New 48V battery, potential issue

[dpappyp]

I have a new LV6548 system installed with a 48V 150AH battery that I built from 16 cells from batteryevo. They were listed as refurbished with a 3 year warranty. They arrived at pretty much the same voltage so I did not do a balance. After seeing the voltages deviate from each other during conservative use I did a top balance on them. I started out with setting my bulk charge at 54.5V and float at 53.1V, trying to be "conservative". The voltages stayed nicely in line with one another. My charge/discharge limits are set at 30A (spec sheet allows up to 50A). The battery seems to be operating fine with these settings, but I am obviously not getting the full use of the battery. So, I changed the settings to more common ones: 56.4V bulk and 54.1V float. The battery charged up to a peak voltage of about 56.8V and settled down to 54.1-54.4V. Then the inverter flipped to discharge mode. Looking at the watchpower app's history the discharge amps peaked at 14A. The battery immediately dropped to 52.6V when it started discharging. It remained discharging for only 22 minutes with amps ranging from 10-14A and then the BMS shut it off. When I checked the battery it was at 52.8V and the individual cells were all within 0.017V of each other. Any idea what happened/is happening? Do I possibly have a bad cell(s) that dropped drastically under load and that's why the BMS shut it down? Is it normal for the pack to immediately drop ~1.5V when starting to discharge (I do not believe so)? The BMS is set to shut down if cell voltage drops below 3.00V or goes above 3.65V or there's a difference in voltage between cells greater than 0.50V. Could a cell(s) have dropped below 3.00V under load and then bounced back after the load was disconnected?

Thanks for any help or guidance anyone can provide.

Thanks,
Dan

 

[SparkyJJO]

You did a top balance with all the cells in parallel to 3.65v per cell until current was nearly 0, then put back in series for your pack, right?

Was there a reason logged for the BMS cutoff, like one cell dropped below threshold?

If testing by meter indicates cells are all close to each other, I'd be thinking you've got a bad BMS balance lead connection someplace.

 

[dpappyp]

The balance was at 3.35V at 0 current before connecting back to series. When charging from there the voltages did stray, but then the BMS performed a top balance and the voltages have been consistent ever since.

No, the BMS has "Status information 0". For reference, I'm using the Daly bluetooth app with the VNSZNR BMS.

Hmm, could be. I'll recheck the connections tonight. Shouldn't the BMS register a low cell voltage though if that were the case? Could the BMS itself be faulty?

Thanks for your input.

 

[Ampster]

The balance was at 3.35V at 0 current before connecting back to series

Was that the termination voltage or resting voltage after power was removed. It is too far from the knee to be effective as a charging termination voltage.

 

[dpappyp]

I originally started using the battery without balancing since the cells were of a similar voltage when I got them (I know, I should have top balanced to start). I observed, however, that the voltages started deviating. So, with the current state that they were at I connected them in parallel and balanced them to 3.30V (the highest cell at the time + 0.01V) Sorry, I mistyped before with the 3.35V. Then I connected them back in series and let the BMS top balance them after they charged up. After that they have remained in balance. Since the recent event described above the cells range from 3.297V to 3.314V.

 

[SparkyJJO]

You need to top balance to 3.65v in parallel to actually top balance. 3.3 or 3.35v is just "somewhere" in the wide flat SoC range and won't do much at all.

Read the top balance how to on the site for more info and details. It is time consuming, but important.

 

[dpappyp]

You need to top balance to 3.65v in parallel to actually top balance. 3.3 or 3.35v is just "somewhere" in the wide flat SoC range and won't do much at all.

Read the top balance how to on the site for more info and details. It is time consuming, but important.

The BMS performed the top balance and the cells have all been in alignment since.

 

[dpappyp]

There should be one or more bad battery in the pack. You need to find out them and replace them. If you want to check out if the refurbished battery is good or not you need to check also the inter-resistance, not even the voltage with no load on battery.

Is there a good "how to" for determining a bad cell(s)? Is there more to it than just the resistance? Thanks for the suggestion.

 

[400bird]

So, with the current state that they were at I connected them in parallel and balanced them to 3.30V (the highest cell at the time + 0.01V).

The "top" in top balance is about the top of the voltage range. This wasn't a top balance.
Like others have said, go to 3.65, or even 3.6 to achieve a top balance. Something like top, not random middle balance.

The BMS performed the top balance and the cells have all been in alignment since.

Also, not a top balance. But, if the cells are even at high cell voltage, mission accomplished.

For your initial issue, what were the cell voltages (min and max) when the charge finished? What was the cell voltage when the discharge stopped?

When charging, the cell voltage might go up to 3.65 volts. But, the normal fully charged resting voltage is around 3.4 volts. If you start discharging with cells above 3.4 volts, yes it is normal for the cells to quickly drop below 3.4 volts.

 

[dpappyp]

For your initial issue, what were the cell voltages (min and max) when the charge finished? What was the cell voltage when the discharge stopped?

When charging, the cell voltage might go up to 3.65 volts. But, the normal fully charged resting voltage is around 3.4 volts. If you start discharging with cells above 3.4 volts, yes it is normal for the cells to quickly drop below 3.4 volts.

Good question, I did not have access to the BMS at the time to check individual cell voltages. I wasn't home when it charged up, leveled off, and then started discharging.

Sounds like I need to try charging it up again when I can be there and get a reading on the cell voltages, thanks for the suggestion.

 

[RCinFLA]

You likely either have imbalanced condition between cells or cells may not be matched well, assuming you don't have a cell terminal connection issue. Besides the 3.6v for top balancing, at battery terminals, you also have to be sure the charging current drops to low level indicating all cells are fully charged.

After you top balance them, load the inverter so you have about 30 to 40 amps of fairly constant discharge current and after 3 minutes check the voltage on each individual cell while under the constant current load.

If cells are well match they should all have similar current loaded voltage. If you see a variation of more than 20-30 mV between loaded cell voltages the cells are not well matched. If any slumps down to below 3.33v there is a very poor condition cell.

 

[dpappyp]

You likely either have imbalanced condition between cells or cells may not be matched well, assuming you don't have a cell terminal connection issue. Besides the 3.6v for top balancing, at battery terminals, you also have to be sure the charging current drops to low level indicating all cells are fully charged.

After you top balance them, load the inverter so you have about 30 to 40 amps of fairly constant discharge current and after 3 minutes check the voltage on each individual cell while under the constant current load.

If cells are well match they should all have similar current loaded voltage. If you see a variation of more than 20-30 mV between loaded cell voltages the cells are not well matched. If any slumps down to below 3.33v there is a very poor condition cell.

Thanks for the suggestion, I'll give it a shot.

 

[dpappyp]

Update:

I reset the BMS and the inverter showed it was charging again, but only at less than 1 amp (battery at 52.6 volts or so). So, I decided to try to discharge the battery first before charging back up. I disconnected the grid and solar so it was battery only and turned on the furnace fan. I was drawing up to 42A. Cell voltages stayed close to each other and nothing was getting warm. After maybe an hour BMS abruptly cut out again. No error status, charge and discharge disconnected, voltage difference between high and low cell is only 0.001 volts. I previously checked the BMS connections to all the cells and everything seemed secure. I think I have a faulty BMS. I swapped the BMS for the non-Bluetooth version I first started with. Now testing with that. If that cuts out too then it isn’t the BMS and back to the drawing board.

 

[dpappyp]

Old BMS performing fine, but when the battery was charges up there was one cell off from the others. All cells were at 3.30-3.32 volts while one was at 3.60. It was the first one in line connected as the positive, not sure if that means anything.

 

[Ampster]

not sure if that means anything.

It could mean either the pack is not balanced or that one cell has lower capacity.

 

[dpappyp]

It could mean either the pack is not balanced or that one cell has lower capacity.

Sorry, I meant that the cell with the differing voltage happening to be right at the positive end of the bank meant anything.

 

[Ampster]

No worries. Location in a pack should have no influence on cell voltage.

 

[Berseker]

so how did you resolve this?

 

[dpappyp]

Bad BMS, hooked up my old one again.

 

[Samcat]

When setting BMS cells max and low V you should set it example 2.7V low 3.450 V high and set Inverter pack voltage to stop above this limits. Example 3.000 V X 16 = 48V low cut off for inverter.
3.45V X 16 = 55.200 V high voltage and if you have ability for absorption time set 3 hours and if tail current is available 0.8A .This way you will have inverter as #1 defense and BMS as #2 defense to keep your cells in check. Inspect all bus bars and re torque all terminals after normal current run for 1-2 hours. Wait for terminals to cool down. Cut off voltage for BMS cells low 3.000 V is not good settings. It will trigger more time than necessary protection. Use Inverter settings to cut off before BMS settings.