Possible BMS Issue?

Tjurgensen

New Member
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
 

gtxltd

Solar Enthusiast
Does anyone have a clue as to what issues I could be running into?
I don't. I'm a beginner but I would separate the parallel batteries and one at a time try to resurrect them. Apply a small amount of charge current and see if that brings them back. Apply a small discharge load and see if that works. Hopefully something you do will bring the BMS back to normal operation.
 

MisterSandals

Participation Medalist
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.
I am pretty sure your EPever needs the battery(s) connected and powering it before connecting solar panels.

Similarly, the solar should be disconnected from the EPever before disconnecting the battery. Having a breaker to the panels makes this a lot easier than a fuse (but a fuse and a switch are fine too).

There have been numerous discussions as to whether having only solar connected will damage SCC's but the general consensus is that following the best practice is battery first to connect, last to disconnect.
 

Tjurgensen

New Member
Thank you for the replies. I have been working with the batteries separately since the initial mishap. I've now 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.
 

MisterSandals

Participation Medalist
I turned the PV off so as not to mismatch the batteries and when I shifted the battery to plug in the second battery,
This is not clear what you are doing? How do you have this wired, exactly?
Are you connecting the second battery in parallel or series? Hopefully parallel otherwise you'd be changing quite a bit without explaining it.
Have a picture of your wiring with 1 battery (working) and with 2 batteries (not working)? Sometimes it takes a couple sets of eyes to see a problem.
 
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Tjurgensen

New Member
So here is the main wiring scheme and how the batteries are connected in parallel. Note that when I've been messing with it, the inverter has not been connected as it is in the picture.

The top battery seems to function properly... some of the time. I can plug only it in and the cc and 12v converter may turn on for a minute or two, or 20 then it cuts out again. Unplugging the battery and resetting the BMS on this battery will sometimes work. Sometimes it takes a few unplugs/replugs of the BMS to get it back to normal.

When its functioning, the voltage across the bms at b- and c- is identical. After its tripped, the c- is always a half volt less. I'm not sure if that means anything or not, it's just something I've noted.

All that said the bottom battery won't seem to reset at all since I initially connected the pv and scc.
 

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MisterSandals

Participation Medalist
Wow, there is a LOT going on there. Can you simplify things to narrow down the scope of the system?
It seems like the BMS is getting in the way of just a basic connection between a battery (or two) and the SCC.

Can you bypass the BMS and connect the battery (only) to the SCC? With a reasonably charged battery, your SCC should start up and stay running (with ONLY a battery connected). This should determine whether your SCC is the culprit.

Next I'd introduce the smallest amount of solar that you can to see if that functions for any amount of time. Remember, when running these tests, YOU are the BMS and responsible for the battery. If this works, disconnect the solar first, then the battery. Introduce the BMS to the system and see if just the battery and BMS work.
Then introduce a small amount of solar.

If that works, try starting over with 2 batteries (only) and progress. Let us know how it goes.
 

Tjurgensen

New Member
It seems like the BMS is getting in the way of just a basic connection between a battery (or two) and the SCC.
I'd say you've got it.

When I bypassed the bms (in about the most redneck way possible I might add,) I was able to power on the scc and apply charge. Adding the second battery went without a hitch and all was functional.
I pulled my bypass connections and reconnected the batteries as they were intended and again nothing happened.

Also, after some more digging I found an old YouTube comment from techdirect themselves saying that this bms only provides over charge, discharge, and current protection with no balancing. I'd say I need to get a couple new bms's coming my way.
 
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