Possible BMS SCC Issue?

Tjurgensen · 05-03-2021, 08:15 PM

I am just putting together my first system with two 24v 8s BYD batteries, connected in parallel, from techdirect. These came with preprogrammed chinese BMS's: Better Shenzhen Better New Energy BT.25M2020-MWJ115152. I had previously connected these to my step down converter and ran some 12v appliances/lights w/o a hitch. Today I was ready to connect my solar through my EPever Tracer 10415AN cc, had all four leads connected, battery plugged in with the breaker turned off, and the solar isolator switch in what I mistakenly thought to be the off position. After making the PV connections on the roof I see the cc on and after about 10 sec. I realized it shouldn't have been and quickly flipped the breaker, connecting the batteries. The cc was indicating all was well but after a few more seconds the warning light appeared and showed there was no battery connection so I disconnected the solar. Now the batteries show ~26v (what they were @ before) until they're connected to any sort of load: cc, or converter after which I read a volt or two. I attempted to make the PV connections again this time in the correct order and got the same ~10 sec. of green status until it switched to show no battery connection. When connecting the cc w/o PV, the screen flashes on for a split second before going back to blank. I have also tried jumping the b- and p- on the BMS to possibly reset but to no avail. Does anyone have a clue as to what issues I could be running into? Any help/suggestions are much appreciated.
Thanks for reading,
TJ

rffffffff · 05-04-2021, 01:28 PM

Most likely fried the charge controller. It’s hard
To decipher and I’m
Not reading carefully but charge controllers with pv and no batteries are a no-no.

Some member here recently suggested eliminating a fuse or breaker between the charge controller and batteries for this very reason and I think it’s wise. Mine are direct to the batteries, albeit with the main 400a fuse in the way (and maybe that’s even dumb) and the shunt on the negative side but that’s it, no separate disconnect.

If the cc doesn’t work with batteries connected and and no pv it’s fried.

Tjurgensen · 05-04-2021, 02:20 PM

Well the cc is good. Today I've checked all the cells and they are in good balance. After resetting the BMS via unplugging/replugging the sense cable, I was able to power on my 12v converter (with nothing else hooked up.) At this point I only have one battery plugged in. I slid the cc lugs onto the shunt and + bus and it powered on. I was able to program the charge parameters and settings and after flipping the PV switch, it was charging and functioning. I turned the PV off so as not to mismatch the batteries and when I shifted the battery to plug in the second battery, everything powered off again and I read ~3v at the battery terminals. Now it seems I'm back where I started. I'm relieved that it doesn't seem I've damaged anything although I'm still at a loss. I apologize if I speak unclearly, please let me know if I can clarify anything.

Rucker · 05-04-2021, 02:29 PM

A couple of things-perhaps you've already checked these.
-CC configured for your battery type?
-Solar array configured so it doesn't exceed CC spec? Have you checked voltage at the PV leads that connect to the CC to triple check configuration?

Also, are you saying that the battery no longer shows ~24V when connected to a load independent of the CC? Are they fully charged?

Do you have any readouts on the BMS? Might be helpful to see if any given cell is above or below BMS cutoff.

(edit: I sent this just before you posted your update)

Rucker · 05-04-2021, 02:36 PM

Quote:

Originally Posted by Tjurgensen

I shifted the battery to plug in the second battery, everything powered off again and I read ~3v at the battery terminals. Now it seems I'm back where I started. I'm relieved that it doesn't seem I've damaged anything although I'm still at a loss. I apologize if I speak unclearly, please let me know if I can clarify anything.

First, good to hear you didn't fry anything.

I'm inclined to say this is related to your BMS. I'm not the expert but is it possible to disable the BMS then check the voltage? Other than some loose connection (unlikely from your description) I can't think of why you would get that kind of reading.

To be clear, do you get ~3vDC when BOTH batteries are connected (in parallel, I assume), or when you physically move the battery, BEFORE the second one is connected?

Bert06840 · 05-04-2021, 04:17 PM

Well, your BMSes are both disconnected. What does your phone app tell about what is going on? They report overvoltage, undervoltage, overcurrent, short circuit, MOSFET trip cause, and so on.

Guessing is rarely effective!

Tjurgensen · 05-04-2021, 05:00 PM

So the solar array is confirmed to be within specs.

The batteries aren't fully charged but they're both sitting at ~26v. The batteries, when connected are in parallel. Unfortunately these bms's are fairly rudimentary. They were preprogrammed and have no display or interface aside from a total voltage readout.

It turns out on one of the batteries there was a loose connector. It has been secured and it seemed to fix the issue with that one. No such luck on the second battery though. All the connections are tight and resetting doesn't seem to have any effect.

At one point I had both batteries plugged in and the cc charging but soon realized only the one battery was charging so I shut it down again. I'm thinking there may be a bad connection further down the board towards the cells?

rffffffff · 05-04-2021, 05:10 PM

Can you measure each cell? If you start at one end and measure each terminal it should add ~3.2v each time, you may find where the math doesn’t add up and be able to locate the issue, perhaps both on the bms and the battery itself. No clue if the terminals are easily available but it sounds like the problem on battery one so maybe you can check battery 2 like so?

Rucker · 05-04-2021, 07:15 PM

Quote:

Originally Posted by Tjurgensen

So the solar array is confirmed to be within specs.

The batteries aren't fully charged but they're both sitting at ~26v. The batteries, when connected are in parallel. Unfortunately these bms's are fairly rudimentary. They were preprogrammed and have no display or interface aside from a total voltage readout.

It turns out on one of the batteries there was a loose connector. It has been secured and it seemed to fix the issue with that one. No such luck on the second battery though. All the connections are tight and resetting doesn't seem to have any effect.

At one point I had both batteries plugged in and the cc charging but soon realized only the one battery was charging so I shut it down again. I'm thinking there may be a bad connection further down the board towards the cells?

Loose connection! Ha!

I took a quick look at a youtube video of your brand of battery. My impression is that there could be a BMS wire broken or somehow compromised. I think that would prevent the battery from charging/discharging, though it may show voltage.

Maybe do a little research on youtube and carefully inspect that defective battery. Hopefully just a simple fix!

Regarding frying the CC, I didn't think you could fry it quite so quickly, so I didn't think that was the issue and glad to hear you are okay with it. This equipment has at least some practical safety features. Having said that, all electronics need to be carefully connected.

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