Idk whether it's because I used the wrong gnd (everything except the servo was on its gnd, the servo was on the Arduino gnd but the Arduino gnd was on the batteries) or because I hot glue the wires to the breadboard so they wouldn't fall out in 2 seconds
The second I plugged in the 12v for the l829n it started smoking and I unplugged everything.
I gutted everything, looks fine besides a selected few wires (has melted gunk in them) and I might need to use a different l829n.
Wires only melt like this when the battery + is connected to the battery -. This is not because you used the wrong GND, it's because your positive was connected to GND.
This is a concern as it implies that you had a choice of GND.
Unless you have a specific reason to have separate GND connections, there should only be one GND and thus no reason to make a comment like that.
As others have said, the wires will only melt if too much current is going through them, and that will typically happen as a result of a short in your circuit.
Either way, you probably want to carefully review your circuit and compare it to your design. And if you didn't already create a schematic, do so now from your wiring as it may well (likely will) reveal the flaw.
Li-ion batteries catch fire when they are overcharged or punctured. When they are shorted they deliver a lot of energy and can catch whatever causes the short on fire, not the battery.
Your 3.7v batteries are a reasonably energy-dense battery chemistry, but prone to fire if punctured or damaged. LiFePO4 is another li-ion chemistry with somewhat less energy density, but much safer. LiFePO4 also has a lower voltage (3.2v) but a much flatter discharge profile than li-ion.
In either case over discharge of the battery would result in an uncontrollable failure. The battery does say "protected" so it has a BMS module built into the battery, so it's fine. Also Li-ion is not a chemistry, it's a category. It is often used as a title and then you have to dig to figure out what the chemistry is. LiFePo4 is going to be the default for 18650s you buy today.
Well, LiFePO4 is significantly more resistant to fire than other li-ion chemistries, but otherwise the corrections are appreciated! And I also misremembered the relative voltages - LiFePO4 has a lower voltage (~3.2) vs. the 3.7v of the OP's batteries. I'll edit accordingly. Thanks!
Your right about the voltage, and I was mistaken - most 18650s seem to be INRs rather than the LiFePO4. I'm not sure they make 3.2v 18650 batteries, I've only ever seen them 3.7 (30Q and 35Es are what I use, mostly).
In a few years we'll have sodium ion batteries so that'll be exciting.
You're right - the cell says "protected" which means it has a built in protection circuit. So a short circuit event would trigger something in there to interrupt the connection before the battery became over discharged.
Gauge right? How thick the wire is? Yeah I wasn't thinking about that at all when I bought these.
I just assumed it was fine because why would they sell something that couldn't handle the electricity. I read the reviews and it did happen to another guy so I guess I'm not alone in this.
So if I wanted more power I'd need to replace these wires wire myself.
let's assume that some Arduino and 2 dc motors will take the most current, so there's up to 6 amps in peak. those are li-ions so they actually will give this amount. I would suggest to use at least 20awg or even 18awg wire (0.8-1mm diameter) . actually pretty any wire with multiple threads in it. it won't melt moresoever won't even get warm
Wires on these things would typically be 24AWG size. This is good for about 3 Amps continuous, so good for most applications. When your wires got toasted they were seeing 10s of Amps. It's difficult to change wires on these things. The wires are permanently crimped into place and the plastic melt if you try to solder wires to them. You could add a fuse to protect from the short circuit case.
You probably shorted it out, but that looks like not larger than 22 AWG wires. 18650 cells will easily put out enough juice to burn them out. You'll want to look at how much stall current all of your servos will draw to get your worst-case current, and the power wiring needs to be sized appropriately.
Regardless of what other grounds you've got set up, there is still only one ground wire going to the battery pack - and one positive wire. The total current of your project is carried by those two wires.
Always check the resistance between the power rails and any switched loads to make sure you have sensible resistance values and no short circuits. If you’re using batteries then remove them before making the resistance measurements.
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u/negativ32 9d ago
Nice fuse.