Cell deviation during discharge after top balance

[ericfx1984]

So I was bored yesterday and I decided to use my EG4 chargeverter to charge my 16S, 280 amp hour Eve cell to 57 volts

Once that was done I disconnected the BMS and the bus bars and reconfigured in 16P

From there I used my bench power supply to charge all the cells to 3.65 and held them there until the current was 0.033 amps...

From there I reattached the bus bars and BMS, and the cell deviation was about 0.00x volts, and started the inverters... I had a load of about 200 to 400 watts

But I noticed is that at about the 3.4 mark I had a cell deviation of 0.037v

The majority of the cells were within 0.01x



Any cause for concern?

 

[sunshine_eggo]

What's it look like now, 40 minutes after your post (assuming you left it running that way).

A small load like that may take a bit to pull the cells down into the operating range. I would expect them to be < 10mV 40 minutes later.

Any time voltage readings are a concern, it's important to confirm them with a separate meter.

 

[ericfx1984]

What's it look like now, 40 minutes after your post (assuming you left it running that way).

A small load like that may take a bit to pull the cells down into the operating range. I would expect them to be < 10mV 40 minutes later.

Any time voltage readings are a concern, it's important to confirm them with a separate meter.

0.007v

 

[sunshine_eggo]

there you go.

Under load and BELOW 3.4V is where you expect the cells to come together. The upper and lower knees of the voltage curve are consistently inconsistent.

 

[ericfx1984]

there you go.

Under load and BELOW 3.4V is where you expect the cells to come together. The upper and lower knees of the voltage curve are consistently inconsistent.

Consistently inconsistent I like that

 

[ericfx1984]

Well so far the results aren't all that great... I'm charging to about 3.45 volts for cell

Everything looked pretty good until a little after 3.41 volts per cell... Then number 16 kept pulling ahead until it got to almost 3.51... While cell number 14 pretty much stayed at 3.41 for quite a while

The other cells raised at a pretty predictable rate and were within about 0.02 X of each other

By the time The EG4 started to drop in current, we started to see the Delta closing... Sell 14 had started to rise, while sell 16 had started to drop ever so slightly. Right now we have about five amps coming in and every cell, including cell 16 has a pretty close Delta... Cell 14 is still lagging behind some... I'm going to try holding it and letting the battery management system do its thing. I think I'm sitting at a 0.06x Delta

I think I'll just keep an eye on it. I'm not completely worried about it... I'm sure it's close enough for government work... I may have to pull everything apart and do a top balance again... But... Maybe an active balancer could be helpful to clean things up for a period of time

 

[ericfx1984]

 

[ericfx1984]

Well, by the time we got to zero amps going in this is where we finished... Just not sure how a proper top balance brought me to these results?

 

[ericfx1984]

Well it's looking a little better this charge... So far

 

[sunshine_eggo]

looks pretty luscious to me.

 

[ericfx1984]

looks pretty luscious to me.

You wanna hook up with my battery, lol

 

[ericfx1984]

I went ahead and held the battery at 55.4v. After a few hours we were at about 0.00x Delta...

 

[wpns]

Can you hold it at 55-ish volts and let it finish balancing?

 

[ericfx1984]

Can you hold it at 55-ish volts and let it finish balancing?

See post 13

 

[wpns]

See post 13

You mean 12. We must have posted nearly simultaneously

 

[ericfx1984]

You mean 12. We must have posted nearly simultaneously

Hehehe yeah

 

[upnorthandpersonal]

One thing to look out for are bad connections. I've recently made a post that could help find these in case you have multiple packs in parallel.

Thread 'Detecting bad connections'

Sep 15, 2024

I was looking for a way to detect bad cell terminal connections in multiple parallel battery packs. I found out that this is actually very easy to spot if you graph the cell delta on the packs. I introduced a bad connection in one of my packs and let it run for a day. It went through charge and discharge, and seemed fine. No hot spots or anything. The arrow in the following screenshot show when the bad connection was fixed. Both screenshots are of the same data, just zoomed in.






Having bad connections in multiple parallel packs also show up with this kind of...

• upnorthandpersonal
• Replies: 9
• Forum: DIY LiFePO4 Battery Banks

• 
• 

While this won't work for a single battery, I would still suggest double checking any and all connections.

 

[AshleyL]

I went ahead and held the battery at 55.4v. After a few hours we were at about 0.00x Delta...

Now, without any load, when you stop the charging, which cell falls down to the rest voltage the fastest?

 

[ericfx1984]

Now, without any load, when you stop the charging, which cell falls down to the rest voltage the fastest?

No clue really... I'll have to plan a day to do that

 

[ericfx1984]

So I did another charge today... The results are a little better... Compared to yesterday anyway

Although the one thing that remains constant is that cell 14 takes the longest to charge

While other cells are sitting at 3.45 and above... Sell 14 is sitting at 3.18. And by the time we get to a slower current... It's sitting at 3.38

The good news is the deviation was not nearly as bad as yesterday... I'm curious what would happen if cell 14 was moved to a different spot in the pack

I suspect that this will kind of fix itself as it's held at a high state of charge and allowed to balance out

So far it seems to get a little better with each charge

 

[ericfx1984]

Additionally I'm noticing that it's taking significantly less time for the battery management system to balance the battery after a charge when compared to yesterday

 

[wpns]

Additionally I'm noticing that it's taking significantly less time for the battery management system to balance the battery after a charge when compared to yesterday

Good news, though that is kinda how it's supposed to work.

#ILoveItWhenAPlanComesTogether

 

[sunshine_eggo]

So I did another charge today... The results are a little better... Compared to yesterday anyway

Although the one thing that remains constant is that cell 14 takes the longest to charge

While other cells are sitting at 3.45 and above... Sell 14 is sitting at 3.18. And by the time we get to a slower current... It's sitting at 3.38

3.18?

Is this a typo? If not, confirm that voltage with a meter.

 

[ericfx1984]

3.18?

Is this a typo? If not, confirm that voltage with a meter.

Yep typo... Good catch 3.418

 

[sunshine_eggo]

Yep typo... Good catch 3.418

whew... 3.18 is a pretty low state of charge.

Possibilities:
1) it's just the lowest SoC cell.
2) it has a little more capacity than the others.
3) it loses charge slightly faster than the others.

With the numbers it's presenting, it doesn't matter which of the above it is. Normal use with regular 1-2 hours of absorption time above 3.40V should improve or at least maintain what you have.