I was recently handed a malfunctioning DJI Phantom Quadcopter that anytime the owner tried to lift off, it would just flip backward and crash. So, I went about recalibrating, and figuring out how to actually fly a Quadcopter. Anyway, I figured it out ((The “front” of the Phantom is determined by which end you point “down” during the compass calibration. According to their rather lackluster drawings, the “front” is the battery door side)). Anyway, I flew the unit around for a bit, then when the battery got low, the unit landed and showed me it’s wonderful flashing red light.
So, I brought it in, and plugged in the dead battery. As “luck” would have it, though, the place the unit was ordered from sent a free extra battery (RC-2220C3S) made by some other company (WATSON). So, I figured I’d try that. I checked the unit with a multimeter based on lead coloring, and saw it was at 11.2V, which is fine. So, I plugged the battery into the Quad, and was immediately greeted with flames, and a huge amount of “blue” smoke. I quickly unplugged the battery to figure out what the heck went wrong. An XT60 connector is polarized, so there’s no way to plug in backward. But, if you’re eagle-eye’d, you’ll notice the picture for this post, and something I didn’t even think about prior to plugging in the battery. The pos/neg (red/black) are wired backward! So basically 11.1V and all but unlimited amps (Lithium Poly battery) went up the ground plane, and anything that didn’t have polarity protection was fried (or at least the first thing to short was fried).
I know the reseller will replace the unit, so I don’t want to pull it apart to check when went up in flames. But after thinking about it, I’m guessing the motor controllers have some tantalum caps for decoupling. Tantalum caps are rather vigorous in their death when you wire them reversed. So those probably failed early, and may have saved other things. I did attempt to hook the good battery back up after that, and noticed the motors twitch, and the digital side of things fire up, which makes sense if the motor controller ICs survived, as well as the digital section of things (pretty much ALL voltage regulators have reverse polarity protection). I wish I could pull the unit apart to see what failed, but I don’t want to risk not getting it replaced.
Anyway, the moral of the story is: Don’t trust someone else to wire your RC batteries correctly. Check them. Make sure things look right before you plug them in. If you have a multimeter, make sure you get a positive voltage when you check the battery based on the connector orientation, NOT based on the wiring. That was my mistake (I just assumed the red was wired to the positive terminal).
UPDATE: Called the vendor, and about 2 hours prior they had gotten a call from someone complaining of the exact same issue. So, WATSON apparently let a bad batch through whatever they call QA. Scary. =/
