Experienced Battery Builders Help! - JBD smart BMS random voltage drop from 48v down to 15-17v

[carltoza]

The BMS is a JBD Universal Smart BMS type AP21S001

It can do both LFP and Li Ion and series connections between 6s and 20s

Out of the blue, about a dozen times now it decides to stop working. The voltage at the connector ends drops to 15-17v while the pack voltage remains normal at around 50+ v fully charged with perfectly balanced series modules.

The only way to fix it is to unplug and replug the JST connectors for the balance leads. There are 2 sets of JSTs for the balance leads, the main one with the larger group of strings however does have a broken clip on top. That being said, it fits snugly in place and even though I'm getting the low voltage at the connector head end everything checks out and looks fine in the bluetooth app.

when I wired it up the first time I admit that I had some of the balance leads wrong but only for a few minutes. For the smart BMS, for lower series count batteries you have to connect 8 of the balance leads into one. on Sriko batteries website though they had the manuals for the 20s version and the 21s version reversed. Could this be the culprit and have damaged the BMS?

I did take off the heatsinks from both sides and examined the board. No signs of any fried components.

What I can say is that the problem is not: The cells, the spot welding, the configuration of the balance leads, short circuits in the balance leads, continuity in the balance leads between the male JST and the its series connection, or poor connections to B- and C-.


Any wisdom is appreciated! Happy to send more details about the issue.

 

[amberwolf]

The voltage drop you see is the BMS shutting down the output. This shutdown is normally because of an out of limits conditions being detected, or an error or fault condition. All of these should be shown to you in the app or program during this state (some BMS also log these for later, some only do it if the app is running and talking to it at the time, and some don't log at all).

If it's a cell-level issue (regardless of source being cell, wire, BMS channel), you would still have the channel causing the shutdown show an out-of-limits voltage or error condition in the BMS app / program during the shutdown state.

If it's a pack-level issue (overcurrent, etc) you would have an error condition indicating this in the app/program during the shutdown state.

If you get no error or out-of-limits conditions, even momentarily, then it's either a firmware bug or an intermittent hardware issue.

FW you'd have to get hold of JBD to see if they have a different FW you can flash to it; there's not much you can likely do outside of that.

Hardware...it could be a solder or trace / via defect to the gate drive of the FETs, if the MCU is not detecting any error conditions.

If it is detecting error conditions that aren't true, then the defect is probably between the MCU and the point where the error is supposed to be actually sensed.

when I wired it up the first time I admit that I had some of the balance leads wrong but only for a few minutes. For the smart BMS, for lower series count batteries you have to connect 8 of the balance leads into one. on Sriko batteries website though they had the manuals for the 20s version and the 21s version reversed. Could this be the culprit and have damaged the BMS

If balance leads were actually swapped on cells in the series string so that more positive cells were wired to more negative channels, or vice-versa, damage to channels could occur...but:

If the BMS sense channels were damaged, it would probably not operate correctly at all, not just inconsistently. Even if it was inconsistent, you would still have the channel causing the shutdown show an out-of-limits voltage or error condition in the BMS app / program during the shutdown state.

If balance leads that were swapped were not connected to any cells, and/or they were not swapped in a way that "shorts" across cells, then it's unlikely the channels would be damaged.

To be sure, we would have to have a detailed schematic of their BMS and specs for the parts they use to monitor the channels to see if it is possible to damage things in whichever way they were wired, but without that (which you'll probably never get from them) I'd guess that the above are how it would respond.