches

Add some holes for cooling (or just for looks). Throw some finish on the outside of the enclosure to help keep it clean from grubby fingers. Plug in your phone or other USB device and happy charging!

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42 Discussions


10 months ago

The Adafruit UBEC, used here on an 18V battery, is only rated up to 16V...
This device , for 2 dollars more, is rated for up to 24V and has TWO --> 2.1A sockets that you can use at the same time! (Total: 4.2A)

There are even better options, starting at around 13 dollars, like a trio of similar panel-mounted devices that will give you a normal dual USB socket with 5V 1A/2.1A (Total 3.1A), a 12V Car lighter socket*, and a Voltmeter to keep an eye on your battery level...

* Why a Car lighter socket?
-- LED lights often use 12 volts.
Also, there are many other accessories that are made for use inside a car.
-- Charge extra USB devices with a car charger plugged into it.

One final suggestion: If this is truly meant as an emergency solution, then you might want to start thinking about how to charge this power-tool battery after you've drained it with all your toys! --> Next project: Charge your 18V Power-Tool Battery from your Car! ;-)

1 year ago on Step 4

I hope to do this (with a helper) for before the next hurricane hits, but I need to order the parts first. What components need to change for 20c or 40v? I'll have access to 2 40 volt lithium batteries for an electric lawnmower. I think. I know it is 40v and I know it has 2 batteries. Whether it's 2 20v or 2 40v is unclear, as I haven't actually seen the batteries yet. .


Reply 2 years ago

Same way: recalculate resistors, or use stabiliser with heatsink.


3 years ago

Thank you for this great instructable. I have done some electronics tinkering, but this will be the first "utility" electronics project that I will be using to supply power to expensive things like tablets and phones. For that reason, I want to ask, what UBEC provides the most consistent 5v voltage with the lowest tolerance? And also, how risky is it to supply 5.3v or 5.4v to an ipad or expensive tablet? Can you lower the battery capacity by doing this? Thank you for your help.


3 years ago

how would i do this with a 20v dewalt battery?


4 years ago on Introduction

The website I am looking at offers 22k 20k and 6.2k resistors at 1, 1/2, 1/4, and 1/8 watts each, I'm not sure if the factions are what I need, would it be simple 1 watt resistors all around?

Jeph Diel

4 years ago

if you had a 12 volt battery you could simply take the guts out of a cigarette lighter to usb adapter.

wavey davey

5 years ago

this is excellent and has spured me on to have a go at one. my drill is 24 volt so i was thinking of getting a 24v truck to 12 car converter and running it to a lighter socket then i could run all the leads i use in my van onsite. i cant see why this wouldnt work ?

1 reply

The more voltage conversions you do the less effect you'll end up with. In other words - you loose capacity.


Reply 5 years ago on Introduction

Linear regulators like LM7505 are very inefficient. If we are using LM7505 to step down 12v to 5v then out of 12watt taken from battery only 5watt will be used by the phone to charge and the rest of the 7watt is lost as heat in LM7505. So use a DC-DC converter to stepdown voltage.


5 years ago

I was able to reuse a broken charger for my black and decker


5 years ago on Introduction

This looks well thought out. I don't know what the amp-hour rating on my Ryobi portable batteries, but I'm certain it would keep a phone charged through the 72 hour "you're on your own" post disaster period.

As an alternative to the Adafruit BEC, I've soldered up a RECOM switching regulator. ( ) It's rated for 500mA, but I've tested them up to 1500mA with almost no voltage drop. The only indication is that they get slightly warm. (Probably reduces the MTBF though.)

Good information about the 3V data lines. I've had several items that needed their own proprietary charger. I bet that was why. (As an experiment, I've added a 3.3V module to see if that will work.)

I'll post the results.


5 years ago on Introduction

I made something similar. My project was for a portable adjustable DC power supply. I used one of these to lower the voltage, and then I sacrificed a Ryobi One flashlight as the enclosure (it gave me a nice on-off switch). I never tried to use it as a USB charger, but that was on my todo list with it. Good to know about the 3v trick.


5 years ago on Introduction

Instead of a resistor divider, or even a linear regulator, use a car
cigarette lighter charging adaptor. They typically have a wide input
voltage range and are much more efficient.

All in all, a very handy addition to anyone's tool box, especially for long hours on the road!


5 years ago on Introduction

Great idea to use the picture frame hooks, I already have an "every-possible-input-to-USB" converter, but I might use the frame hooks to tap into my drill batteries. It's better than using an elastic band to hold wires on the terminals.