Smart BMS incorrect voltage reading (any fuses/ diodes?)

[Corrida victim]

Hi everyone,

I couple days ago I received the famous smart BMS by LLT power. It worked flawlessly for a day, but after tidying up the balance wires (hot gluing and such), two cell readings are way off. Cell6 reads 47mV and cell7 reads 5.8v. And yes, I was stupid enough to leave the BMS connected.

I guess I must have shorted something without realizing with the hot glue gun, or it's just plain bad luck.

Anyway, here's what I tried:

At first I disconnected the wire between cell6+ and cell7-. There was no difference at all in voltage readings from the BMS, so I suspected this wire to be bad.

But:
- I measured every neighboring wire at the balancing connector (all read ±4.1v as they should)

- Measured every wire from the negative battery pole , and very wire gave a logical 4.1v increment. (4.1v, 8.2v, 12.3v....)

- Suspecting a high impedance from the cell6+ cell7- wire, I took a 47ohm resistor and connected it between that wire and an adjacent wire, expecting a voltage drop. But the multimeter still measured 4.1v


All in all, I'm pretty sure the wiring is just fine. So I guess the BMS itself is probably the culprit.


I know this BMS is quite popular in the modding community, so does anyone know of any repair guides? Schematics? Or is this a common issue? Any fuses/diodes that might have blown?

 

[amberwolf]

have you checked the posts about that bms?

these are the ones i can find by the name you gave:
https://endless-sphere.com/forums/search.php?keywords=LLT+power&terms=all&author=&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
i didn't read any of them so i don't know if they have the answers you want, just linking them in case you haven't seen them.

fwiw, it sounds like you have a bad connection between the cell and the sense input of the bms on that bad channel.

 

[Corrida victim]

Hmm yes, I suspected bad wiring too, but i think I can rule that out.

Here is a picture of every connection u measured.
From right to left:
4.1v
8.2v
12.3v
16.4v
20.5v
24.6v
28.7v
32.8v
36.9v
41.0v
45.1v
49.2v

So I guess the problem arises somewhere after that point, in the bms?

 

[amberwolf]

i see a giant solder bridge between two of the channels on the bms. between bc8 and bc9. i can't tell but it looks like the wire is also missing from bc9.

there is another bridge between bc13 and bc14, and perhaps another missing wire at bc14.

there is an extreme amount of solder on bc7, which may have damaged the pads and traces there, and potentialy shorted to bc5, if not on the solder side, perhaps underneath the connector itself on the opposite side.

with those solder issues, it implies there are possibly connection problems wherever teh solder is heavy, and could be at any of the channels.

this is a strange way of wiring things, which implies there might be a problem with those channels, which may also imply there is a problem with the bms in general.

or it is wired this way because the original builder incorrectly skipped some channels when wiring up the sense wire harness, and instead of fixing the problem just bridged across their mistakes.

either way, i don't know if these will cause problems with bms operation, since it is probably not designed to skip channels like that (even if it is ok to not use some of the channels at the most positive end, those usually have to be consecutive, with no cells more positive than the skipped channels).


is the channel you are having problems with one of those channels? or one next to them?