Possible to top balance and stored alot of LiFePo4 cells? Hundreds of cells?

[zedconnor]

As the title says. I am a battery pack reseller in local.

Can I top balance the cells,
Then bring it down to 3.37Vpc(using EBK 40AL charger/discharger)
And store it? For maybe several months?

Reason for wanting to do this is so that I can quickly assemble the cells for a particular order without requiring alot of time for balancing and stuff.

All I want to do is the cells are around equal or very slightly different SoCs so that I can quickly assemble them.

Will it be harmful to the cells?
(PS: Currently there are cells that are not matched and their SoC maybe all over the place.)

 

[sunshine_eggo]

First, discharging to 3.37V is pointless, They will almost instantly discharge to that level.

The general wisdom is that Lithium cells shouldn't be fully charged and stored; however, Will recently tested a battery he left outside fully charged in Las Vegas (hot). He now recommends you fully charge them even when in a mostly standby use. Many LFP battery manufacturers recommend fully charging and recharging every 3-6 months (probably more about slow drain by the BMS).

Additionally, @RCinFLA has some comments on fully charging LFP in general:

resting SoC in Battery Backup system and battery health

If this has been discussed before I would love a link to the discussion ... I couldn't find it with several searches I tried. With LiFePo4 batteries what is the preferred resting SoC for a battery backup system ? Should I be cycling the battery every X days down to a certain SoC and then...

diysolarforum.com

Furthermore, the effects of any high SoC storage is mitigated with storage in cooler temps (room temp or lower).

IMHO, if you fully charge them (3.65V @ 0.05C tail current) and store for a few months in a climate controlled environment for the purposes of reduced time for top balance at assembly, it will be fine.

 

[zedconnor]

First, discharging to 3.37V is pointless, They will almost instantly discharge to that level.

The general wisdom is that Lithium cells shouldn't be fully charged and stored; however, Will recently tested a battery he left outside fully charged in Las Vegas (hot). He now recommends you fully charge them even when in a mostly standby use. Many LFP battery manufacturers recommend fully charging and recharging every 3-6 months (probably more about slow drain by the BMS).

Additionally, @RCinFLA has some comments on fully charging LFP in general:

resting SoC in Battery Backup system and battery health

If this has been discussed before I would love a link to the discussion ... I couldn't find it with several searches I tried. With LiFePo4 batteries what is the preferred resting SoC for a battery backup system ? Should I be cycling the battery every X days down to a certain SoC and then...

diysolarforum.com

Furthermore, the effects of any high SoC storage is mitigated with storage in cooler temps (room temp or lower).

IMHO, if you fully charge them (3.65V @ 0.05C tail current) and store for a few months in a climate controlled environment for the purposes of reduced time for top balance at assembly, it will be fine.

Damn you learn new and totally contradicting stuff everyday on this forum!

However, I got a problem with the cells dropping to resting voltage.

I have parallelled 10x 202Ah "CATL" cells. (Notice the quotes)
They were mostly at about ~3.25Vpc
And charged it for over 40+hours with the charger@40A (3.6V@0.05C EndCurrent)

The batteries got charged. And sitting at 3.51V.
I disconnected the charger. And I waited for almost two hours but the voltage never dropped below 3.5V (The cells are still in parallel. And I am also sure that the highest Volt they reached during charging was 3.55V as shown by the graph of the charger's PC software "EB Tester")

What do you make of this? Why hasn't the voltage come down quickly to rest?

 

[RCinFLA]

It can take several days to bleed down surface charge capacitance on an open circuited LFP cell. Full charge rested voltage for LFP is 3.43v. Anything above this voltage, at rested open circuit equilibrium, is surface charge that does not have significant capacity. It does not involve lithium-ion electrode to electrode discharge other than current carried through cell to discharge the capacitance. It is similar to a supercap.

If you want to speed up the surface capacitance charge bleed off, you can load the cell with about an amp for a minute or two. It will take a few minutes after the bleed down to equalize back to rested state.

Construction of lithium-ion battery is very similar to a supercapacitor. Once a lithium-ion battery is fully charged it acts like a supercapacitor for addition charging. Below full charge, most of the charging current goes into lithium-ion transfer from cathode to anode electrodes and very little goes into the inherent supercapacitance.

During actual lithium-ion transfer charging there is also a overpotential voltage bump up that drives the ion migration. This charging overpotential drive voltage decays out within several minutes after cell current is brought to zero. Any surface charge capacitance charge will remain for a few hours to several days depending on individual cell leakage rate.

If you fully charge a cell to 3.65v allowing charge current to drop off, the charging overpotential voltage drops off as cell charging current drops off and the surface super-capacitance is charged. You are effectively trading cell charging overpotential voltage for surface capacitance charging as charge current tapers down.

If you charge at low current rate, you create a smaller cell overpotential voltage bump up and the cell becomes fully charged at a terminal voltage slightly above 3.45v. No need to absorb to 3.65v to achieve full charge. Conversely, if you charge at a very high current rate, you create a high overpotential voltage to support the high cell current and if you immediately stop charging when cell hits 3.65v without allowing time for current to taper down, you will not be fully charged.

Many folks see this variance in surface charge bleed off rate between cells after top balancing and think the cells are not completely balanced. If after top balancing, all the cells stay above 3.45v for first 10-15 minutes after charging current is removed, they are all fully charged and top balanced even if open circuit cell voltage variance ranges from 3.45v to 3.65v.

 

[OffGridInTheCity]

Here's some charts to illustrate @RCinFLA's comment, especially about 'very little power' between max charge and rest....
Re: https://spheralsolar.com/lifepo4-voltage-chart/

 

[shvm]

I have parallelled 10x 202Ah "CATL" cells. (Notice the quotes)

So, basically one 'large' LFP Cell of 2020 Ah Capacity.

They were mostly at about ~3.25Vpc
And charged it for over 40+hours with the charger@40A (3.6V@0.05C EndCurrent)

40 Amps/2020 Ah give a C-rate of ~ 0.02 C. Because, this is already lower than 0.05 C end-current rating , cut-off needs to be done based on rising voltage.


The corresponding LFP cut-off voltage for 0.02 C is around 3.475V. As soon as, voltage rises to this value, charging is considered to be done.

The batteries got charged. And sitting at 3.51V.
I disconnected the charger. And I waited for almost two hours but the voltage never dropped below 3.5V (The cells are still in parallel. And I am also sure that the highest Volt they reached during charging was 3.55V as shown by the graph of the charger's PC software "EB Tester")

What do you make of this? Why hasn't the voltage come down quickly to rest?

Classic case of overcharge or accumulated surface charge whatever you want to call it as I have described above.

Basically the larger the capacity of cells you're trying to top-balance, the lower the cut-off voltage becomes.

Someone familiar with web programming could implement a handy web calculator for these values.
Users should be able to input their cell capacity, the number of cells in parallel, and top-balancing current;
and it should spit out the appropriate cut-off voltage.
I'm not that good unfortunately !

 

[zedconnor]

So, basically one 'large' LFP Cell of 2020 Ah Capacity.

40 Amps/2020 Ah give a C-rate of ~ 0.02 C. Because, this is already lower than 0.05 C end-current rating , cut-off needs to be done based on rising voltage.

View attachment 187759
The corresponding LFP cut-off voltage for 0.02 C is around 3.475V. As soon as, voltage rises to this value, charging is considered to be done.

Classic case of overcharge or accumulated surface charge whatever you want to call it as I have described above.

Basically the larger the capacity of cells you're trying to top-balance, the lower the cut-off voltage becomes.

Someone familiar with web programming could implement a handy web calculator for these values.
Users should be able to input their cell capacity, the number of cells in parallel, and top-balancing current;
and it should spit out the appropriate cut-off voltage.
I'm not that good unfortunately !

As soon as I put a very small load.(the charger can discharge as well.) The surface charge goes away quickly.

 

[MattiFin]

As the title says. I am a battery pack reseller in local.

Can I top balance the cells,
Then bring it down to 3.37Vpc(using EBK 40AL charger/discharger)
And store it? For maybe several months?

Reason for wanting to do this is so that I can quickly assemble the cells for a particular order without requiring alot of time for balancing and stuff.

All I want to do is the cells are around equal or very slightly different SoCs so that I can quickly assemble them.

Will it be harmful to the cells?
(PS: Currently there are cells that are not matched and their SoC maybe all over the place.)

To maximize the lifetime I guess you could top balance your cells, connect temporarily in series, draw off for example exactly 60Ah to get 300Ah cells to roughly 80% SOC.
Later on when you assemble your packs it should be 60Ah short off top balance.

Variation in self-discharge is going to ruin the balance at some point, as well as different time in storage. Cells top balanced and put to storage within 2 weeks of each other would be probably within 1% SOC

 

[wpns]

Can you pick a float voltage, and wire them all up in parallel to an appropriate voltage source (maybe with a one ohm resistor, or a diode to isolate them?). Then they would all be at the same state of charge when you take them off the shelf, no?

 

[OffGridInTheCity]

Can you pick a float voltage, and wire them all up in parallel to an appropriate voltage source (maybe with a one ohm resistor, or a diode to isolate them?). Then they would all be at the same state of charge when you take them off the shelf, no?

Not necessary. I've been working with 18650 cells for over 5 years now... as I build batteries I set the left-overs to ~3.7v and let them sit (good ambient - e.g. home temps/humidity). I've observed that even after 2, 3, 4 years, the cells are still at the 3.6v range. Only self-discharges will drop significantly and they should be discarded. I believe LifePo4 to behave similarly.

I'd just draw them down to the 80% level mentioned above and let them sit. If they are healthy, they should be good to go for several years and ready to assemble into a battery.