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The Demon Razor that Wouldn’t Turn Off

What do you do when a battery-powered appliance won’t turn off? And when it’s a sealed unit, so removing the batteries is impossible? And when its body starts to grow disturbingly warm? That’s the situation I found myself in a few days ago.

 
Riddles in the Dark

I was working at home one night, and gradually became aware of a strange buzzing sound. Initially I thought the sound was outside, but when I went to investigate, I discovered it was coming from the bathroom. My skull shaver, plugged in and recharging, had mysteriously turned itself on and the blades were spinning away. Pressing the on/off button had no effect. Unplugging the charging cable had no effect. The body is a single piece of molded plastic, so there was no non-destructive way of opening it. Nothing could stop the whirrrrrrrrrr of the blades, and the shaver was noticeably warm.

I started to panic that the razor would explode. The internal battery is likely lithium polymer, and from my days with RC cars and aircraft I know that defective or damaged LiPos can fail catastrophically. Like literally go boom and eject flaming molten goo everywhere that burns down your house.

I quickly took the razor outside, and set it on the concrete patio, blades whirring this whole time. A couple of minutes later, I began to fear that it was still too close to the house if it exploded, so I moved it to the street. Thankfully it didn’t explode, and those blades kept whirring for 90 minutes, during which two people stopped to ask what the horrible noise was.

 
A Tale of Two Chargers

So what caused the skull shaver to go crazy? Bad charging. Besides this manly pink skull shaver, I also own a more conventional Norelco cordless shaver. I’d never noticed it before, but the chargers for the two shavers have the same plug at the end of their cords:

A quick check confirmed that yes, I’d accidentally plugged the skull shaver into the Norelco charger. Is that bad? You might think that the plug shape is standardized, and that all charger plugs with this shape are designed for the same voltage. Let’s check. Here’s the skull shaver charger, which is nicely labeled. 5V output, max 1000 mA:

And here’s the Norelco charger. Instead of a label, its specs are molded into the charger body using impossible to read tiny-sized black-on-black lettering. Yuck.

But if it’s tilted at just the right angle to the light, and you get your reading glasses, here’s what emerges:

15 volts! Ouch! I charged a 5 volt device with a 15 volt charger.

I’m suddenly nostalgic for the days when real on/off switches physically disconnected the power. Many of today’s electronic appliances have a soft on/off switch that’s really just an input to some controller circuitry. When soft switches work, they’re great. But when something goes wrong with the control circuit you suddenly have a zombie appliance that can’t be shut off. In the case of this razor, the 15 volts apparently killed the control circuitry before the LiPo battery could be damaged to the point of explosion by over-charging. And the failure mode of the control circuitry was to fail ON.

Have you ever made a similar charging mistake, or exploded a battery through mistreatment? Leave a comment below and tell your story!

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