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Can I be lazy when I top ballance my batteries?

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MNotLyon

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Sep 8, 2020
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I've built six 12 volt Lithium Iron batteries. Each has it's own BMS.
Do I have to remove the bms's and the bus bars when I top ballance, or can I just leave them, remove all loads and add some temporary cables to link them all in parallel while I charge them at 3.65 volts?
 
MNotLyon said:
add some temporary cables to link them all in parallel
Click to expand...
If you do this while the batteries are built up in series, please wear goggles and film it. The show will be dangerously spectacular.

Seriously, they do not need to be lined up neatly if that is what you are asking. But the series connections and BMS need to be removed first, before making parallel connections.
 
Some folks here think top balancing is overrated. I think that a lot of people damage their cells top balancing and capacity testing.
If you have them built up and the cells are all at similar voltages you should consider giving them a go as is.
Keep a VERY close eye on them near the upper and lower charge states until you are sure they are behaving.
 
MisterSandals said:
Some folks here think top balancing is overrated. I think that a lot of people damage their cells top balancing and capacity testing.
If you have them built up and the cells are all at similar voltages you should consider giving them a go as is.
Keep a VERY close eye on them near the upper and lower charge states until you are sure they are behaving.
Click to expand...
Put another way. Give the bms a chance to top balance the cells.
If they are too unbalanced then do it the laborious way.
 
MisterSandals said:
Some folks here think top balancing is overrated. I think that a lot of people damage their cells top balancing and capacity testing.
If you have them built up and the cells are all at similar voltages you should consider giving them a go as is.
Keep a VERY close eye on them near the upper and lower charge states until you are sure they are behaving.
Click to expand...
Thanks, I'm still learning.

I have put my cells into service. I'm not happy with the way they are performing when I charged them. They went way out of balance, even though they were very close before I started charging them. That's why I decided to try top balancing. I'm hoping that fixes my problem.

Presently, I'm only charging to 13.3 volts, and they have balanced themselves very well. My thought was to run them like that until my charger shows up.
 
smoothJoey said:
Put another way. Give the bms a chance to top balance the cells.
If they are too unbalanced then do it the laborious way.
Click to expand...
That made me very nervous. The first time I charged, the bms shut the battery down 9 times because one cell went over voltage.
 
MNotLyon said:
That made me very nervous. The first time I charged, the bms shut the battery down 9 times because one cell went over voltage.
Click to expand...
The bms did the important thing correctly.
Do you recall the cells voltages when the bms tripped?
Did you leave the cells to settle afterward?
If so for how long?
And what were the cell voltages after resting?
 
smoothJoey said:
The bms did the important thing correctly.
Do you recall the cells voltages when the bms tripped?
Did you leave the cells to settle afterward?
If so for how long?
And what were the cell voltages after resting?
Click to expand...

The BMS shut off the battery when the highest cell reached 3.65 volts. It keeps the battery off until the cell drops to 3.5 volts. That happened within a couple of minutes the first few times it did it. It took quite a while the last time.
The other cells were somewhere around 3.3 volts.

I did not let the cells rest after charging. Instead, I changed the charger's settings to float at 13.3 volts. Presently, all cells are within .003 volts.
 
MNotLyon said:
The BMS shut off the battery when the highest cell reached 3.65 volts. It keeps the battery off until the cell drops to 3.5 volts. That happened within a couple of minutes the first few times it did it. It took quite a while the last time.
The other cells were somewhere around 3.3 volts.

I did not let the cells rest after charging. Instead, I changed the charger's settings to float at 13.3 volts. Presently, all cells are within .003 volts.
Click to expand...
The cells appear to be middle balanced.
That's not a thing.
 
Label your batteries and keep track of everything of note. I recommend using letters as cell labels. This helps because you may be noting the positions a cell is in, which is a number.
“Cell A in position 3 hit 3.65v causing BMS shutdown. Other cells at 3.3v”

Good notes will help find patterns and hopefully a cause and solution down the road.

Fwiw, its pretty common for cells to go divergent REALLY quickly above 3.4-3.5v.
Same for discharging down below 2.9v.

From what you’ve described, i think its time to strongly consider a true top balance. You initial post did not contain any of your prior experiences with the cells/battery.
 
MisterSandals said:
Some folks here think top balancing is overrated. I think that a lot of people damage their cells top balancing and capacity testing.
If you have them built up and the cells are all at similar voltages you should consider giving them a go as is.
Keep a VERY close eye on them near the upper and lower charge states until you are sure they are behaving.
Click to expand...
so for example, I'm getting a set of 8 batteries, if i measure all the batteries, they are all 3.3v +/- 0.01v can I just run them as is? Well band 4 together and make a 12v cell. Add the BMS and run them without any balancing? I see many posts and videos about getting new batteries and ballancing them before using.
 
MisterSandals said:
Label your batteries and keep track of everything of note. I recommend using letters as cell labels. This helps because you may be noting the positions a cell is in, which is a number.
“Cell A in position 3 hit 3.65v causing BMS shutdown. Other cells at 3.3v”

Good notes will help find patterns and hopefully a cause and solution down the road.
Click to expand...
great idea... thanks, I will when i get mine.
 
MisterSandals said:
Fwiw, its pretty common for cells to go divergent REALLY quickly above 3.4-3.5v.
Same for discharging down below 2.9v.
Click to expand...
Is it safe to assume this could be worse if you're dumping a lot of amps at the batteries?
I'm running duel victron multipluses. They can dump 180 amps total at my battery bank, which is 6 batteries of 190 amp hours.
 
jvbutter01 said:
so for example, I'm getting a set of 8 batteries, if i measure all the batteries, they are all 3.3v +/- 0.01v can I just run them as is? Well band 4 together and make a 12v cell. Add the BMS and run them without any balancing? I see many posts and videos about getting new batteries and ballancing them before using.
Click to expand...
That sounds like what my batteries looked like (bank of 6 @ 12 volts). They got pretty silly when they got near full charge the first time.
I've let them set at 13.3 volts for a couple of days, and they seem very well balanced. I'm bringing them slowly up to 13.5 volts now, and they're still doing great. I'm still waiting on a charger to show up so I can top ballance, but I'm wondering if I bring them up slowly if I'll really need to.....
 
jvbutter01 said:
so for example, I'm getting a set of 8 batteries, if i measure all the batteries, they are all 3.3v +/- 0.01v can I just run them as is? Well band 4 together and make a 12v cell. Add the BMS and run them without any balancing? I see many posts and videos about getting new batteries and ballancing them before using.
Click to expand...

The battery voltage doesn't mean anything when it comes to parallel top balancing. The SOC of the batteries could be vastly different when you receive them even if all the voltages are equal.
 
MNotLyon said:
I'm bringing them slowly up to 13.5 volts now, and they're still doing great. I'm still waiting on a charger to show up so I can top ballance, but I'm wondering if I bring them up slowly if I'll really need to.....
Click to expand...

I am wondering as well. Please keep us posted.
 
Will's latest:

diysolarforum.com

"New, Class A" batteries stuck on lower voltage

All the time. Most of the 12 volt gear I work on can pull up to 20 amps. LED power supplied, Car audio gear. Relay drivers. Can you recommend a solid linear 20 Amp bench power supply for under $400? That would be great.
diysolarforum.com diysolarforum.com

For perspective, a 70mA bms balancing current (or resistor bleed-off) can consume 0.07*24 = 1.7Ah/day.

Smoov's BMS balancing tutorial (for 24V, adjust for 12V/48V accordingly):

diysolarforum.com

Big battery A123 24v cells

@MrNatural22 . Any update regarding your battery replacement? I am facing same problem right now. One of the my A123 batteries have two weak cells (tested 4 charge-discharge cycles), communicating with BigBattery right now.
diysolarforum.com diysolarforum.com
 
MNotLyon said:
Is it safe to assume this could be worse if you're dumping a lot of amps at the batteries?
Click to expand...
My gut feeling is yes.
Also, by slamming a lot of current in the battery(s) you give the BMS less time to balance.

Since you have a lot of cells, you can rearrange them into groups that perform similarly so the charge and discharge is more balanced.

Perhaps running a few charge and discharge cycles will reveal enough outliers with similar “rogue-ness” (aka lower capacity) to put together.
I did this with 12 cells with pretty good results forming 12v batteries (206ah).

Again, its important to have good notes and sound testing with cell position rotation (randomness with small sample sizes).

Possibly a lot easier with a quality internal resistance meter but i do not have good one ($$$).
 
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