This guide will help you assemble a portable charging solution capable of charging your OneWheel with an 18V power tool battery. I chose an 18V battery as it suits the input voltage range of the Car Charger supplied by Future Motion, which we will use as the DC to DC charging adaptor.

Please note newer boards including the Pint do not support Charge and Ride. This solution is only intended to charge your board during rest stops.

Disclaimer:
I am a hobbyist with some degree of knowledge in basic electronics. Although confident this DIY build is safe when followed correctly, I am in no way a professional and do not accept any liability in damages you may incur to your self or equipment. Note that some basic soldering skills will be required. Proceed with caution.

Supplies

Below is the list of components I used in making this build. I live in Australia, so you may need to find equivalent products available in your region. I chose Ozito as their batteries and chargers are relatively inexpensive and reliable. If you decide to go with another battery manufacturer, ensure it has an integrated BMS for under voltage protection to prevent over discharge.

1x FM Car Charger ($160 AUD)
2x 18V 5.2Ah Ozito batteries ($198 AUD)
2x 18V Ozito battery chargers ($38 AUD)*
1x DC Car Cigarette Lighter Socket ($3.95 AUD)
Total = $400 AUD ($275 USD)

The Pint has a 148 Wh internal battery, and the FM car charger outputs at an estimated 84% efficiency, which is why I have chosen 2x 5.2Ah batteries. This should bring the Pint up to a full charge in 90 minutes.
If you're building this for an XR, you will require 21Ah of 18V batteries to meet a full charge, which depending on your budget may not be as practical.

*** You may substitute one of the battery chargers for the USB Power Station if you decide to use the alternative method.

Tools Required:
Soldering iron + solder
Heatshrink
Wire strippers
Torx screwdriver set
Lighter
Multimeter (optional)

Step 1: Method 1: Power Tool Battery Charger

The reason I specified purchasing 2x power tool battery chargers is that we will be modifying one to act as a battery interface for our car charger. The other is to be left intact to charge the 18V batteries when not in use as it was intended.

*** I have since built another design which utilises the USB Power Station instead of the battery charger. If you are using this method you may skip to the next step. ***

Begin by disassembling one of your 18V battery chargers. I had to use a Torx screwdriver set on my Ozito to remove 4 screws underneath. We will be gutting the charger, removing the inverter and re-attaching the power lead wires directly to the battery connector contacts.

Once the housing is apart you will want to snip the wires as close to the PCB as possible. Two of these wires are the positive and negative DC inputs (Red and Black). The other two are the AC output wires (Brown and Blue) connected to the extruding power lead. I had an additional comm wire (white) which is responsible for the charger communicating with the battery. We won't be needing this wire, so you may remove it completely.

You can then proceed to completely remove the PCB from the housing by removing the screws holding it in place and discard it. Strip the ends of the 4 wires and solder the RED wire to the BROWN wire and then the BLACK wire to the BLUE wire. Remember to place your heatshrink over the wires before soldering them together. We now have a direct connection from the battery contacts to the extruding power lead. DO NOT PLUG IT INTO A POWER SOURCE.

Cut the AC plug off the end of the power lead and strip back the insulation. Disassemble your cigarette lighter socket by unscrewing the two ends. You will want to solder the BROWN wire to the positive (centre) contact tab and the BLUE wire to the negative (outer shell) contact tab. As you can see in the picture provided, I tied a loose knot towards the end of the power lead before doing so. This is to take any potential strain off the solder joints, in the event the cord is pulled aggressively when the socket is re-assembled. You can now screw the cigarette lighter back together, and re-assemble the power tool 'charger' housing.

Step 2: Method 2: USB Power Station

Rather than gutting and modifying a battery charger, it is possible to utilise the Ozito USB Power Station instead. This is my new preferred method, as it's more compact and still retains the USB functionality.

Simply dismantle the housing by removing the 4 Torx screws and drill a hole through the side of the cover, wide enough to pass the cigarette lighter power cord through. Then solder the positive and negative wires onto the PCB according to my illustrated diagram, taking care to match the correct polarity. Once soldered I decided to cover the solder joints with silicone just as a preventative measure to avoid any potential shorts.

Step 3: Testing

Before we connect the Onewheel Car Charger, you will want to test the voltage coming from our modified power tool battery interface. Slide on your 18V battery and turn your multimeter onto DC voltage. Place the positive probe against the rear centre of the cigarette lighter socket, and the negative probe against the outer shell. You should get a reading of 18V.

Once this checks out okay connect your Onewheel car charger, leaving it initially disconnected from your skateboard. The light should turn green. If this appears to be working as expected, you can now connect the car charger to your Onewheel. It should begin charging!

You may want to purchase a cover to place over the cigarette lighter socket to prevent an accidental short when carrying the charger around (see image).