Melted my battery pack, but unsure how

[SketchyDan]

Hi All,
I managed to melt my battery pack and not totally sure how, so was wondering if anyone could point me in the direction of what I might have done to cause it. I'm pretty new to the hardware side of things, so I'll explain the path I took to get to the pack melting, and you may spot other issues I created.

I have an L298n hooked up to a couple of TT DC motors, a pico powered from a 5v usb, and 4 AA duracell batteries powering the motor controller from a separate battery pack (The one that melted).
I had been playing around with this setup for a while with no problem so decided to add a couple of photo-interrupter sensors to try controlling the speeds of the motors based on some error correcting after following this guide - https://projects.raspberrypi.org/en/projects/robotPID/7

At this point I had tested the motors could spin both directions etc, and controlling the speed through the pwm pins was working so I'm pretty sure all of the wiring there was working out. I had even tested that the photo interrupter sensors were working manually without being attached to the motor and values were being passed through pretty well. So I hooked up the code to control the speed of the motor based on the error correction code from that guide, and let it run.

So this next part is what I'm guessing caused the pack to melt, but I'm not sure if how I went about it was just dumb, or if something else caused it, or both.
This is where I realised I had forgotten to put the batteries into the pack, so while the pico was powered and the program running, i started putting the batteries one by one into the battery pack (I had done this many times before with no issue, was this just lucky?), and where usually there was a light on the motor controller after putting in the 4th, there was none this time.
So I pick up the battery pack again to check the batteries are all in properly, and felt a fair amount of heat coming off the pack, and then noticed the springs starting to melt through the plastic, fairly quickly too.

This is where I quickly unplugged everything to try and find out what happened, but cant really tell what I messed up here haha.

Is it possible for me to tell the motors to draw too much power from batteries based on the program running or anything like that and managed to do this while trying to control the speed? Or is this a case that I've crossed some wires somehwere and sent a load of power back into the battery pack or something?

Any help?
Cheers!
Edit: I was just looking over the driver and saw what looked like a bit of melted plastic with wire embeded within between the VCC and GND terminals, could a stray piece of wire joining these terminals be part of what caused the problem?

 

[Martaine2005]

I would hazard a guess that a short circuit happened somewhere across the batteries. Possibly just a stray strand across the screw terminals.
This would also explain the ‘no light’ in the controller when the final battery was inserted. Could also be the battery holder has shorted somewhere. Have a good look to see if it becomes obvious.

Edit. Your edit is clearly a short across the screw terminals. But check it with a multimeter to know for sure.

Martin

 

[SketchyDan]

I would hazard a guess that a short circuit happened somewhere across the batteries. Possibly just a stray strand across the screw terminals.
This would also explain the ‘no light’ in the controller when the final battery was inserted. Could also be the battery holder has shorted somewhere. Have a good look to see if it becomes obvious.

Edit. Your edit is clearly a short across the screw terminals. But check it with a multimeter to know for sure.

Martin

Ah, I've since pulled out this piece of wire, would I still be able to determine anything with the multimeter now that piece has been removed?

Cheers for the help!

 

[Martaine2005]

Yes. There won’t be a short across those terminals.

Martin

 

[SketchyDan]

Yes. There won’t be a short across those terminals.

Martin

Ahhh ye, that makes sense aha.

Just checked it out, and there's no continuity between those terminals at the moment, which is the correct reading right?
I put the bit of wire back there just to check and my multimeter was then showing continuity between the two.

Just to make sure I understand what happened. After shorting those 2 terminals, I sent too much current back through the battery pack causing it to heat and melt? Did doing this also damage the batteries themselves, they looked ok when I checked them out, but not sure if shorting it out this way could damage them internally or something?

Also is pluggin in the batteries while everything else is powered an "ok" thing to do? Other than me jiggling the wires around and causing the short, can anything else go wrong from doing that?

Thanks again

 

[Bluejets]

I sent too much current back through the battery pack causing it to heat and melt?

No, you shorted the supply which cause a large current drain on the battery.

Other than me jiggling the wires around and causing the short, can anything else go wrong from doing that?

Since you have already caused a catastrophic melt down by powering up without checking, who knows..?
I'd say a distinct possibility.

 

[SketchyDan]

No, you shorted the supply which cause a large current drain on the battery.



Since you have already caused a catastrophic melt down by powering up without checking, who knows..?
I'd say a distinct possibility.

Faaair enough .
Other than checking for shorts between any of the other connections, is there anything I should check before attempting to use this driver again, or would shorting the pack like this through the driver terminals cause some irreparable damage to the driver itself somewhere too?
Like I say, totally new to the hardware side of things, so any tips on what you'd do/check/debug after doing shorting something like this is greatly appreciated too

 

[Martaine2005]

You might be lucky, the board may have good short and polarity protection.
Connect it up again and try it.

Martin

 

[SketchyDan]

Will do.
Cheers for the help, I'll give it a try once I've got some fuses and things hooked up this time.