bms wiring advice

[jimmyhackers]

hello,
i have bought some of these https://www.ebay.co.uk/itm/403530491201

i plan to use them on a 12v (3s 4p) 18650 batt for a razor drift scooter thing. is only 100w.....so hopefully the 10A of these is enough.

there was no circuit diagram and the pictures show the same size nickel strip for every battery terminal.

i will be using cable however....so im wondering if i need to use a thicker gauge cable for every cell terminal or just the main batt + and - from the battery to the board?

 

[lnanek]

Balance wires going to individual cells are usually quite small. Only have to deal with 2-4.2V after all. Usually only measure voltage. Occasionally shunt it away when charging if a certain cell reaches max voltage before others.

 

[harrisonpatm]

Heads up, i used a few of those, 50% of them failed. One of them didn't, but almost, caused a fire in a small pack I had.

The other 50% I used worked great, as far as I could tell.

 

[jimmyhackers]

thats kind of what im getting at....how do you wire them up safely? :lol:

i know balance wires are generally smaller, which is why decent balance chargers usualy limit themselves to 200ma as soon as one cell gets to the corrrect voltage....

but seeing as that's not how they've shown it to be used (in a picture and not a diagram), i had to ask.

i may aswell just wire them all with large gauge wires to be safe.

out of interest....when the pack you had nearly caught fire....which part was it? the board, the wiring , or the cells?
did you use small balance wires?
and on the ones that worked, did they charge to a correct voltage and keep all the cells balanced?

 

[harrisonpatm]

As far as I could tell on my bad failure, a part on the board shorted B+ to B2 a little bit, and caused enough of a connection to burn through the balance wire and scorch the cells externally. Luckily it was a tiny 3s 1ah pack and the wires fried before the battery could continue to damage itself. I think. I don't know for sure, I just came to my workbench one day to see a scorch pile around the battery.

The ones that worked, worked fine. I didn't use them extensively, I just know that they worked enough to cutoff at LVC.

 

[jimmyhackers]

well, after installing....i cant say my enthusiasm for a bms has improved

i noticed on a 2 of the 3 boards i had. there should of been continuity between the terminals from each side of the board....but it was randomly intermittant. easily fixable with soldering...
its possible this would cause an issue as traces are used on both sides, the rear traces seem thicker for more current capacity while the front traces hold all the components

each board has annoying little nickel plates soldered to the terminals on one side...which i ended up removing as it made soldering wires to them bloody annoying.

also the spacing/terminal loactions on the board, made soldering awkward, i managed to "wick up" and lose a tiny surface mount component by accident while attaching the final wire, bricking a board.
i had to desolder every wire and start again.

charging seems fine so far, as it stopped at 4.15-4.17 per cell.
im yet to try a discharge in the device to test the lvc and current capability but i will report back.

 

[lnanek]

each board has annoying little nickel plates soldered to the terminals on one side

Probably means it was intended to be spot welded to the cells rather than soldered.

 

[jimmyhackers]

Rant: for whatever reason the goddam razor powerrider 360 its going in, is screwy

the esc is draining the batt so much it will not charge...

when i connect my 12v 1A charger to the esc battery terminals without the batt connected, it instanly drops its voltage to 0.6ish volts from 12.6v....

really annoying as now i will have to install a battery cutout switch just so the sodding thing will charge