Why my circuit is not working - Edit: it was just a bad soldering joint

[MathCa]

Hello,

When the power supply is connected, everything is fine. The relay close so the batteries are charged and the loads are powered by the power supply.

The problem is when I disconnect the power supply. The relay open but the voltage at the exit of the BMS drop to something like 0.6 volt even if the batteries are fully charged.

Everything is sharing the same ground, and when I tested for continuity everything seemed normal.

So there's surely something I clearly don't understand, and/or I inhaled too much lead smoke.

Thanks!!

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[james_s]

There's no ground connection between the battery pack and your load, that can't possibly work.

[MathCa]

There's no ground connection between the battery pack and your load, that can't possibly work.

But they all share the same ground and when I test for continuity from the negative of the load and the negative of the BMS, it beep, so it should be fine?

[james_s]

Is that true when the charger is not powered too? This should be very simple to troubleshoot, use your multimeter to check for voltage at each node, find out where it stops.

[MathCa]

Is that true when the charger is not powered too?

Yes :S That's why I really don't understand. The voltage drop from ~16.8v to ~0.6v at the output of the BMS. Could it be related to the diode?

[james_s]

You have't got P- connected, are you trying to use the BMS only for charge management? I don't know how it's wired internally but it looks like it expects power in/out to be on one pair of terminals and nothing except for the battery connected directly to the battery wires.

[MathCa]

You have't got P- connected, are you trying to use the BMS only for charge management? I don't know how it's wired internally but it looks like it expects power in/out to be on one pair of terminals and nothing except for the battery connected directly to the battery wires.

It's so strange, I tried it again and it worked normally! Hopefully nothing became warm and there was no smoke 😅

I still don't understand, I guess it's related to the BMS.

[shapirus]

What kind of BMS is this? Some types need to be activated after they are wired for the first time.

[MathCa]

What kind of BMS is this? Some types need to be activated after they are wired for the first time.

It's this one: https://www.aliexpress.com/item/4000527872921.html

I just saw something very strange for me, when I measure the voltage out of the power supply, I see a normal value for a fraction of a second and it decrease fast to 1! But if I measure the voltage from the battery, the result stay at the same value... But the power supply is working, it's charging my cellphone even if I remove the batteries!

Edit: it was my fault, I forgot to change the range on the multimeter 🤡

But it stopped to work again when I disconnect the power supply.

Maybe there's a ghost...

[james_s]

I think it's the way you have the BMS connected. Many of them do not enable the output until a charger has been connected. It's related to the way the shutdown circuit works when the battery is fully discharged. You seem to be trying to defeat this shutdown protection with the way you have your circuit wired. It isn't a ghost, you simply aren't using the BMS the way it was intended to be used, one of the terminals is not connected.

[MathCa]

you simply aren't using the BMS the way it was intended to be used, one of the terminals is not connected.

I would love it if it was the problem, but it's possible to share the same port for the load and charger like we can see in the description. And it worked normally for a moment, so that's more strange for me.

[james_s]

I don't think it's strange that you have the BMS wired up to the load incorrectly and something isn't working.

[coppercone2]

Sorry your circuit is complicated. I recommend removing parallel elements until you get the simplest circuit with minimum elements then gradually add elements and test as you go,


Try with out the parallel parts if you are getting intermittant operation. Since it looks like you wired this up with screw terminals, it might be faster to gradually build the system up with say 10x cycle test from the simplest configuration possible, up to maximum complexity with no parallel elements, then start adding parallel elements, then to wait for the forum to decypher it. Lots of proprietary black boxes in your diagram.

systems engineering you find that simple looking shit does not work and its not simple

[MathCa]

I don't think it's strange that you have the BMS wired up to the load incorrectly and something isn't working.

If you look at the description of the seller, you will see it's not wired incorrectly lol

And it's working again, it was a soldering problem 😅

[MathCa]

I recommend removing parallel elements until you get the simplest circuit with minimum elements then gradually add elements and test as you go,

Good idea! Hopefully it was only a wire not soldered correctly to a copper bar between 2 batteries 😅

[coppercone2]

that is common because did not overcome enough thermal mass, whenever you are soldering something more then a light gauge PCB you need to do extra inspection, cleaning, fluxing

I suggest getting either
1) preheater (hot air gun, small torch if applicable (preferably with palladium mesh, its like a high heat hot air tool), hot plate
2) big iron (i.e. automotive type), old school manual iron (blow torch) or if you are pro an actual big iron like a american beauty with a big heated element thats heater heated and not just a red hot wire
3) spot welder

and if good cleaning is possible, a aggressive flux

https://americanbeautytools.com/soldering-iron-3158 ,used with a large variac and a tip temperature tester

And there are proboly more advanced versions with actual temp control, but it should beat out the red hot wire soldering iron for quality work anyway....

https://www.ftm-technologies.com/produit/300w-heavy-duty-soldering-station/?lang=en

this one looks regulated

For a single solder joint, you can heat the old school 'poker' type iron with a torch and do a joint because the tip has substantial thermal mass, so long you use a big enough one

[MathCa]

Great suggestions!

It was my first time but I remembered to heat the copper bar enough first, so 4/5 was solid 🤭

I heated my soldering iron to 400°C (752°F) and left it for several seconds with some lead to increase the contact surface. I tried afterwards to pull on each cable to see and everything seems solid now 🤝 A real beginner!