Boom, Smoke, Help

Hi Folks,

looking for some advice on why/how I just fried my Pixhawk 2.4.8.

I’ve built a boat as a Windsurfing Marker see it here and everything was going brilliantly. The buoy was behaving itself and loitering well. I had it powered from an 18volt Makita battery from one of my drills.

But that battery wasn’t going to cut it for the buoy to stay on station for 5 hours - it was dead flat in 20 minutes.

But that’s OK I thought - I just grabbed and 75amp hour 12volt AGM deep cycle battery and hooked that up. But as soon as I flicked the on switch I fried the pixhawk and all the special blue smoke escaped. I didn’t even have the motors attached at the time.

I have a 35amp BEC - that seems to have survived.

What I don’t understand is why it fried the pixhawk. The BEC must have let through waaaay too much power on startup. But why?

Clearly electrons are not my strong suit.

Can anyone help illuminate this electrically challenged bot builder please as I have a new pixhawk module and would really like to not fry this one?

check the polarity of the connection that you have not reversed it.

A few more details and even a pic or two would help.
I am currently writing up on Ideal Diodes because I had a similar event.
So your info will help.
For me it was the BEC’s that exploded.

Well Go Figure

I almost added in my post that I had double checked all the new wiring for the new battery as I added an extra switch. Good thing I didn’t as your reply made me think - OK, check it again…

Ta Da - I managed to reverse the polarity. As GhostBusters said - “is crossing the streams dangerous?”. Yes, yes it is. I feel so stoooopid.

Thanks for the poke - clearly I needed it.

Turns out I have fried the BEC as well - new one on the way.

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Whilst it is good having found actual cause of your problem, another thing to watch out for is the basic current calculation that needs to be checked when dropping supply voltage like you did:
P=UxI (Power=Voltage times Current)
So in order check how much current will be drawn you change formula to: Current=Power divided by Voltage.
So, using an example with your different batteries the values look like:
Using the original 18V battery and just assuming your motor and equipment uses 1000W

  • 1000W divided by 18V = 55.55 Amps
    However, when dropping down to the 12V the whole thing changes
  • 1000W divided by 12V = 83.33 Amps

…just mentioning it as so many others fallen for this sort of “trap” and blown up equipment after dropping supply voltage. - Just imagine you had two motors with each ESC rated at 30A. So with first battery (18V) everything is fine. But when using the 12V battery it won’t take long before the ESC’s would give off smoke signals.

@Karl_Schoelpple while thats technically true for a fixed thrust output. the lower voltage will generally spin the motor slower, causing it to draw less amps at full throttle.

Good to know - thanks for the heads up - and now I have learnt a new formulae :-).

My motors are only small

  • Voltage: 12-24V
  • Current: 20A
  • Power: 30-200W
  • Motor KV: 1000KV

So at max power, I believe it was 200 / 18 = 11.1amps
Now 200 / 12 = 16 amps

My ESCs are rated to 30amps so I should be fine.
My craft will also be sitting spending most of it’s time loitering quietly in place and I can limit the power if it does become an issue

you should be fine on 12v, the only thing is your turning gains might have to changed a little due to the motors lower thrust but its probably close enough not to matter.

hahaha - you’re assuming I’ve tuned them to begin with :slight_smile:

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That is correct.
But the point I’m trying to make is that the whole current situation changes when the supply voltage changes.
This doesn’t just affect motors and ESC’s but also wiring. Further to that there is the risk of additional strain on motors with low startup voltage, especially in applications where high torque is required.
Just trying to provide some “food for thought” without going into every little detail.

If it was a wheeled rover it would be more of an issue but you dont need a lot of torque to turn a propeller at low speed.