Balancing issue

[boondox]

Hi All,

I have a 4S, 1200wH pack of Fortune 100aH aluminum case batteries. A couple of weeks ago a left on load dropped the battery pack to where the BMS shut it off. Oops. I charged it back up and a week or so later I looked at the BMS software (the one in this thread https://diysolarforum.com/threads/f...4-bms-with-low-temp-charging-protection.2884/). The battery voltages which had always been spot on are now pretty far off:

1 - 3.370
2 - 3.338
3- 3.370
4- 3.360

In the "battery view" the column marked "balancing" is blank. The only two parameters I see in the BMS software that relate to balancing are:
Balanced turn on 3.4 volts
Balanced precision .03

Since the batteries are under 3.4V they won't be trying to balance. Should I change that to 3.3v? I'm guessing I need to pull the pack apart and top balance them. As luck would have it I am getting ready to go on a week long trip using the batteries and it would throw a wrench in the works if I need to pull the pack apart and top balance it. Can I get away with doing it when I get back?

Thanks!

 

[John Frum]

What is the low cell cutoff?
What is the low pack cutoff?
There is only ~40mv differential between the high and low.
IMO That is not to shabby at all.
Your bms won't even try to balance if the differential is <30mv.
You should enable balancing on the charge cycle only and charge to the top.
That is 3.65 volts per cell for a typical lifepo4 cell(check your specific doco).
Make sense?

 

[boondox]

@snoobler I am at work for a 48 hours shift, I will check the low pack and cell cut offs and report.

Good to hear that the cells are not too far out of whack. This is my first LFP battery pack and I appreciate the help. I didn't see a way to enable balancing during the charge cycle only, unless that is through the "Balanced turn on voltage". I will look at all the parameters when I get home. I'll check the battery spec sheet and if appropriate I will up the "balanced turn on voltage" to 3.65.

Yes, I *think* it makes sense. You spoke slowly and used small words so I at least stood a chance!

 

[AussieSim]

@snoobler I am at work for a 48 hours shift, I will check the low pack and cell cut offs and report.

Good to hear that the cells are not too far out of whack. This is my first LFP battery pack and I appreciate the help. I didn't see a way to enable balancing during the charge cycle only, unless that is through the "Balanced turn on voltage". I will look at all the parameters when I get home. I'll check the battery spec sheet and if appropriate I will up the "balanced turn on voltage" to 3.65.

Yes, I *think* it makes sense. You spoke slowly and used small words so I at least stood a chance!

I would set the balance turn on voltage around 3.39

 

[Bob142]

I will look at all the parameters when I get home. I'll check the battery spec sheet and if appropriate I will up the "balanced turn on voltage" to 3.65.

This is your milk crate power cube, right? (I'm the other milk crate guy.)

I have the same BMS on a 4S 280Ah cell pack. I suggest you leave the Balance Turn on Voltage set to 3.4V (where you have it now). Unless you changed settings from the defaults, the balancer should be enabled on charging. When you start charging, the BMS will try to keep the cells that are at a higher voltage from running away from the others. That's when you'll see an indication that a cell is "balancing".

 

[John Frum]

I suggest you leave the Balance Turn on Voltage set to 3.4V (where you have it now). Unless you changed settings from the defaults, the balancer should be enabled on charging.

I suggest to leave balancing totally disabled until you see if the cells with hang together(not drift further apart) over a discharge/charge cycle.

 

[snoobler]

@snoobler I am at work for a 48 hours shift, I will check the low pack and cell cut offs and report.

Good to hear that the cells are not too far out of whack. This is my first LFP battery pack and I appreciate the help. I didn't see a way to enable balancing during the charge cycle only, unless that is through the "Balanced turn on voltage". I will look at all the parameters when I get home. I'll check the battery spec sheet and if appropriate I will up the "balanced turn on voltage" to 3.65.

Yes, I *think* it makes sense. You spoke slowly and used small words so I at least stood a chance!

Did you actually confuse me for smoov????

FWIW, I agree with him completely.

Though, I am concerned that he hasn't changed his avatar for what feels like forever.

 

[Bob142]

You should enable balancing on the charge cycle only and charge to the top.

I suggest to leave balancing totally disabled until you see if the cells with hang together(not drift further apart) over a discharge/charge cycle.

Does not compute.

 

[John Frum]

Does not compute.

I'm glad you said something.
If you don't get it other won't too.
For the first test we want to see how the cells diverge/converge without the bms interfering.
After that then sure turn on the balancing during charge above the resting full voltage which in turn should be above float.

 

[snoobler]

@smoothJoey

Thank you Mr. Thursten Howell, III.

 

[boondox]

Dang, Sorry SJ! Dunno how I mistook that jerk snoobler for someone as cool as Thurston Howell III.

Sorry Snoobler, couldn't help myself.

Thanks for all the advice gents. Leaving the balance off and see how it does for a cycle makes sense to me. Since I am going for a trip to the desert next week I will see how it does over a week. I am assuming the purpose is to see how well the cells are aging together? I have only a couple of cycles on the pack and didn't really properly top balance them so it will be an interesting data point.

 

[boondox]

This is your milk crate power cube, right? (I'm the other milk crate guy.)

I have the same BMS on a 4S 280Ah cell pack. I suggest you leave the Balance Turn on Voltage set to 3.4V (where you have it now). Unless you changed settings from the defaults, the balancer should be enabled on charging. When you start charging, the BMS will try to keep the cells that are at a higher voltage from running away from the others. That's when you'll see an indication that a cell is "balancing".

Yes, it is the milk crate power cube. So you don't think letting them drift and see what they do makes sense?

 

[Bob142]

Yes, it is the milk crate power cube. So you don't think letting them drift and see what they do makes sense?

I'm certainly not against that if you have time. I was reacting to your comment about changing your balance turn on voltage when it seemed like @smoothJoey was suggesting you turn on balancing and charge your pack to full. I didn't want you to change your balance turn on voltage to 3.65V because that's a fully charged cell so the BMS wouldn't have much opportunity to balance.

 

[Bob142]

I have only a couple of cycles on the pack and didn't really properly top balance them so it will be an interesting data point.

Just noticed that part. Now I'm not sure what you're going to learn by doing a discharge/charge cycle without BMS balancing other than what you already know - that they likely need a proper top balance to keep them closer together than you have them.

 

[John Frum]

Just noticed that part. Now I'm not sure what you're going to learn by doing a discharge/charge cycle without BMS balancing other than what you already know - that they likely need a proper top balance to keep them closer together than you have them.

I just want to see a discharge/charge cycle to see how the diverge/reconverge without the balancer interfering.
If that has already been done great.