The Philips OneBlade razor is great, but the charger that comes with it is annoying to carry when traveling, since it is relatively bulky and the prongs do not retract. Since the AC adapter on the one I have (QP2520) is rated 4.3v at 70mA, it was within the range of USB. For what it is worth, when I measure the output of the AC adapter directly, it shows 6v with no load, despite being marked 4.3v, so they are likely counting on some dropoff under load.
NOTE: Apparently there are other versions of the OneBlade that use a different voltage. I have no idea for those, so check the AC adapter that came with your OneBlade to make sure it is within 5v USB range.
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Step 1: Snip and Solder
We snipped the connector off about 6 inches from the end of the Philips AC adapter, stripped the wires, and soldered them to the stripped wires of a charge-only (2 wire) standard USB-A connector cut off of an old USB cable. Make sure to not reverse power and ground, so use a multimeter to check the polarity of the AC adapter and the stripped USB cable before you start.
You need to heat shrink the individual wires and then the entire assembly, so don't forget to put all three pieces of heat shrink on before you solder the wires together.
Step 2: Double Check the Polarity of the Assembled Cable!
Make sure the polarity is the same when you connect it to USB, before plugging it into the razor! Note that the connector only fits into the razor one way, so make sure you have the positive and negative on the correct sides. If they are reversed. it will probably destroy your razor.
Step 3: Working!
When connected, the USB power meter shows a little over 5v, at 0.08A, or about 80mA, which is similar to the rating on the AC adapter that came with it. It is drawing more than the 70mA you might expect since the USB power meter draws a bit of power as well. Note the voltage is also a bit higher than the 4.3v AC adapter rating. I've charged the rasor a couple of times with the cable, and have not noticed any ill effects, but your mileage may vary - this mod certainly voids your warranty, and is at your own risk.
If I did it again, I'd make the overall cable shorter so it would pack even more easily, but this is much better for travel than the original AC adapter!
Thanks to my 9yo son Cory, who did this project with me.