Is it ok to top balance LiFeP04 cells if solar system won't be assembled for a month or two?

[RCinFLA]

I would not leave a new battery at full charge for several months. A used battery would be no problem.

Especially with many recent LFP cells using pre-lithiated anodes, it may need a few moderate charge/discharge cycles to fully develop its SEI protection layer.

If they are truely new cells with pre-lithiated anodes, they can bloat a little if fully charged and left to sit at full charge state for a couple of months.

 

[NorthTown2022]

I would not leave a new battery at full charge for several months. A used battery would be no problem.

Especially with many recent LFP cells using pre-lithiated anodes, it may need a few moderate charge/discharge cycles to fully develop its SEI protection layer.

If they are truely new cells with pre-lithiated anodes, they can bloat a little if fully charged and left to sit at full charge state for a couple of months.

Well you had me curious so I googled that one.. solid electrolyte interphase. Got an idea of why new cells should be cycled now. Thank you.

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[Dzl]

It may need a few moderate charge/discharge cycles to fully develop its SEI protection layer.

Can you characterize more specifically what you mean by this (a few moderate cycles)? Full cycles, but low C-rate?

 

[RCinFLA]

Original way to grow SEI layer,which greatly reduces electrolyte damage during charging, requires manufacturer to do a several days to over a week long step charging/discharge process. It cost a lot for charging equipment and time. It is called 'charge forming'

It grows the SEI layer while minimizing electrolyte damage. The forming of SEI layer consumes about 5% of available cathode lithium so cell capacity is reduced by about the same amount. Just cost of building a cell.

For a couple of years, about the time welded studs started to show up, some cells manufacturers started using pre-lithiated anodes. This is done during negative graphite screen printing paste to copper foil. It sprinkles a lithium compound on the graphite that will react to create a SEI protection layer on its own. It takes about a week or more for it to react and make the SEI coating around graphite granules. It also does not use the lithium from cathode so it saves the 5% capacity loss of original charge forming process.

It is not quite as complete SEI build as the original lengthy precharging process. The cells are also totally assembled so there is sometimes a bit of electrolyte breakdown and gas bloating on first few charge cycles. Many folks have complained about 304 AH cells showing some small amount of bloating.

SEI continues to grow every time cell is charged and it repairs cracks in SEI layer. It is part of normal cell aging. Over time it grows in thickness restricting ion flow so it increases cell impedance and consumes available lithium which reduces cell capacity over time.

The purpose SEI is to restrict electrons entering negative electrode during charging from jumping to electrolyte which create electrolyte decomposition. The greater the state of charge the more likely the electrons will make the jump into electrolyte.

 

[Just John]

Just received 16 280ah grade A EVE cells which are all sitting at about 3.29Voc. It will be a month or two before I have my panels and inverter wired in.

Just leave them until you need them, they are at storage voltage (more accurately, a storage state of charge). They should be cycled every 6 months, then be brought to 20 to 50% state of charge for storage. Most are shipped at 30%.

I was told not to bother with a top balance, to just hook them up in parallel and let them sit for a week or so before building my battery. Thoughts?

This person is giving you bad advice. This will not balance your cells.

I purchased the 10A Longwei power supply and have the time. Would it be ok to top balance if cells will be sitting for a month or two at a full SOC?

No, it is actually bad to store them fully charged.

 

[Just John]

Don't put them in parallel.
Put a number on each cell and make a note on your computer of the voltage of each cell.
Consider the next month or 2 a self discharge test.
Check the voltage again and compare the results.
This is a good test to run on the cells before you make them into a battery.
If you get one or more cells that leak like a sieve it will inform your subsequent actions.

This is good advice. ?

 

[Just John]

I cannot see any benefits to top balancing them now and storing them at a full SOC for two months.

In fact you will indeed cause battery degradation storing them full. Ask me how I am 100% certain of this.