How bad are these voltages?

[walter-]

I build a 4S, 12V, 272Ah battery bank.

I charged the battery, did a capacity test and charged again.

This is now what the BMS shows:

It has total 0.25V difference between cells after loading.
Is this bad?
When I start discharging, the cells quickly equalize and difference becomes around 0.05V (I think about the measuring fault of the BMS).

Should I a top balance? Or is this fine?

One thing that surprised me: the BMS -daly smart 250A, is not activating the balancing: any idea why this is the case?
Balancing seems only active when charging or discharging...

Walter

 

[RGIvy]

This is reasonably standard for a Daly even with good cells.
You're not losing much capacity because at those voltages your SOC is very close to 100% (if you charged each cell individually to 3.65 they would probably all reach that in minutes).
TBH Daly balancing is not that good - it's about average. If it really bothers you (as id did me) go for an active balancer.

 

[walter-]

You mean an active balancer on top of the Daly BMS?

My reasoning is at the moment that the difference is not important as once the batteries have to work they allign to the same voltages quickly.
Does this reasoning make sense?
Also during the discharge test , the batteries stayed very close to each other (in the range of 0.1-0.01 volts).

Ps: active balancer: how is the daly balancing working differently then an active balancer?

 

[John Frum]

Should I a top balance? Or is this fine?

Its not good.

Did you top balance prior to your capacity test?
What was the result of your capacity test?
What was the delta when the pack was discharged?

 

[walter-]

Capacity test was 270A running at about 2kw/180A; limitations was the inverter stopping at 10.4V;
At the end of the test the batteries where at close to each other voltages.... (with 0.05V). Which actually suprised me.

Top balancing: best in series at 12V (or higher) or in parralel once at 3.5-ige volts?

 

[John Frum]

Capacity test was 270A running at about 2kw/180A; limitations was the inverter stopping at 10.4V;
At the end of the test the batteries where at close to each other voltages.... (with 0.05V). Which actually suprised me.

I'm gonna guess that you didn't top balance even though you didn't explicitly answer the question.
Suggest you top balance and repeat the test.

Top balancing: best in series at 12V (or higher) or in parralel once at 3.5-ige volts?

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To get the paper, click on the orange at the top of this page. This is a tutorial with detailed steps on how I top-balance my LiFePO4 Cells using a low-cost benchtop power supply. To get the tutorial, click on the orange "Download" button...

diysolarforum.com

 

[walter-]

I'm gonna guess that you didn't top balance even though you didn't explicitly answer the question.
...

No, indeed; I used a dc-dc charger to charge without the BMS first. That made a decent 'top' with equal cells but not a formal top balance...

 

[John Frum]

No, indeed; I used a dc-dc charger to charge without the BMS first. That made a decent 'top' with equal cells but not a formal top balance...

Sorry not following.
What does "made a decent 'top' with equal cells but not a formal top balance" mean?

 

[RGIvy]

You mean an active balancer on top of the Daly BMS?

Yes, it's a controversial thing but quite a few people go for an active balancer.

My reasoning is at the moment that the difference is not important as once the batteries have to work they allign to the same voltages quickly.
Does this reasoning make sense?
Also during the discharge test , the batteries stayed very close to each other (in the range of 0.1-0.01 volts).

Ps: active balancer: how is the daly balancing working differently then an active balancer?

It's quite common for all the cells to be very similar voltage during discharge and charge, and only go to different voltages at close to 100% SOC or 0% SOC.
Even if they have been top balanced!
If my cells are at, say 75% SOC (discharging or charging), my delta is often around 0.05 or less. Sometimes around 0.001v delta. It's only in the last few minutes that the delta increases. One of my cells always reaches my cutoff before the others with around a 0.25v delta.

If I charge quite rapidly, say above 10a, the delta grows very quickly to around 0.25v
If I charge much more slowly, say around 3a, the BMS seems to have more "time" to do some balancing. As it's not an active balancer all it's doing is running charge out of my highest cell.

All I'm saying is that while a delta of 0.25 is not ideal it's not that uncommon when close to a 100% SOC. Unless it bothers you ignore it. If it does bother you then consider an active balancer which will balance more rapidly, probably start the balancing much sooner than your Daly and continue balancing after the Daly has stopped allowing charge and therefore stopped balancing. The active balancer will continue transferring charge from highest to lowest cell after the Daly has stopped allowing charge.

But as others have said, it's really important that you do a complete top balance because this may help with the delta you have.

 

[RGIvy]

The other thing I'd add is to make sure your connections are OK.
Many people have had unacceptable deltas and have stripped everything off their cells and rebuilt - to find that this eliminated their delta.
Important to ensure your busbars are level and clean, and that terminals are clean. Tighten everything on properly.

...but only after a good top balance!

 

[John Frum]

Independently verify cell voltages and balance lead integrity by walking the pins and the jst connector with your dvom.

 

[Ampster]

The other thing I'd add is to make sure your connections are OK.
Many people have had unacceptable deltas and have stripped everything off their cells and rebuilt - to find that this eliminated their delta.

I am one of those "many people" although I only went as far as replacing the cell I thought was bad. It was still a pain getting tha cell in balance with the others. As you also say in an earlier post the big deltas only show up near the top and bottom. The top is a good place to burn off some capacity with a resister to get them more in balance once you know your connections are good and your readings are confirmed.

 

[John Frum]

I am one of those "many people" although I only went as far as replacing the cell I thought was bad. It was still a pain getting tha cell in balance with the others. As you also say in an earlier post the big deltas only show up near the top and bottom.

With an effective top balance the big delta should be at the bottom and a relatively small delta at the top.

All that is required to top balance the cells is to individually charge each cell to 3.65 volts as measured at the cell terminals at a consistent current.
It can even be done without breaking the pack.

 

[walter-]

Sorry not following.
What does "made a decent 'top' with equal cells but not a formal top balance" mean?

I meant that I had charged the battery and they all had similar voltage. It looked like an OK to balance.

 

[John Frum]

I meant that I had charged the battery and they all had similar voltage. It looked like an OK to balance.

You description is still ambiguous.
Did you charge the cells individually or in parallel to 3.65 volts?
That is a top balance.

Did you charge the pack in serial configuration to system voltage?
That is not a top balance.

 

[walter-]

But now I realized; I had charged initially with my inverter; this one resulted in the cells being balanced when it stopped charging. The inverter is an Ective SSI 20, charging at 20A;
And now with a dc-dc convertor- renogy 50- and if was charging at 35A . At the end the BMS stopped the charging; at that moment charging was still at 20-25A.
With the inverter , the inverter stopped the charging. And it stoppen after some time charging at low current - 1A and many lower.

Is it normal that the BMS stops the charging and not the charger?
For the DC-DC charger, it stopped when it was at about 13.8 volt before one of the cells hit the 3.6V max the BMS is configured.

 

[walter-]

You description is still ambiguous?
Did you charge the cells individually or in parallel to 3.65 volts?
That is a top balance.

Did you charge the pack in serial configuration to system voltage?
That is not a top balance.

I charged the battery; not cells.

 

[John Frum]

Is it normal that the BMS stops the charging and not the charger?

That indicates your cells are not balanced.

 

[walter-]

I will indeed once do a proper top balance and see the difference.

 

[John Frum]

I will indeed once do a proper top balance and see the difference.

Please report back with test results.
Capacity (should be improved)
Delta after discharge should be higher.
Delta after recharge should be lower ideally <10mv.