Ok you win... Cell imbalance

[ericfx1984]

Well they're still not back to perfect balance... But every single charge cycle they seem to be a little closer before I was seeing nearly two-tenths of a volt now I'm seeing less than a 10th of a volt delta

 

[740GLE]

I’d be happy to see .010v delta if cells are above 3.45v.

 

[ericfx1984]

I’d be happy to see .010v delta if cells are above 3.45v.

I'm still thinking of taking this pack off line and doing a (re) top balance

 

[740GLE]

You’ll be chasing the impossible if you want everything perfect

 

[ericfx1984]

You’ll be chasing the impossible if you want everything perfect

Well that's what I thought... But then as I approached about 3.41... That same cell started running away again

 

[Quattrohead]

That will be your ginger headed child for the rest of your life LOL. I think if everyone really looked at their batteries there would be under and over performing cells in every single unit. That's why we have a BMS and balancing.

 

[ericfx1984]

That will be your ginger headed child for the rest of your life LOL. I think if everyone really looked at their batteries there would be under and over performing cells in every single unit. That's why we have a BMS and balancing.

I suppose the other option would be to lower my charge voltage from 13.8 to 13.6... The cells seem to track to a 0.00x Delta until just above 3.4 volts per cell

 

[740GLE]

How much SOC are you using each day? No harm in lowering the charge voltage a bit if you don’t need every bit of SOC to carry for a few days.

 

[ericfx1984]

How much SOC are you using each day? No harm in lowering the charge voltage a bit if you don’t need every bit of SOC to carry for a few days.

It's usually the evenings that cause me to use a lot of charge... I'll go through anywhere from 10-40% of my storage over the evenings

It really just has been rough because it's been so hot... I usually recover most of it

It's been very hot and cloudy the last 2 weeks and after 2 weeks of that I finally decided to run the generator... But even then I was only setting at about 50% state of charge

So missing out on a few percentage points of storage I won't really notice

I figure it will at least keep everything relatively safe until I can spend some time getting up to a higher state of charge and following the procedure that eggo mentioned

 

[ericfx1984]

So I took the big 4p4s pack offline...

Started out by first charging it to the point that cell number two started running away

Then I took it offline and attached the act of balancer

Then I attached my power supply and started by adjusting it to 14.1 volts and let it run for a few hours

I came back and found that the Delta was just under four 100ths of a volts... Quite an improvement from 2/10 of a volt

However I still found that cells 1,3,4 are pretty darn close... Only showing a couple thousands of a volt delta

But now, with all 4 cells hovering at the 3.5x range, cell number two is actually the lowest cell... Whereas before it was the runaway

 

[MisterSandals]

Then I attached my power supply and started by adjusting it to 14.1 volts and let it run for a few hours

Its a known issue that adjusting the power supply voltage while connected to a battery is very inaccurate. Who know what the voltage really was. I don't know if this had any side effects, just commenting on the procedure.

I was going to comment on one of my favorite topics of grouping parallel cells for use in a battery. The key issue is needing the 4 cells' capacities, in each 4P group to equal the combined capacities of the other groups.

Its clear that what you call "cell 2" has the lowest combined capacity. If you happen to have capacity tested your cells you could reshuffle cells to create equivalent groups. Otherwise grouping is rather hard without any data.

 

[ericfx1984]

Its a known issue that adjusting the power supply voltage while connected to a battery is very inaccurate. Who know what the voltage really was. I don't know if this had any side effects, just commenting on the procedure.

I was going to comment on one of my favorite topics of grouping parallel cells for use in a battery. The key issue is needing the 4 cells' capacities, in each 4P group to equal the combined capacities of the other groups.

Its clear that what you call "cell 2" has the lowest combined capacity. If you happen to have capacity tested your cells you could reshuffle cells to create equivalent groups. Otherwise grouping is rather hard without any data.

I should probably call it block 2

 

[Skypower]

Are you sure that some oxidation or corrosion hasn’t occurred between the terminal and buss? It was good for a year so the battery is still quite young but it’s plenty of time for a bad connection to happen.

 

[ericfx1984]

Are you sure that some oxidation or corrosion hasn’t occurred between the terminal and buss? It was good for a year so the battery is still quite young but it’s plenty of time for a bad connection to happen.

Honestly I don't think so... If I look at my notes it seems that this was an issue after I first assembled the pack... I think I was just in denial ?

 

[ericfx1984]

I'll let it rest for the evening...then disconnect the active balancer when I wake up in the morning...

 

[ericfx1984]

Everything is looking spectacular after an evening of rest

Basically I left the active balancer on while the output of the power supply was set to 14.6 volts... I also watched during the come up just to make sure that none of the cells were running away

During the process while everything was only a few thousands of a volt delta... One of the cells hit the high voltage disconnect which I expected would happen... From there the balancer just kind of did its work and from what I can see a couple times the battery dropped below that threshold and topped itself back off

Thank you all... But more importantly I'd like to give a special thanks to @sunshine_eggo

 

[ericfx1984]

I think I better understand what was happening... Because my cells oftentimes don't spend a lot of time at full charge they never actually have a chance to really balance out

Especially with just the BMS doing the balancing

Furthermore the fact that I wasn't spending a lot of time at a full charge and never giving this battery a chance to go to zero load has a lot to do with the results... It's virtually impossible for the active balancer to do its thing if you're constantly adding or taking away from the battery

 

[740GLE]

I’d say you got that last 1% of capacity out of it.

Now the question will be how many cycles/weeks/months before that delta starts to grow.

 

[ericfx1984]

I’d say you got that last 1% of capacity out of it.

Now the question will be how many cycles/weeks/months before that delta starts to grow.

Time will tell

 

[ericfx1984]

Well crap... It didn't solve anything... We still have cell block number two reaching 3.65v on the BMS (which is more like 3.575 accord to my volt meters) with the other blocks only reaching around 3.412v

It's not a huge difference in overall capacity... And the cell blocks stay within a few thousands of a volt until about 3.4xx volts per cell block

At some point I probably need to disassemble this pack into a capacity test... Especially on the cells in block number two

On another note the manufacturer of these cells has agreed to send me another cell only asking that I cover shipping costs which seems reasonable to me

I don't anticipate that this will be much of a problem once this pack is broke down and reconfigured to 16S... I think at that point the battery management system and the active balancer (If I choose to use one) should be more than capable of handling a 304 amp hour battery with a miss balance between cells

The problem right now is that a 1216 amp hour cell block is not particularly easy to balance...

I have a 30 amp discharge cell tester that is completely automated... I think I will fire that up and run All the cells through a capacity test once it cools down around here

I have also noticed that when I'm on a much slower rate of charge that I don't run into this runaway cell issue

For now I think I will just run the system at a lower charge voltage to avoid any runaway cell blocks

After all the difference between 3.4 volts and 3.65 volts really is only about 1% of capacity... And the difference between 3.4 and 3.4125 is probably damn near immeasurable with any equipment I own