Batteries with different state of charge?

[Scorch]

I currently have 6 Rosen powerwalls (48V, 200Ah), though 1 is not working (but thats for another post, and it is now offline). I only visit this property once every few weeks, and it seems like every time I look at the SOC of the 5 individual batteries, some number of them will not be discharging.... for example, I'll have 2 of them at 60% SOC, 2 at 70%, and 1 at 100%....Rosen claims this is normal operation, but then a few weeks ago, one of the batteries seemed to be taking all the load, dropped down to 0% SOC, and shut down my entire system, while the other 4 batteries all had 20-40% charge left. When they are fully charged, all 5 batteries are showing 96-99% SOC

Is this normal operation? I had assumed they would all maintain the same SOC. For reference, I have 2 DEYE 8k inverters, and I have the batteries linked to inverter 1 BMS. Is there something in the inverter settings I need to change? Or maybe I should use the battery BMS instead of inverter BMS?

 

[sunshine_eggo]

Some imbalance is normal. 30% imbalance (70-100%) seems off. Pictures of the wiring?

 

[Scorch]

Some imbalance is normal. 30% imbalance (70-100%) seems off. Pictures of the wiring?

I don't have any pictures..... but they are run in parallel... installed by a professional solar installer based on schematic from Rosen (attached)

 

[sunshine_eggo]

That's a sketch. Pictures would be more helpful.

From the sketch, is the round trip cable distance to each battery the same?

Have you confirmed that all connections are properly torqued, and all crimps are of high quality?

Please provide additional information on this "combiner box."

 

[Scorch]

Cable lengths are pretty similar. I can double check all the connections etc. I didn't really go down that path since I was thinking it's a problem with the BMS and how they are communicating. This is the only pic of the combiner box I have handy (I’m not at the location right now)

 

[sunshine_eggo]

To clarify, the sum of the (+) and (-) cables need to be identical, or you have a different resistances between loads/chargers and batteries. Batteries connected with longer cables will tend to lag the shorter ones in both directions - lower current and last to fully discharge/last to fully charge.

Worth checking all the connections to the bus bars. Make sure all copper is clean and none of the shrink is pinched between the terminal and the bus bar.

Lastly, what's under the electrical tape on the black wire where the shroud is absent?

Basically, scrutinize the shit out of everything between the battery terminals and the inverter terminals. If the copper isn't shiny with full contact between components, and the fasteners aren't properly torqued, you may be dealing with a bad connection.

 

[Scorch]

To clarify, the sum of the (+) and (-) cables need to be identical, or you have a different resistances between loads/chargers and batteries. Batteries connected with longer cables will tend to lag the shorter ones in both directions - lower current and last to fully discharge/last to fully charge.

Worth checking all the connections to the bus bars. Make sure all copper is clean and none of the shrink is pinched between the terminal and the bus bar.

Lastly, what's under the electrical tape on the black wire where the shroud is absent?

Basically, scrutinize the shit out of everything between the battery terminals and the inverter terminals. If the copper isn't shiny with full contact between components, and the fasteners aren't properly torqued, you may be dealing with a bad connection.

 

[Scorch]

Thanks. I’ll check that out

 

[sunshine_eggo]

what's under the electrical tape on the black wire where the shroud is absent?

 

[Scorch]

I’m not sure. I sssume it’s where they may have had to splice the cable together. I’ll have to take a look

 

[sunshine_eggo]

I’m not sure. I sssume it’s where they may have had to splice the cable together. I’ll have to take a look

"They" suck. If you paid somebody for this installation, there's no excuse for something like that. If it's possible those are splices, I'd be peeling that tape back to investigate.

 

[Ellcon123]

If one battery stayed at %100 while the others went to 60~70% seems to indicate BMS is stopping discharge? Do the BMS's have any logs that show intermittent operation?

 

[Scorch]

"They" suck. If you paid somebody for this installation, there's no excuse for something like that. If it's possible those are splices, I'd be peeling that tape back to investigate.

I'll have a look.

 

[Scorch]

If one battery stayed at %100 while the others went to 60~70% seems to indicate BMS is stopping discharge? Do the BMS's have any logs that show intermittent operation?

I don't have (or know how to access) BMS logs.... but this is more what I was thinking the issue was.... something with the BMS, because it doesn't always seem to be happening all the time. sometimes they all discharge the same, and othertimes there will be one or two that are not discharging at all, and sometimes it's 1 or 2 not discharging as much as others.

 

[sunshine_eggo]

Cable lengths are pretty similar. I can double check all the connections etc. I didn't really go down that path since I was thinking it's a problem with the BMS and how they are communicating. This is the only pic of the combiner box I have handy (I’m not at the location right now)

I didn't address this.

The BMS don't regulate anything. They are simply observers and drawbridge operators with a different bridge for each direction of traffic. They watch for things to go out of limits. When it does, they raise the drawbridge prohibiting the traffic in the offending direction.

The BMS in no way influence how much any battery discharges or charges beyond cutting it off if it's too much.

 

[Scorch]

I didn't address this.

The BMS don't regulate anything. They are simply observers and drawbridge operators with a different bridge for each direction of traffic. They watch for things to go out of limits. When it does, they raise the drawbridge prohibiting the traffic in the offending direction.

The BMS in no way influence how much any battery discharges or charges beyond cutting it off if it's too much.

So if one battery isn't discharging, couldn't it be the BMS shutting it down (raising the drawbridge)? Similarly if there is a difference in SOC, couldn't it be that BMS is shutting off some of the batteries at some time after they started to discharge which would account for the difference in SOC?

 

[sunshine_eggo]

So if one battery isn't discharging, couldn't it be the BMS shutting it down (raising the drawbridge)? Similarly if there is a difference in SOC, couldn't it be that BMS is shutting off some of the batteries at some time after they started to discharge which would account for the difference in SOC?

It could be, but when you say all 5 are 96-99% that strongly suggests that all are working. Connection problems are far more often the cause of all types of odd behavior.

After you've checked all connections, at your next full charge when all batteries read > 95%, it might make sense to take the system offline and reset each of the batteries.

 

[Scorch]

It could be, but when you say all 5 are 96-99% that strongly suggests that all are working. Connection problems are far more often the cause of all types of odd behavior.

After you've checked all connections, at your next full charge when all batteries read > 95%, it might make sense to take the system offline and reset each of the batteries.

Perfect! I'll try that..... With these batteries, I also had to replace all the BMS boards (since they had a batch of bad diodes), so maybe that is what is going on.

 

[sunshine_eggo]

Perfect! I'll try that..... With these batteries, I also had to replace all the BMS boards (since they had a batch of bad diodes), so maybe that is what is going on.

That info certainly changes the perception and shifts concerns to the BMS

 

[Scorch]

That info certainly changes the perception and shifts concerns to the BMS

I never had them working properly before the BMS boards got changed. Essentially batteries started dropping dead from day one, so they shipped me new BMS boards (since the bad diodes was a known issue) and I installed those new boards myself.... but never did any type of reset on them