What is my BMS telling me about my cells?

[PipHelix]

I purchased 4 cells from Ezealco back in February. After 6 or 8 weeks of ownership, I top balanced them to 3.65V and installed the battery in my camper with an Overkill BMS and a 30 amp Victron DC to DC charger. At rest, there’s generally a 6mV-9mV spread between the lowest and highest cells. However, when I charge the battery, two of my cells get up to 3.5V before my other two can even reach 3.4V. The cell that hits 3.5V first will trigger the over volt warning on the BMS and charging stops so the cells never re-balance.

I’m not sure if I have mismatched cells and I need to order new ones or if I need to rebalance or something else.

Here are some screen shots for some data points. The first is after sitting overnight before driving






This is after charging during a drive




This is after sittting overnight after the drive above





And this is after driving for about 9 mins

 

[AshleyL]

.....The BMS is telling you that your top-balancing is a failure.
Your jbd-sp04s020 only have passive balancer. My question, how long do you think it will take to balance those cells with only 50-60mA current?

Can you post your settings as well?

 

[time2roll]

I would add an active balancer. And even these are mostly slow but 10x the speed of that passive balance.

I assume even with the given number the battery will produce very close to the rating.

To really check relative capacity run the battery down until a cell is hitting 2.6v 2.7v range and see where they all stand. If the high cells at full become the low cells at discharge.... this would tell there is a capacity issue.

 

[PipHelix]

.....The BMS is telling you that your top-balancing is a failure.
Your jbd-sp04s020 only have passive balancer. My question, how long do you think it will take to balance those cells with only 50-60mA current?

Can you post your settings as well?

Here are my BMS settings. I haven’t made any changes to whatever Overkill does. I did notice that it says to only balance while charging which won’t help if I have two cells that trigger the over voltage shutoff.

 

[PipHelix]

I would add an active balancer. And even these are mostly slow but 10x the speed of that passive balance.

I assume even with the given number the battery will produce very close to the rating.

To really check relative capacity run the battery down until a cell is hitting 2.6v 2.7v range and see where they all stand. If the high cells at full become the low cells at discharge.... this would tell there is a capacity issue.

I did the initial top balance with a variable power supply set to 3.65V and 10 amps. The pack sat for 5 or 6 days and I turned off the power supply when the amps dropped well below 1….probably somewhere around .5 amps. I probably didn’t wait long enough at the low amperage because I wanted to get the pack installed.

Should I re-do the top balance with the variable power supply as well as add an active balancer?

 

[AshleyL]

Here are my BMS settings. I haven’t made any changes to whatever Overkill does. I did notice that it says to only balance while charging which won’t help if I have two cells that trigger the over voltage shutoff.

View attachment 218111View attachment 218112

In the screenshot, now we know the problem..........this is Lifepo4 cell, correct? Your current setting just undone your top balance.

Cell full voltage should be 3.45v to 3.65V (Depending on how high you want go, but I suggest 3.6v)
The balancer start voltage = 3.45v (reason, only at this voltage level that lifepo4 exhibit the deep steep curve that indicates it is charged)
And you should turn off "balance only when charging".

I did the initial top balance with a variable power supply set to 3.65V and 10 amps. The pack sat for 5 or 6 days and I turned off the power supply when the amps dropped well below 1….probably somewhere around .5 amps. I probably didn’t wait long enough at the low amperage because I wanted to get the pack installed.

Should I re-do the top balance with the variable power supply as well as add an active balancer?

Yes, you should re-do your top balance.
No, active balancer is not needed in this case.

 

[PipHelix]

In the screenshot, now we know the problem..........this is Lifepo4 cell, correct? Your current setting just undone your top balance.

Cell full voltage should be 3.45v to 3.65V (Depending on how high you want go, but I suggest 3.6v)
The balancer start voltage = 3.45v (reason, only at this voltage level that lifepo4 exhibit the deep steep curve that indicates it is charged)
And you should turn off "balance only when charging".



Yes, you should re-do your top balance.
No, active balancer is not needed in this case.

Thanks for your reply and your suggestions. Yes, this is definitely a Lifepo4 battery. I was led to believe that the default settings on the Overkill BMS were already optimized. I guess not!

I’ll pull the battery and start re-doing my top balance.

 

[Quattrohead]

Turn off "balance only when charging".

 

[time2roll]

Should I re-do the top balance with the variable power supply as well as add an active balancer?

I don't recommend pulling the pack apart. Active balancer works but takes time. Active balancer is only working when there is a differential of maybe 20+ mV so not when the battery is sitting at 40% to 80% unless way way out of balance. Need to hold at full charge several hours a day to make significant progress at the beginning.

Otherwise OK to charge the battery and boost charge the low cells individually until they slightly exceed the top cell. This can be done while everything remains connected and operating.

Consider both if in a rush. Hopefully the active balancer can better maintain the cell voltages.

 

[AshleyL]

I don't recommend pulling the pack apart. Active balancer works but takes time. Active balancer is only working when there is a differential of maybe 20+ mV so not when the battery is sitting at 40% to 80% unless way way out of balance. Need to hold at full charge several hours a day to make significant progress at the beginning.

Otherwise OK to charge the battery and boost charge the low cells individually until they slightly exceed the top cell. This can be done while everything remains connected and operating.

Consider both if in a rush. Hopefully the active balancer can better maintain the cell voltages.

Oh yes, this too.
Perhaps OP should install active balancer after all. Reason being each cell has its own self-discharge rate. If the rate is higher than the BMS balancing current, then those cell will be never be "balanced" or take forever to balance.

Time to get programable NEEY 4Amp active balancer?
.......Unless if OP prefer to buy another BMS with integrated 2Amp active balancer instead?

 

[time2roll]

In the screenshot, now we know the problem..........this is Lifepo4 cell, correct? Your current setting just undone your top balance.

Cell full voltage should be 3.45v to 3.65V (Depending on how high you want go, but I suggest 3.6v)
The balancer start voltage = 3.45v (reason, only at this voltage level that lifepo4 exhibit the deep steep curve that indicates it is charged)
And you should turn off "balance only when charging".



Yes, you should re-do your top balance.
No, active balancer is not needed in this case.

The balancing does not start until both 15mV delta and 3.400 mV cell voltage. So in the first screen shot the delta was only 7 mV and no balance is running at 93% charged.

 

[Pierre]

I would add an active balancer. And even these are mostly slow but 10x the speed of that passive balance.

I assume even with the given number the battery will produce very close to the rating.

To really check relative capacity run the battery down until a cell is hitting 2.6v 2.7v range and see where they all stand. If the high cells at full become the low cells at discharge.... this would tell there is a capacity issue.

I agree. OP , get yourself a 5amp Heltec active cell balancer. I have one running on a 48v system with a JBD bms. Much against popular opinion, it is connected 24/7 and has been running like that for 18 months. The balancing function on the JBD’s is a non-event in any case.

 

[PipHelix]

Okay, if I’m understanding things correctly, I should

1. turn off the “balance only when charging” setting on my BMS

2. add an active balancer to keep things in balance over the long term

However, my cells are not so out of balance that these balancers will work right now so I should

3. charge the lower cells individually and use the active balancer to keep things balanced?

 

[Brett V]

I did the initial top balance with a variable power supply set to 3.65V and 10 amps. The pack sat for 5 or 6 days and I turned off the power supply when the amps dropped well below 1….probably somewhere around .5 amps. I probably didn’t wait long enough at the low amperage because I wanted to get the pack installed.

Should I re-do the top balance with the variable power supply as well as add an active balancer?

When you did the top balance, did you measure the voltage with a decent digital volt meter or rely on the display on the variable power supply? The display isn't always accurate in my experience. you could individually charge each cell to 3.65 without taking everything apart. Shouldn't take too long since your pack is mostly charged.

 

[TomC4306]

I did the initial top balance with a variable power supply set to 3.65V and 10 amps. The pack sat for 5 or 6 days and I turned off the power supply when the amps dropped well below 1….probably somewhere around .5 amps. I probably didn’t wait long enough at the low amperage because I wanted to get the pack installed.

Should I re-do the top balance with the variable power supply as well as add an active balancer?

When you performed this top balance, were the four cells connected in parallel? All positives connected together and all negatives connected together.
These 4 cells would probably be better balanced if they went untouched out of the box.
Whatever your top balance method was created a mess.

 

[AshleyL]

Okay, if I’m understanding things correctly, I should

1. turn off the “balance only when charging” setting on my BMS

2. add an active balancer to keep things in balance over the long term

However, my cells are not so out of balance that these balancers will work right now so I should

3. charge the lower cells individually and use the active balancer to keep things balanced?

1. Yes. Remember 3.45v start balance.
2. Perhaps, depend on your cell quality though.
3. For now, just manually top balance all those cells one by one again and see how it goes with new BMS settings.

 

[mikefitz]

Before you consider adding an active balancer or carrying out a top balance there are alternative techniques that will help the cells balance.
Check over the cell interconnections and the balance lead crimp connections, re torque connections. The oxide on the cell terminals can cause issues, as can imperfect buss bar surfaces.
Set the charge voltage, absorbtion equal to float, to just below the point where protection takes place. Use the battery for a few days.
Whilst the battery is under charge above the point where the cell volts start to diverge, manually discharge the high cell whilst monitoring the cell volts, use a suitable DC load, ( 48 watt car headlamp filiment bulb).

 

[PipHelix]

When you did the top balance, did you measure the voltage with a decent digital volt meter or rely on the display on the variable power supply? The display isn't always accurate in my experience. you could individually charge each cell to 3.65 without taking everything apart. Shouldn't take too long since your pack is mostly charged.

I used my Fluke VOM to verify the voltage.

It wouldn’t be too much of a hassle to pull the whole pack and bring it indoors for individual cell charging. That sounds like a good way to solve my problems while also keeping the pack together.