How long to leave cells connected after top balancing ?

[squowse]

After the top balance, the voltages slowly settle down to around 3.4V ish. I’m wondering if it’s beneficial or not to leave them connected in parallel during this period? I can’t put them together in series for a week anyway.

16 Lishen 272Ah cells to make 16S battery.

 

[pollenface]

I would personally leave them connected

 

[squowse]

I would personally leave them connected

For how long? Till they settle to 3.4ish?

 

[42OhmsPA]

For how long? Till they settle to 3.4ish?

Doesn't matter, until your ready to use them. Image if you were building an xpxs pack... You'd have cells in parallel all the time.

 

[John Frum]

Disconnect them after the top balance is complete and leave them for a couple of weeks.
Its called a self discharge test and it will help you to determine if your pack will be viable and how much balance current you will required.

Since your cells have been abused you may have some duds.

 

[RCinFLA]

A rested open circuit cell voltage on an LFP cell above 3.43v is fully charged.

Charging until current taper off with an absorb voltage of 3.45v-3.65v will create a layer surface charge mostly in the negative graphite layer of cell. The charging overpotential voltage rise caused by charging current is traded for this surface charge when charging current tapers off.

This surface charge voltage has no significant capacity to cell. It amounts to about 0.01% of cell AH rating.

It will bleed off on its own. Depending on cell, it may take a few hours to a few days to bleed off on its own with no load placed on cell. You can put a load on cell, like one amp for 60 seconds, and it will bleed off most of the surface charge quickly, after which cell will settle down to 3.43-3.50v of rested open circuit voltage.

Many folks see this voltage drop after top balancing to 3.65v, with different cells showing between 3.45v and 3.65v and incorrectly think the cells are not balanced. If they stay above 3.43v open circuit voltage, they are fully charged and SoC balanced.

 

[squowse]

Disconnect them after the top balance is complete and leave them for a couple of weeks.
Its called a self discharge test and it will help you to determine if your pack will be viable and how much balance current you will required.

Since your cells have been abused you may have some duds.

Ive already weeded out a few weaker cells. This is balancing the replacements with the existing.

 

[squowse]

A rested open circuit cell voltage on an LFP cell above 3.43v is fully charged.

Charging until current taper off with an absorb voltage of 3.45v-3.65v will create a layer surface charge mostly in the negative graphite layer of cell. The charging overpotential voltage rise caused by charging current is traded for this surface charge when charging current tapers off.

This surface charge voltage has no significant capacity to cell. It amounts to about 0.01% of cell AH rating.

It will bleed off on its own. Depending on cell, it may take a few hours to a few days to bleed off on its own with no load placed on cell. You can put a load on cell, like one amp for 60 seconds, and it will bleed off most of the surface charge quickly, after which cell will settle down to 3.43-3.50v of rested open circuit voltage.

Many folks see this voltage drop after top balancing to 3.65v, with different cells showing between 3.45v and 3.65v and incorrectly think the cells are not balanced. If they stay above 3.43v open circuit voltage, they are fully charged and SoC balanced.

So do you think it makes any difference if I leave them connected whilst the surface charge dissipates?

 

[RCinFLA]

So do you think it makes any difference if I leave them connected whilst the surface charge dissipates?

Doesn't matter unless you have a bad cell that has a lot of true leakage due to internal metal dendrite growth. Separated this would show up quicker in weak cell. Together it will be masked by other good cells supporting the cell with excessive leakage current.

 

[zorlig]

Mimic your BMS settings, that's where they'll end up anyway.

I would do whatever was fastest, preferably skipping top balance all together.

The only time you really need to top balance is if you have no BMS, and in that's case match your charging profile.

 

[squowse]

Doesn't matter unless you have a bad cell that has a lot of true leakage due to internal metal dendrite growth. Separated this would show up quicker in weak cell. Together it will be masked by other good cells supporting the cell with excessive leakage current.

Sorry I thought I had replied. I decided to disconnect them from each other and monitor how they settled. They all settled slowly and fairly evenly ? thanks